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Winter injury of fruit trees : an analysis of factors responsible for the 1949-50 winter injury to cherry,… King, Earl Maurice 1954

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W I N T E R I N J U R Y OF F R U I T T R E E S An analysis of factors responsible for the 1949-50 Winter Injury to Cherry, Peach, and Apricot trees in the Okanagan Valley of British Columbia with Recommendations for the care of injured trees* B T E A R L M A U R I C E K I N G A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE IN AGRICULTURE In The Department of Horticulture We accept this thesis as conforming to the standard required from candidates for the degree of Master of Science in Agriculture. Members of the Department of Horticulture The University of British Columbia May 1954 ABSTRACT  Winter Injury of F r u i t Trees. This report contains an analysis of the factors responsible f o r the 1949-50 winter i n j u r y to cherry, peach and a p r i c o t trees i n the Okanagan V a l l e y of B r i t i s h Columbia, Canada. Included i n the report are sections dealing with the h i s t o r y of winter injury, various theories of tlie causes of winter in j u r y , and a description of the many forms of injury. The non-climatic and cl i m a t i c factors a f f e c t i n g the i n t e n s i t y of i n j u r y are discussed i n d e t a i l . Recommendations f o r minimizing the s u s c e p t i b i l i t y of trees to winter i n j u r y under Okanagan V a l l e y conditions are included, together with recommendations f o r the care of trees a f t e r i n j u r y has occurred. Reports on the r e l a t i v e hardiness of s p e c i f i c stone-fruits are presented i n some d e t a i l . The report i s l i b e r a l l y supplied with tables i n d i c a t i n g the extent of crop damage and tree damage i n each d i s t r i c t of the Okanagan Va l l e y . These tables are based on observations made i n over two thousand orchards. The s t a t i s t i c a l analyses are based on detailed observations carried out i n over four hundred orchards. The published l i t e r a t u r e dealing with winter i n j u r y has been f r e e l y consulted, and reference i s made to many of the important papers dealing with the winter i n j u r y complex. A complete bibliography i s included. T A B L E OF C O N T E N T S Page FOREWARD 1 ACKNOWLEDGMENT 2 (I) INTRODUCTION 3 (II) HISTORY OF WINTER INJURY 4 (III) THE OCCURRENCE OF FROST INJURY 5 (A) HOW FROST KILLS PLANTS 7 (B) FROST RESISTANCE IN PLANTS 11 (C) METHODS OF ASSESSING FROST RESISTANCE OF ANY VARIETY 14 (D) WINTER HARDINESS - A COMPLEX 16 (IV) TYPES OF WINTER INJURY 19 (A) BUD INJURY 19 (B) TWIG INJURY 20 (C) TRUNK AND SCAFFOLD INJURY 21 (D) "BLACK-HEART" 23 (E) SUN-SCALD 23 (F) ROOT INJURY 25 (V) DELAYED EFFECTS OF WINTER INJURY 27 (VI) NON-CLIMATIC FACTORS AFFECTING INTENSITY OF INJURY 28 (A) SITE AND SOIL 28 (B) FERTILIZING PRACTICES 29 (C) CULTIVATION, COVER CROPS AND MULCHES 31 (D) PREVIOUS CROPS 33 Page (E) PRUNING PRACTICES 33 (F) CONDITION OF TREES 35 (G) DEFOLIATION 36 (H) OTHER FACTORS 37 (VII) RELATIVE HARDINESS OF SPECIFIC STONE FRUITS 39 (A) CHERRY 39 (B) PEACH 41 (C) APRICOT 44 (D) PRUNE AND PLUM 46 (VHI)THE 1949-50 WINTER IN THE OKANAGAN VALLEY 47 (A) NATURE OF DAMAGE 47 (B) RECORD OF TEMPERATURE AND PRECIPITATION 57 (IX) MATERIALS AND METHODS 61 (X) RESULTS AND DISCUSSION 64 (A) EFFECT OF DISTRICT, SOIL TYPE AND KIND OF FRUIT 65 (B) RELATIVE COLD INJURY TO SEVEN VARIETIES OF APRICOT 68 (C) RELATIVE COLD INJURY TO SIX VARIETIES OF CHERRY 70 (D) RELATIVE COLD INJURY TO EIGHT VARIETIES OF PEACH 72 (E) EFFECT OF LATE IRRIGATION 73 (F) EFFECT OF AGE OF TREE 75 (G) EFFECT OF PRUNING TECHNIQUE 77 (H) EFFECT OF VIGOUR 79 (I) EFFECT OF CULTIVATION 81 (J) EFFECT OF SNOW MULCH 82 (K) EFFECT OF PREVIOUS CROP 83 Page (XI) MEANS OF OFFSETTING WINTER INJURY 83 (A) SITE 83 (B) SOIL 84 (C) HARDY ROOTSTOCKS, FRAMEWORKS AND VARIETIES 85 (D) HANDLING YOUNG TREES TO PREVENT SUN-SCALD 86 (E) COVER CROPS OR MULCHES VS. CULTIVATION 86 (F) FERTILIZER PRACTICES 87 (G) PRUNING PRACTICES 88 (H) THINNING THE CROP 89 (I) IRRIGATION PRACTICES 89 (J) OTHER PRACTICES 90 (XII) CARE OF TREES AFTER INJURY 91 (XIII) SUMMARY 94 BIBLIOGRAPHY 97 APPENDIX A 108 APPENDIX B 124 F O R E W A R D Periodically, during its history, the Okanagan Valley of British Columbia has been subjected to a winter weather pattern which has caused untold low-temperature damage, not only to the abundant crops of fruit which can be produced in this fertile valley, but also to the fruit trees which constitute its major economy. The vast devastation of crops and trees during the severe winter of 1949-50 was manifest throughout the entire valley, Tet, during the spring of 1950, there could be seen marked contrasts in the survival of various orchards. One orchard of a specific kind and variety of fruit would show almost nothing but dead trees, while an adjoining orchard, comprising exactly the same kind and variety of fruit, would show almost one hundred percent survival of trees bearing at least a partial crop. This phenomenon and others of its kind were responsible in part for prompting the present study, which was planned in an endeavour to determine the pre-disposing factors responsible for such marked differences in the extent of winter injury from one orchard to another. - 2 -A C K N O W L E D G M E N T The writer wishes to express sincere appreciation to Dr. A.F. Barss, Professor and Chairman of the Department of Horticulture at the University of British Columbia for his valuable assistance throughout the study of this subject and during the preparation of this thesis. Appre-ciation is also extended to Dr. G.H. Harris and Dr. C.A. Hornby of the Department of Horticulture for their assistance in the background and statistical aspects of this study. - 3 -(I) INTRODUCTION Winter injury over a period of years has caused a heavier financial loss to Okanagan Valley orchardists than has any other single factor. When the injury i s confined to the fruit buds and so affects only the current seasons crop, the loss i s serious enough, but when this injury reaches out to k i l l thousands of trees and to injure severely thousands more, the economic security of the entire valley is devastated. Not until replacements for the dead and injured trees have passed through the various stages of orchard culture to become bearing trees, can the orchard industry flourish in anything like i ts normal manner, and never can the new trees produce heavily enough to compensate the orchardist for the losses that winter injury has thrust upon him. Experimental data dealing with the nature of winter injury, and the resistance of woody plants to i t , indicate that certain cultural practices tend to lessen the severity of this injury to fruit trees and thereby to reduce the accompanying economic loss. Yet, so complex is the nature of plant hardiness and winter injury, that many of the thousands of experiments dealing with these phenomena have produced inconclusive results. The pattern of winter injury to stone-fruit trees in the Okanagan Valley during the winter of 1949-50 indicated that certain combin-ations of factors resulted in a high percentage of tree survival, while other combinations resulted in a low survival rate. If i t could be shown which combination of factors cculd be consistently relied upon to minimize winter injury under Okanagan conditions, i t might then be feasible to establish a stereotype procedure in handling orchard trees so that economic loss from winter injury would be greatly reduced. Although some of the great volume of published data dealing with winter injury is found to be conflicting, i t does, nevertheless, form - 4 -a good working background for the present comprehensive survey of stone-fruit orchards, which was carried out during the spring and summer follow-ing the 1949-1950 freeze. (II) HISTORY OF WINTER INJURY Throughout North America, meteorological records indicate that the weather has been, from time to time, very prone to extreme changes. Such extreme changes, especially in temperature, invariably wreak havoc among orchards in the particular areas affected. Fraser (39) states that even •though some areas have been described as "frost-free", yet under certain conditions "any and a l l parts of the temperate zone in North America are sub-ject to frost. No part of the mainland of United States or Canada has abso-lutely escaped freezing temperatures". According to Hedrick (47), in the twenty-five year period beginning 1881 and ending 1905, the peach crop was destroyed or seriously damaged over a large part of New York in thirteen separate seasons. Cowart and Savage (27) report that winter injury is the outstanding cause of tree losses in a l l peach-growing sections of Georgia and i t is unusual to find orchards ten or more years old s t i l l with a good stand of trees. Gourley and Howlett (42) report that during a period of one hundred sixty years, there have occurred in the northern United States at least nineteen recorded winters, or approximately one year in nine, in which abnormally low temperatures have been attained. The severity of some of these freezes is well illustrated by Maney (56), who states that the winter of 1940-41 killed from 80 to $0% of a l l apples and 957? of all peaches in Iowa, Northwestern Missouri, Nebraska and Kansas. The Okanagan Valley of British Columbia has been subjected to a slightly greater frequency of severe winters than have orchard districts - 5 -in the United States. Records indicate that in this area approximately once every six or seven years a winter of unusually severe temperatures occurs. Mann and Palmer (59) report severe winter injury to orchards in 1909, 1916, 1924, 1929 and 1935. Again in 1942 and 1949, unusually low temperatures caused severe injury to trees. The winter of 1949 was especially disastrous, the B.C. Fruit Growers Association (68) reporting tree losses ranging from 25?? to 100?? in certain orchards throughout the entire valley, and a total crop loss value in 1950 of $5,652,183. The B.C. Department of Agriculture (8), following an official survey of damage during that winter, reported that 336,110 fruit trees were killed outright and that many trees were le f t so permanently weakened that they became susceptible to diseases and breakage which further reduced their economic value in subsequent years. (HI) THE OCCURRENCE OF FROST INJURY Frost injury in Okanagan orchards usually results from one of two main types of cold air movement. The most common type of frost injury i s caused by radiation frost, which usually occurs at night when the air is calm and the sky is free from clouds. Under these conditions, heat which the earth has absorbed from the sun during the day, passes by radiation into the air mass adjacent to the ground. Thus, a l l soil and plant surfaces exposed to the air, cool. The warm air mass i s quickly replaced by denser cold air from above i t , and when the temperature of this air falls below the freezing point of water, we say that there has been a frost. Because the heat was lost by radiation, i t i s called a radiation frost. Such frosts are a feature of a l l arid regions due to the intense radiation made possible by the generally clear skies and lack of moisture in "the atmosphere. The cold air layer near the ground is usually quite thin. Above i t lies a layer of warmer air, perhaps 10° warmer, to an altitude of up to a thousand feet. It is not usually until the early - 6 -morning hours that the temperature of the air reaches the freezing point and its duration may not be long enough to cause injury to plants. According to Day (30), the air is often so dry when radiation occurs that frost does not form at 32° F. or even at several degrees below^  32° F. It is readily apparent that orchardists may be able under certain conditions to cope with radiation frosts and thereby to offset much of the damage which the frosts might cause. In some orchard areas i t is quite common practice to raise the temperature of the air itself or to move the thin low lying layer of cold air by using heaters in the orchards. Convection currents set up by such heaters tend to mix the upper warm air mass with the cold air adjacent to the ground, thus raising the temperature of the air surrounding the trees. Even more fundamental is the knowledge that different vegetation cover varies in the amount of heat radiated. Experiments conducted by Comford (26) indicate that meadow grass gave rise to the coldest air, while bare soil gave rise to the warmest, the difference in temperature being 6° F. between the two extremes. Comford explains that grass forms a very large surface from which heat radiates and as the stems are poor conductors of heat, they tend to prevent the heat in the soil from passing up to replace the heat lost by radiation, so the grass blades are greatly cooled and the air in contact with them is cooled too. Bare soil, on the other hand, has the smallest possible surface from which to radiate heat and any heat radiated is quickly replaced by heat from the deeper soil levels, so that the air above bare soil i s not cooled as much as that above grass. Since radiation frosts can be offset to some extent by various orchard practices, and since they are a menace for only a short period around blossom time in certain years, and only in certain orchards, they usually cause only minor scattered losses in the Okanagan Valley, - 7 -Of much more serious nature is the injury caused by a general lowering of temperatures far below the freezing point. These low temperatures usually accompany the southward movement of a high pressure polar air mass. Such cold air movements may occur at any season, but are usually confined to the winter months. They are characterized by a general lowering of temper-atures over a wide area and are often accompanied by drying winds. If the cold air mass is relatively stationary, radiation may occur within that mass, thereby causing localized differences in temperature. The nature and extent of the damage to fruit trees caused by these massive cold air movements depends largely upon the growth stage of the tree, the temperature pattern preceding and during the freeze, the duration of the freeze and absolute minimum temper-ature attained and many other factors, a l l of which contribute to the complex-ity of the winter injury problem. The winter of 1949-50 in the Ok an ag an Valley was marked by prolonged periods of low temperatures and severe drying winds. Consequently, damage which was recorded following this particular winter resulted from this latter type of freezing and not from radiation frost. (A) HOW FROST KILLS PLANTS The most widely accepted theory of how frost k i l l s plants has evolved from a number of highly speculative theories, each of which probably contains some truth. Levitt (54) has consolidated masses of evidence into a fairly sound theory of frost injury. His theory asserts that as the temperature of a plant falls below its freezing point, its aqueous contents undercool to some degree. Since frost cannot injure a plant in the undercooled state, some species of plants are seldom injured by frost. They have the ability to undercool to about-20° C. and below this temperature their c e l l contents vitrify. To this end, Chandler (19) points out that such tissues as pollen - 8 -and seeds when dried until there is no water to form ice, may not be killed at temperatures as low as-328° F« There may, however, be ice formation in the tissues of even the most resistant plants. Usually ice begins to form in plant tissues i f that plant cools below i t s freezing point. The ice crystals originate on the cell walls since the walls are covered with pure water. The cell sap, which contains many dissolved substances, does not freeze, but loses water by exosmosis. This water forms ice crystals mtracellularly, leaving the cell sap progressively more highly concentrated and progressively more resistant to freezing. Even when the temperature does drop below the freezing point of the cell sap, the plasma membrane and cel l wall act as effective barriers against inoculation by the ice crystals in the intercell-ular spaces. As the water moves.out of the cells, the cells shrink. The formation of ice in the intercellular spaces i s accompanied by expansion. Dehydration of the plant cells may increase the consistency of the protoplasm to a point where i t wil l coagulate and become irreversible. During the thawing process, however gradual i t may be, the change in cell form due to endosmosis is likely to cause a fatal rupture. A l l this applies only to injury caused by extracellular ice formation, which may have been the type which occurred in Okanagan fruit trees in 1949-50. But i f ice formation occurs inside the cells, due to a sudden temperature drop, the crystals may lacerate the protoplasm and destroy cell structure. Hardy plants are known to resist extensive ice formation within cells by virtue of their great protoplasmic hydrophily which reduces the volume of ice crystals forming within the c e l l . The more capable a plant is of preventing intra-cellular freezing, the hardier i t i s . Whether or not the ice will be confined to the intercellular spaces depends on whether or not the freezing point of the ce l l contents can drop as rapidly as its temperature. For i f the ce l l cools below its freezing point, its undercooling point will be reached and ice will form. Thus the speed of the cooling and the speed of exosmosis of water from the ce l l deter-mine the freezing resistance of that c e l l . Rapid freezing of a plant cell is more injurious than slow freezing since i t induces intracellular ice formation. In tender plants, cell permeability to water is very low and intracellular ice formation may occur even when the temperature drop is gradual. Hardy plants, on the other hand, have high permeability rates and can withstand rapid temperature drops without intracellular ice formation. Even after a plant is fully thawed, frost injury may progress for some time since the cells may be injured though s t i l l alive. Repeated freezing and thawing of a plant must increase the opportunity for intracellular ice formation unless each freezing i s suffi-ciently gradual to prevent i t . Repeated expansion and contraction of the c e l l may injure the protoplast much more than a single contraction and expansion. A gradual decrease in temperature produces increased permeabil-ity, sugars and osmotic pressure within the limits of the kind of plant tissue exposed. Thus protoplasmic consistency and permeability may be considered the key factors in frost resistance. A hardy plant owes its high cellular permeability and its low protoplasmic consistency (even when dehydrated) to an increased protoplasmic hydration. Its plasma membranes have larger aqueous pores than those of tender plants and permit more rapid passage of water and other substances. The protoplasm as a whole is able to retain a relatively large quantity of water even at low freezing temperatures and therefore does not coagulate so soon. Severe cases of cel l shrinkage will often cause cracks in the wood of a tree. Chandler (20) points out that this results from the greater - 10 -contraction of the tissue of the tree tangentially than radially. Cells of the medullary rays, which are situated between wedges of rigid tissue, shrink to a greater extent than any of the cells situated radially. Thus the tendency is for the wood of a trunk or branch to crack longitudinally. During long cold periods, buds and small twigs are often killed outright. This "type of injury can probably be attributed to the evap-oration of moisture from these buds and twigs. This cannot be replaced by the roots because the conducting tissue of the tree is frozen. Killing of this kind seems to be associated with regions having strong prevailing winds and continuous low winter temperatures. Dorsey (32), in an examination of freezing phenomena within peach flower buds, discovered that during any period when temperatures are dropping, cellular water moves away from -the vital flower parts (stamens and pistil) to lower bud scales, thus increasing the protoplasmic consistency and frost resistance of these vital parts. Sudden low temperatures, however, caused ice formation within the cells of the vital flower parts and their subsequent death. Following the break in the rest period, the cells of vital parts failed to release water as readily as before and were thus more suscept-ible to intracellular ice formation and subsequent frost damage. Dorsey found that cells in peach leaf buds are able to release water more rapidly than those of fruit buds and thus have more resistance to cold. Levitt (54) found that frost injury to cell structure was closely related to the particular weather pattern of any given region. In one region the low temperature may be severe but constant. It may come gradually and leave gradually. In another region, the minimum temperature may not be so low; yet the fluctuations may be sudden and frequent. In the former, a hardy plant is one that is capable of surviving tremendous extra-cellular ice formation. In the latter, this is not necessary, but the - 11 -essential thing is an ability to prevent intracellular ice formation and injury from the rapid thawing. Even the rate at which the plant hardens off may vary sufficiently from one variety to another to play a decisive role. This may depend on the reactions of the plant to the photoperiodic conditions which prevail, i.e. on its developmental stage at the time when the frosts occur. Chandler (19) suggests that low-temperature killing of plant tissue may occur when the harmful effects are not due to ice formation within the tissues. The cells may be killed by products of respiration at temperatures just above freezing. He points out that some plants die slowly when grown where the summer and winter temperatures, although above the freezing point, are below that which they require for optimum performance. (B) FROST RESISTANCE IN PLANTS The portions of a tree most likely to be injured by low temper-atures are determined by the circumstances of the freeze and the condition of the trees. (42) Generally, the root and crown are the most susceptible parts, but since vegetation and snow cover usually protect them from the same extremes of temperature to which the remainder of the tree is exposed, they often escape damage from freezes which k i l l less susceptible parts. According to Palmer (65), the roots and crown are usually damaged most by early f a l l freezes. In general, fruit buds, spurs and twigs are more readily injured by lov; temper-atures than are the trunk, crotches and main limbs. Auchter and Knapp (5) observe that in well-hardened wood, the pith is usually the least frost resist-ant tissue, followed in order by sapwood, bark and finally cambium tissue. Knowlton and Borsey (52) and Chandler (19) have demonstrated that blossom buds become resistant to cold towards the end of the growing season. An autumn or early winter freeze may k i l l leaf buds or other tissue - 12 -in the tree when i t does not k i l l blossom buds. However, during the middle of winter i t is generally accepted that blossom buds tend to be a l i t t l e less resistant to cold than do leaf buds or cambium. During this dormant period, blossom buds of the more tender species such as apricot and peach have been shown to withstand temperatures as low as-22° F.$ those of hardy species such as apple, have withstood temperatures as low as-40° F. Chaplin (21), experimenting with peach buds, found that blossom buds withstood a temperature of 8° F. with l i t t l e injury even though the leaves had not yet fallen from the trees. This cold resistance increased by several degrees just after leaf f a l l and greatest hardiness was achieved about thirty days later. This point of greatest hardiness seems to co-incide closely with the end of the rest period. Following this point of greatest hardiness, the buds lost resistance with the advent of each warm spell and regained some resistance with the advent of each colder spell, although they never did regain their maximum cold resistance once the rest period was broken. It would therefore appear that peach buds, i f not all blossom buds, lose hardiness with each warm period during the late dormant season. Knowlton and Dorsey (52) found -that there was considerable variation in the degree of development of the blossom buds on different parts of the tree by mid-winter, as measured by their degree of pollen differentiation. In general, buds on the bases of terminal twigs were latest to develop, buds in the mid-portion of twigs next and terminal buds far-chest advanced. The fruit buds borne on the short spurs of the inner parts of the tree were, on the average, slightly ahead of the basal buds on the outer terminals. In a l l cases, the buds which were farthest advanced, were the least hardy. Experiments with cherry buds led Roberts (70) to conclude that the rate at which blossom buds develop after differentiation determines their frost resistance. Too rapid development of the blossom buds tends to promote - 13 -the presence of large vacuoles i n the cytoplasm of the bud c e l l s . Thus the cytoplasm lacks the high density which i s associated with frost r e s i s t -ance. Chandler (19) agrees with this point of view, stating that flower buds of some varieties are more resistant to cold i f the differentiation of flower parts does not advance too far before the cold weather arrives. Trees that cease growth very early i n summer may cause their flower buds to advance too far by the end of autumn for maximum cold resistance. Most investigators agree that the rate of temperature f a l l greatly influences the amount of tree injury. (Table I (18)) This view i s directly i n accord with the theory of a plant 1s a b i l i t y to undercool i t s c e l l contents to a point below i t s actual freezing point. Blossom buds retain TABLE I Effect of slow and rapid temperature f a l l on freezing to death of plant tissues Kind of buds Manner of freezing Date No. of buds % k i l l e d Montmorency cherry Slowly to - 20° c. Mar. 2 163 3.0?? Montmorency cherry Rapidly to- 20° C. Feb.29 120 96.0?? Early Richmond cherry Slowly to -20° C Mar. 9 297 5.0?? Early Richmond cherry Rapidly to- 20° c. Mar.14 263 98.0?? a f a i r l y high degree of frost resistance long after the breaking of the rest period provided that the weather i s not warm enough to cause them to swell. As they take up water from the vascular system, the cytoplasm becomes less dense and their resistance drops rapidly u n t i l the flowers are f u l l y open. At the f u l l bloom stage, a blossom which could withstand temperatures below '30° F. i n mid-winter may now be k i l l e d by a temperature of 24° F. or higher. Following f u l l bloom, there i s a further decline i n frost resistance i n most species, so that small f r u i t s may be destroyed by two or three degrees of frost. - 14 -Chandler (19) relates the importance of foliage i n deter-mining the frost resistance of a tree. I f the foliage i s damaged or removed by insects or by pruning i n late summer, a l l the above ground portions of the tree become less cold-resistant than i f the foliage had remained. I t appears that some substance i s translocated normally from the leaves to the wood and that this substance i s v i t a l to the cold resistance of a tree. This theory appears to be borne out by a study of the relative acquisition of cold resistance of different portions of the tree. The l a s t wood to acquire maximum cold resistance i s usually the basal part of the trunk which i s farthest from the leaves. The trunk, crotch and framework are next in order of lateness, while the small branches and twigs acquire frost resistanbe quite early i n the winter* Much of the explanation of frost resistance i n various tissues i s highly speculative. The present knowledge of the nature of cytoplasm i s incomplete and hence the present knowledge of the changes i n cytoplasm i s incomplete. I t appears to become more permeable to water during the hardening process, but this i s l i k e l y to be only one of many changes that occur i n the physiology of the plant. (C) METHODS OF ASSESSING FROST RESISTANCE OF ANY VARIETY Some writers have said that i t takes at least ten years to determine the frost resistance of a variety because only i n this length of time i s there apt to occur a sufficient number of winters severe enough to cause cold injury. This i s an understatement of the fact. As Levitt (54) points out, "It i s no exaggeration to say that present methods never succeed i n establishing the relative hardiness of a variety under a l l conditions". Within the past few years, however, new techniques for establishing the relative frost resistance of any variety have been developed. - 15 -Levitt and Scarth (53) found that c e l l permeability was of practical value i n predicting the hardiness of woody plants. Immersing plant material i n a strong electrolyte such as KNO^  , they found that the difference in permea-b i l i t y of the cells of various plants afforded an easily applied measure of their potential frost resistance, thus offsetting the necessity of waiting for a test season. On the basis of this permeability test, they classified the varieties as non-hardy, semi-hardy, and hardy. Swingle (77) describes the exosmosis method of determining injury from low temperatures. This method i s based on the assumption that the release of electrolytes by the c e l l , measured by electrical conductivity-constitutes a direct reading of the amount of injury i n f l i c t e d by a given treatment. This, i n turn, i s used as a measure of the frost resistance of the plant i n question. Meader, Davidson and Blake (60) used the exosmosis method for rating hardiness of peach f r u i t buds. They used controlled a r t i -f i c i a l freezing of dormant f r u i t buds to provide a rapid and reliable method for estimating the relative cold hardiness of f r u i t buds of different varieties of peach. When varietal samples were so frozen that only 1% to 15/2 of the buds of Elberta, a criterion variety, remained alive, other varieties tested had percentages of l i v e buds that compared favourably with their previous response i n the orchards. Variations occurred, however, i n the hardiness of buds of the same variety on qualitatively different twigs. These variations emphasize the importance of care i n the selection of samples used i n such comparisons. The results of these tests indicate the advisability of testing the cold hardiness of a variety two or more times during the dormant season, since weather conditions prior to collection of samples do not affect a l l varieties i n the same way. These workers (60) have found that the structure and composition of the wood are directly related to hardiness, as i s the moisture content of - 16 -buds. Hardiness i s also related to the freezing point lowering of the c e l l sap, moisture content of a l l c e l l s , ash determinations and the adsorption • by c e l l s of certain dyes. Cold hardiness has also been measured by the enzymatic activity and by the plasmolytic method for determining osmotic pressures. These are a l l steps towards a common end and there i s now a great need for co-ordination of these methods into one framework of tests which w i l l establish relative hardiness of varieties i n a short period of time. Once established, such a series of tests would not only furnish information which now takes many years to obtain, but would also permit a prediction of the behaviour of a variety wherever long-time climatic records are available. Such predictions would be of great value to plant breeders who want to know the value of new varieties when they arise and who constantly endeavour to plan their breeding programs along.certain directed l i n e s . (D) WINTER HARDINESS - A COMPLEX Many hundreds of articles have been written on the winter hardiness of deciduous woody plants. From these articles has developed the concept that hardiness i s a complex of several specific factors. Brierley (13) defines the hardiness of a woody plant as i t s "overall a b i l i t y to escape injury during the varying conditions of winter weather over a period of several years". There seems to be no particular temperature at which k i l l i n g of wood and buds i s certain. Campbell (17) and others have shown that the c r i t i c a l temperature at which death occurs i s not a definite point for any species, variety, or individual plant, but that i t i s governed by a complex of conditions. Among the prime requisites of hardiness i n any plant are f u l l maturity at the onset of cold weather, healthful condition, and the ab i l i t y to withstand desiccating winds. A plant must be mature before i t can r e s i s t - 17 -cold, but the fact that i t has matured does not guarantee hardiness, since some other factors may prevent i t from developing i t s highest degree of cold resistance. Then, too, a well-grown healthy tree, with adequate food reserves, medium-sized crops, and freedom from parasites, w i l l invariably survive cold weather better than trees which are devitalized, have cropped heavily or been weakened by the presence of parasites. In addition to this, plants which are protected from winter desiccation by windbreaks and shelter belts are more l i k e l y to survive cold temperatures than those which are exposed to the drying winds of winter. But, i f a plant i s to be considered "hardy", i t must be able to survive drying conditions as well as low temperature. And i f the cause of such winter drying i s not accurately determined, the hardiness factor becomes more complex. . Hardiness ratings of woody plants are very d i f f i c u l t to assess owing to great variations i n rest periods, dormant periods, the time and rate at which they develop cold resistance, their ultimate cold resistance, and their ability to lose or retain cold resistance. The rest period of many plants i s readily broken by a spell of warm weather following a cold s p e l l . Late i n the dormant season, for example, the rest period of apricots i s more easily broken by warm weather than i s that of the apple. Consequently, apricot trees are more readily injured by late cold snaps than are apple trees. Similarly, the various species of woody plants dif f e r i n their emergence from dormancy. Although most species are unable to grow at temperatures below 41° F«, a few woody plants actually commence growth at temperatures between 35° F. and 40° F. Since growing plants are more susceptible to cold injury than are dormant plants, i t i s readily seen that the dormancy characteristics of woody species have a direct bearing on their hardiness. The time at which a plant develops cold resistance, bears directly on i t s hardiness rating as well. If a certain species or variety - 18 -f a i l s to develop cold resistance early enough to protect i t s e l f against the temperatures prevailing i n the d i s t r i c t i n which i t i s growing, i t obviously lacks hardiness for that d i s t r i c t . In the same way, a plant may f a i l to develop i t s cold resistance quickly enough to cope with the rapidly f a l l i n g temperatures which may be characteristic of a certain area. Again i t lacks hardiness for that area. Plants have been shown (13) to lose hardiness with the onset of each mild spell during the dormant period. They have also been shown to regain some of that hardiness.when the temperature drops again following the mild s p e l l . But plants vary greatly i n the speed with which they lose and regain hardiness. One that loses hardiness slowly and regains i t quickly i s said to be hardy.from this point of view. The ultimate cold resistance of plants determines their absolute hardiness and i s a genetic characteristic. Plants vary greatly i n their ultimate cold resistance. Among the tree f r u i t s , Chandler (19) reports that i n general, apricot, almond and peach are not as hardy as apple and pear. Campbell (17) reports that thirty varieties of peaches survived a temperature of-32° F., while some apple trees did not. This exception, however, would appear' to relate to some other factor of the hardiness complex than the ultimate hardiness. With so many variable factors entering into the hardiness complex, there i s l i t t l e wonder that so much has been written about winter injury and so l i t t l e accomplished to counteract i t . Perhaps scientists should attempt to evaluate only one variable at a time, instead of attacking the hardiness problem as a whole. Certainly the hardiness complex i s l i k e l y to remain a complex u n t i l i t i s taken apart, factor by factor, and then put together again to give a true picture of the problem. (IV) TYPES OF WINTER INJURY Winter injury to f r u i t trees may be manifested i n many dif f e r -ent ways. Not a l l the effects of low temperatures are quickly recognized, since the injuries i n the trees may linger for many years and may be accom-panied by other complications which obscure the basic trouble. Several types of winter injury, however, are quickly recognizable and a knowledge of their symptoms i s important i n determining the treatment which an injured tree should receive. (A) BUD INJURY Although leaf and f r u i t buds may both be k i l l e d by low temp-eratures, the leaf buds are generally much hardier than the f r u i t buds. Higgins et a l (48) explain that leaf bud c e l l s contain dense cytoplasm with small vacuoles, and do not freeze readily, while f r u i t bud cells generally have less dense cytoplasm with large vacuoles and are therefore more prone to winter injury. Instances have been recorded i n which leaf buds have been k i l l e d or injured while a portion of the flower buds have survived and pro-duced f r u i t i n the absence of leaves. This phenomena has been explained on the basis of lack of maturity of the leaf tissues, while the flower buds reached maturity before the freezing occurred. Fruit buds are usually k i l l e d i f they are subjected to frost i n early winter before they are f u l l y hardened off. Again, they may be k i l l e d by low temperatures following a warm period i n winter which has broken their rest period. And f i n a l l y , they may be k i l l e d i f the temperature drops to a point below their absolute cold tolerance. Very often, the injury to f r u i t buds i s not easily discernible, only the tender p i s t i l having been k i l l e d . In such cases, the flower w i l l break open quite normally and then f a l l off when the f r u i t f a i l s to set. I f the temperature drops below the - 20 -absolute cold tolerance, the flower buds do not swell, but dry up, shrivel and drop off. This characteristic i s sometimes noted i n stone f r u i t s but seldom i n apples and pears, the f r u i t buds of the latter two being well protected i n clusters. In the Okanagan, f r u i t buds of apricot, peach, and cherry have shown more injury from low temperatures than have buds of other f r u i t trees, their absolute tolerance ranging between-15° F. and-30° F., depending on variety and other factors. Prunes and plums follow closely i n their tolerance, while pear and apple f r u i t buds w i l l often tolerate as much cold as w i l l their twigs, and hence their absolute tolerance may be governed by the tolerance of their twigs. Since 90% or more of the f r u i t buds of a tree may be winter k i l l e d without seriously reducing the crop of saleable f r u i t (10), a moderate k i l l i n g of f r u i t buds does not appear to be a serious problem. Certainly i t does not constitute as serious a menace to f r u i t growing as do certain other types of winter injury. (B) WIG INJURY Low temperature injury to small fruit-bearing twigs i s rather common i n the Okanagan Valley. This type of injury probably results from a desiccation of such twigs-by strong drying winds which are usually associated with Okanagan winters. During long cold periods, the twigs tend to give up moisture which cannot be replaced by the frozen vascular system of the tree. When the cytoplasm within the twigs i s coagulated beyond a certain point by evaporation of water, i t i s rendered incapable of taking up water again, and hence the twig dies. A less common type of winter injury to twigs i n the Okanagan i s that which involves the k i l l i n g of terminal twigs through failure of the twigs to harden off before the onset of low winter temperatures, Such injury i s most prevalent i n apricot and peach, and usually occurs where growth has - 21 -continued too late i n the f a l l . The injury takes the form of a dieback from the tip to the more mature wood at the proximal end of the shoot. Higgins, Walton & Skinner (48), working with several varieties of peach, found that twig injury was greater i n trees of low vigour than i n those of moderate to high vigour. Low vigour trees were shown to be low in total nitrogen, but high i n ash and total carbohy-drates. High vigour trees showed higher N content i n the twigs, which was associated with a high degree of cold resistance. These workers suggest that this association may result either from increased quantity of protoplasm with resulting smaller vacuoles i n the cells of the cambial region, or from the nature of the proteins present i n high nitrogen trees. These obser-vations appear to be i n accord with the usual pattern of twig injury as i t occurs i n the Okanagan Valley. In fact, a deficiency of any nutrient appears to weaken trees, and so render them more susceptible to cold injury. (C) TRUNK AND SCAFFOLD INJURY Winter injury to the trunk and main scaffold branches may take several forms. I t may show up as patches of various extent, localized crotch injury, splitting of the woody tissue, or simply as frost rings. A l l of these conditions were manifest i n the trees of the Okanagan Valley follow-ing the winter of 1949-50. Injury to large or small areas of the trunk or scaffold limbs i s most common when trees grow late into the f a l l and a sudden cold wave appears, or when warm periods during the winter encourage growth activity. Sudden cold does not permit time for the c e l l s i n the trunk to undergo the changes which bring about hardiness. Similarly, i f the ground i s not chilled before the f i r s t snow cover arrives, there w i l l be a tendency for the root system to carry on limited activity, which may, i n turn, bring about injury - 22 -to the trunk. K i l l i n g of trunk tissue i s also common i n varieties which inherently lack hardiness and i n trees which are i n poor vigour due to over-cropping practices. Following k i l l i n g conditions, the affected areas w i l l f i r s t appear as sunken, darkened areas. Later on, the bark w i l l often crack and come away from the wood of the trunk. Sometimes tree trunks are badly injured on the side next to the prevailing wind. This type of damage probably results from the increased evaporation rate on the windward side, together with mechanical damage due to the bending of the tree when the bark i s under tension from freezing. Whenever this type of bark injury extends into the crotches or lower scaffold of a tree, i t i s termed "crotch injury". Most investigators now agree that the crotch of a tree i s among the last of the tree tissues to harden i t s e l f for cold weather, and hence i s susceptible to early winter freezes. According to Fraser (39) crotch tissue often matures late because the foliage on the inside and at the "head" of a tree i s insufficient to carry enough of the resistance-building materials to the ce l l s of the crotch, tissue. Evidence also favours pruning the trees to form wide angles for scaffold limbs and to l e t plenty of l i g h t reach the foliage i n the v i c i n i t y of the crotches. Following extremely low temperatures, the woody tissue of the trunk and framework of a tree often s p l i t s . This splitting, which usually causes the bark to s p l i t open as well, i s caused by a greater contraction of the tree tissues tangentially than radially. The tangential contraction i s brought about by the exosmosis of water from the large c e l l s of the wedge-shaped medullary rays, with resulting shrinkage. The c e l l s of other woody tissues are small and do not lose water by exosmosis to the same extent. If the tree trunk could diminish i n circumference to keep pace with the shrinkage i n the thin-walled cells of the medullary rays, splitting should not occur, but most of the woody tissue c e l l s are thick-walled and do not give up moisture readily. Not a l l types of cold injury are easily discernible from outward appearances. Tingely (80) describes the appearance of frost rings i n woody cross-sections of hardy trees. These rings which appear as dark streaks concentric with the growth rings occur at the beginning of the season's growth, being bordered on the inside by the late wood of the previous year. They are therefore thought to be caused by low winter temperatures rather than late spring frosts. Fraser (39) explains the physiological significance of frost rings on the basis that such injury to the conducting tissues of the tree impairs the rate of travel of water and food materials within the vascular system, so impairing the growth activity of the tree. (D) "BLACK-HEART" "Black-heart" i s a well-coined word to describe the appearance of the injured woody tissues of a tree. The dark, shiny appearance of these tissues i n cross-section has been attributed by Steinmetz and Hilborn (74) to death of the protoplasm i n the parenchyma c e l l s followed by an occlusion of the vessels by a substance resembling wound gum. This occlusion prevents further translocation of materials by these woody tissues but apparently does not affect the normal functioning of the cambium layer, for there are many black-hearted trees producing commercial crops of f r u i t i n certain areas of the Okanagan Valley. Large bearing trees, however, may eventually become punky and hollow owing to entrance and widespread activity of fungi, causing them later to topple over. (E) SUN-SCALD Winter sun-scald i s a type of winter injury which invariably Winter Sun-scald Severe s u n - s c a l d on twenty-year-old 3 i n g c h e r r y t r e e . T h i s e i g h t - a c r e orchard was almost e n t i r e l y wiped out, as can be seen i n the background. Sun-scald i n j u r y on upper s c a f f o l d l i m b o f Red D e l i c i o u s apple t r e e . T h i s type o f i n j u r y appeared to be aggr-avated by removal o f s m a l l f r u i t i n g limbs on upper s u r f a c e o f s c a f f o l d l i m b s . - 24 -occurs on the south-west side of the tree trunks. For this reason, i t i s often called "Southwest injury". This type of. injury results from the trunk of the tree absorbing radiant heat from the sun's rays during cold afternoons i n mid-vd.nter. Almost as soon as the sun goes down, the warm thawed tissue on the south-west side i s again subjected to freezing temp-eratures. The tissue may be alternately thawed and frozen by several successive sunny days and cold nights. Such alternate freezing and thawing of tissue exerts strains on the protoplasm which result i n i t s ultimate death. Experimental work by Mix (62) shows that sun-scald injury i s characterized either by a peeling of bark from the affected parts, or a sinking and adhesion of injured tissue to simulate a sunken canker. According to Chandler (19) this type of injury i s most l i k e l y to occur during a s t i l l afternoon of a very cold day when the air tends to be free from moisture or particles that would obstruct the rays from the sun. If there i s no wind to dissipate the absorbed heat, the tissues on the south-west side may absorb enough heat to thaw them, even on a day when the shaded part of the trunk i s as cold as 5° F. Eggert (35), i n an experiment designed to record winter temperatures in the cambium of peach and apple trees with the aid of thermo-couples found that often the temperature of the cambium reached 80° F. or higher on the south-west side of the tree when the surrounding air temper-ature was below 3 2 ° F. A north wind made l i t t l e difference to the temper-ature readings. He found, too, that differences i n temperature between the north and south sides of trees were as high as 50° to 55° F. I t i s obvious that the tissues subjected to such sudden and marked changes i n temperature must undergo severe stresses which bring about mechanical destruction of c e l l s . Upon thawing, the damaged tissues release water readily and give rise to the areas of dead tissue characterized by "south-west injury". Winter Sun-scald. Sun-scald i n j u r y on south-irest s i d e of t h t h i s c h e r r y t r e e lias caused hark to slough away, and ejcpose trunk wood. H e a l i n g i s w e l l under way i n second y e a r f o l l o w i n g f r e e z e . C h e r r y t r e e showing severe s u n - s c a l d on the trunk and lower s c a f f o l d l i m b s . Photo "taken one y e a r f o l l o w i n g the f r e e z e . - 25 -(F) ROOT INJURY Damage to the roots of deciduous woody plants is a fairly common type of winter injury, especially in winters when the ground i s lacking in snow-cover. As demonstrated by Chandler (19) the root system is the most tender portion of a tree, and the roots become progressively more tender from the crown towards the extremities of the root. In a normal winter, of course, the roots do not require as much cold resistance as the above-ground portions of the tree, since the soil cools more slowly than the air and seldom reaches the extreme low temperatures of the air above the ground level. A sustained period of cold weather, causing ever-increasing soil penetration of frost, will usually cause root injury to some extent. According to Chandler (19), a long cold winter without any particularly severe temperatures may cause extensive root injury and no injury to the top of a tree, while a moderate winter with only a few hours of severe cold may cause a reversal of these conditions. Snow, or any other type of soil cover tends to delay the cooling of the soil by reducing the soil contact with cold air. Root injury, therefore, seldom occurs where the soil is insulated by a heavy cover of snow, sawdust, green manure or other mulch. It is well known that different rootstocks exhibit different degrees of cold hardiness. Although most peach and apricot rootstocks are raised from cannery seed, Blake (9) suggests that where hardiness is of particular concern, seedlings of Early Crawford Iron Mountain, New Jersey and Bell are particularly desirable. In the Okanagan Valley, seedlings of Lovell, Muir and Veteran are most commonly used, Childers (22) reports that some promising hybrid peach rootstocks are being developed from the Indian variety Shalil, the Chinese Types o f I n j u r y " B l a c k h e a r t " i n j u r y has permanently weakened many t r e e s . These n u r s e r y peach t r e e s were k i l l e d back to the snow l i n e , b u t sprouted from below, i n d i c a t i n g t h a t r o o t s were n o t s e v e r e l y i n j u r e d . - 26 -variety, Yunnan, and the Russian variety Bokhara. Elberta seedlings generally show poor cold resistance. Winklepleck and McClintock (83) tested various other peach and apricot rootstocks for hardiness and found that the Florida peach, of PlLnto origin exhibits serious injury after only a one-hour exposure at-15° F. He found Myrobalan Plum was more hardy than a l l peach rootstocks except Prunus Davidiana, Marianna plum stock was very hardy and Prunus Americana was hardiest of a l l stocks tested, showing no injury after forty-eight hours exposure at-15° F. Thus i t appears that some of the hardier plum stocks may be of value as peach and apricot root-stocks where cold hardiness in the roots i s a limiting factor to peach and apricot production. Stewart (75) testing the cold hardiness of various root-stocks, concluded that scion roots are generally hardier than seedling stock roots. Analysis of the roots revealed that the hardiest roots contained slightly more sugar and less moisture than tender roots. Repeated checks on hardiness indicated that roots tend to increase in hardiness during the winter, reaching their maximum hardiness in March. With respect to stock-scion relationships, Stewart found that the hardiness of the scion varieties was not measurably affected by the hardiness of the stocks during one year's growth in the nursery. Stock hardiness, on the other hand, was greatly influenced by the scion variety. However, the hardiness transmitted to the stock by the scion bore no relationship to the hardiness of the scion. When the roots of a tree have been injured by low temper-atures, the trouble may not show up immediately. The trees may start to grow normally, but wi l l soon exhibit lack of size in the leaves, or perhaps wilted leaves and blossoms. Very often the trees will appear to struggle along until the f i r s t hot spell, at which time they may quickly die. Other trees, on which only a portion of the root system may be damaged or killed, - 27 -may struggle for years before finally being removed as unprofitable units in the orchard. To growers who are confronted with the problem of extensive replanting following severe winter damage, the use of hardy root stocks offers hope and encouragement. (V) DELAYED EFFECTS OF WINTER INJURY In many cases winter injury of fruit trees does not command attention at the time of its occurrence. It may induce minor injuries, the consequences of which are not revealed before the original cause is obscured. When a l l the fruit buds on a tree are killed, the loss is plain and the damage i s credited to cold temperatures, but when a small portion of the trunk is killed, i t often receives l i t t l e attention until decay of the wood sets in. When decay occurs, the wood-rotting fungi, rather than the winter injury, are usually credited with the damage. This very subtlety of winter injury makes difficult any appraisal of i t s extent. Much of the injury in the Okanagan Valley during the winter of 1949-50 was of the delayed type. In some cases the trees blossomed, set fruit, and produced foliage, but died suddenly with the advent of hot weather. In other cases, the trees survived the heat but foliage was small, fruit was undersized and terminal growth lacking. These trees usually died before the end of the 1950 growing season. S t i l l other trees appeared quite normal until a few days before the expected harvest, at which time the leaves dropped and the fruit shrivelled. Such trees usually showed exudation of gum on the trunk and crotches. The limbs were brittle and broke easily. But not a l l the after effects of the 1949-50 winter became apparent in 1950, Many of the trees were known to have "black-heart11 and were apparently recovering by virtue of the uninjured cambium layer giving Delayed Effect's o f winter I n j u r y F a i l u r e o f i n j u r e d conducting t i s s u e s i n trunk o f t h i s peach t r e e caused death i n f i r s t h o t weather f o l l o w i n g severe w i n t e r . Twenty-year-old c h e r r y t r e e which e x h i b i t e d severe s c a f f o l d i n j u r y on south s i d e . Tree d i e d two y e a r s l a t e r . - 28 -rise to new tissue surrounding the blackened inner wood. According to Bradford (11), this type of injury is not, in itself, likely to cause the death of the tree, but will undoubtedly weaken its structure. S t i l l other trees were showing dead areas on the trunk and scaffold limbs which opened the way for invasion by secondary parasites. There is no doubt that trees will continue to show the effects of this freeze for many years to come, as they did following the winter of 1941-42, described by Brown (16), in Illinois, It is quite likely that many trees will grow for several years and when they finally begin to go to pieces, the evidence to connect the condition with a slight winter injury several years back will be scant indeed. (VI) NON-CLIMATIC FACTORS AFFECTING INTENSITY OF INJURY Since winter damage to fruit trees i s often associated with factors other than the actual degree of cold prevailing, a review of the most important of these factors is imperative. (A) SITE AND SOIL The site of an orchard and the soil associated with i t are undoubtedly the most important factors determining its longevity. Certainly an orchard located on a site where the drainage is good, and where the temperatures are modified by environment, stands a better chance of surviving adverse winter conditions than one which lacks these attributes. Similarly, trees planted on deep, friable, well—drained soil have a better survival rating than those planted in excessively wet, dry or shallow soils. Low temperature usually takes its heaviest t o l l of trees in depressions in valley bottoms and up against natural barriers to air drainage. As a rule, damage is also heavy in orchards which are exposed to desiccating winter winds. The elevation of an orchard above the valley S i t e . Lack o f a i r drainage from the d e p r e s s i o n i n t h i s o rchard a p p a r e n t l y was a major c o n t r i b u t i n g f a c t o r i n causing d e a t h o f peach t r e e s i n the c e n t r a l p o r t i o n o f the orchard. Ten-acre b l o c k o f J H Hale peach t r e e s almost t o - t a l l y k i l l e d . A p r i c o t orchard i n background e x h i b i t e d o n l y s l i g h t w i n t e r i n j u r y . - 29 -floor also determines to some extent its survival rating, as evidenced by the progressively increasing amount of winter injury with increases in altitude* Tukey and Brase (82), in a study of winter injury in New York, found that the greatest injury occurred on sandy and gravelly soils; especially when these soils were located on knolls or in low spots. Similar observations were made by Anthony, Sudds and Clarke (4) in Pennsylvania orchards. Fraser (39) describes injury resulting from trees standing in soil which is too wet while Palmer (65) warns against letting the trees go into -the winter in dry soil. In Michigan, Knight (51) found that considerable freezing injury of roots occurred in surface horizons which were underlain by compact soils, and related the extent of root killing to the nature of the subsoil. (B) FERTILIZING PRACTICES The time of fertilizer application and the amounts applied are recognized as being important factors in determining the susceptibility of trees to winter injury. Normal spring applications of fertilizer have rarely been credited with causing injury. Fall applications, on the other hand, have been a subject of lively debate among several investigators. Most workers agree that late summer or early f a l l applications of nitrogen may cause delayed ripening of wood, which leaves the tree vulnerable to frost damage. But there i s disagreement among workers as to the effect of late f a l l applications of fertilizer. Havis and Lewis (45), Palmer (65), McMunn and Dorsey (61), and Higgins et al (48) found that late f a l l appli-cations of nitrogen did not increase winter injury in any parts of the trees. But Crane (28), Sudds and Marsh (76) and Tingeley et al (81) submit evidence to show that late f a l l fertilizer applications are harmful and do cause increased susceptibility to winter injury. In no case, however, - 30 -has the writer been specific in what he means by a "late f a l l " application and since this term is so loosely used the evidence cannot be regarded as conclusive. Gourley and Howie tt (42) report that nitrogen has been the only element directly associated with the degree of winter injury. Unlike Higgins et al (48) they found that none of the other elements appeared to be a factor in governing the extent of injury and .that nitrogen became a governing factor only i f i t was applied too late in the growing season or in too large quantities. Unless the evidence against late f a l l fertilization of orchards becomes more conclusive, the practice will probably continue. The amount of fertilizer, especially the nitrogenous f e r t i -lizer, applied to fruit trees appears to influence the extent of winter injury only in the extremes. Where the applications are too low to impart satisfactory vigour, trees are extremely vulnerable to frost damage. Similarly, where the applications are so great that the trees are forced into a late production of succulent and pithy growth, those trees are likely to be damaged. Higgins, Walton and Skinner (48) report that trees showing high ash and carbohydrate content (i.e. trees in poor vigour) were very susceptible to cold injury. They found, too, that moderate applications of nitrogen increased the quantity of protoplasm in the cambial cells, decreased the size of the cell vacuoles, and changed the nature of the cell proteins. A l l of these changes are known to increase cold resistance. Within the limits of moderate applications of nitrogenous fertilizers, Edgerton and Harris (34) found that cold hardiness was not appreciably affected at various nitrogen levels. Apparently other tree responses to these f e r t i -lizer treatments, such as yield, colour, maturity date and quality of fruit would be more important practical considerations than would cold hardiness• Delayed E f f e c t s o f Winter i n j u r y B a dly i n j u r e d trunk on t h i s c h e r r y t r e e caused d e f o l i a t i o n and s h r i v e l l i n g o f f r u i t a t f i r s t o nset o f h o t weather. Poor l e a f area and l i g h t s e t o f f r u i t were t y p i c a l o f w i n t e r - i n j u r e d prune t r e e . ITote gummosis on f r u i t . - 31 -(C) CULTIVATION, COVER CROPS AMD MULCHES The amount of cultivation required for the greatest economy in fruit production has been -the subject of prolonged debate. The timing of such cultivation has also been debated, but not to arrive at any unani-mous conclusions. There s t i l l remain two main schools of thought in the matter of when to cultivate. One school persists in a f a l l cultivation while the other settles for a spring cultivation followed by a winter ground cover. From the point of view of winter injury, there should be no doubt about the relative merits of the two systems. Investigators an over North America have been unanimous in their observations that fruit trees suffer greater winter damage under a clean f a l l cultivation than under a system whereby some sort of ground cover is l e f t intact for the winter season. Thus Kelly and McMunn (49), in a survey of winter injury in Illinois, found that wherever orchards were cultivated late or a summer cover crop was disked down in preparation for a winter cover, the trees suffered the most severe damage. Knowlton and Dorsey (52) concur with these findings and explain the lack of cold resistance on the basis that trees growing under clean cultivation were delayed in their hardening-off processes, while trees in sod ripened off early and were in an advanced stage of maturity a l l through the dormant season. Barnett (6) and Gourley (41) both advocate the use of mulches or cover crops in preference to clean cultivation on the basis of their work with ground penetration of frost. Gourley states that the extremes of frost penetration in a New Hampshire orchard soil were, under clean cultivation eighteen inches and under cover crop seven inches. Barnett set up a series of plots to determine the extent of frost penetration under various mulches. As shown in Table 2, he found that greatest penetration occurred on bare ground, whether i t had been cultivated or not, and - 32 -discovered that where snow cover persisted there was l i t t l e difference in penetration between cultivated and compact soil surfaces. A straw mulch three inches deep proved most effective in offsetting frost penetration. TABLE 2 Ground Cover Effect Plot # Soil Surface Max. depth of frost 1. Compact - no snow 25.0 2. Compact - snow 14.0 3. Cultivated - no snow 23.5 4. Cultivated - snow 12.0 5. Rye cover crop - snow 11.0 6. 3" Straw mulch - snow 1.0 7. 3" Straw mulch - snow (2nd replication) 7.0 Havis and Lewis (45), conducting a detailed survey of winter injury in Ohio orchards following the severe winter of 1936-37, found that trees in sod were severely injured in the trunks while those which had been mulched with hay and straw survived in good condition. Trees under clean cultivation were severely injured throughout. Upon further examination they found that the roots of the injured trees had not been directly killed by frost. In this case, i t would appear that there was a more constant moisture supply under mulch prior to and during the cold winter than was the case under sod or clean cultivation. The mulch apparently offset the penetration of frost and thereby permitted an adequate uptake of water by the unfrozen roots during the winter. This may partly account for the difference in injury found between trees in cultivation and trees in sod or mulch. If, however, a heavy snow cover exists, differences in cultural methods are not likely to be significant. - 33 -In general, the use of cover crops and mulches appears to be one of the most effective preventatives of winter injury. They act not only to protect the roots against frost penetration, but also cause the trees to ripen their wood early in the f a l l and assist in the regulation of moisture supply to the root system during the dormant period. (D) PREVIOUS CROPS Trees which bear heavy crops of fruit are more likely to be injured by low temperatures than are trees bearing light crops. To this end, Gourley and Howlett (42) observed that hardy apple varieties which had cropped heavily during the summer were more severely damaged during the following winter than were tender apple varieties which bore light crops. Similarly, Macoun (55) found that of fourteen identical Wealthy trees, the eight which had cropped heavily suffered severe winter injury during the following winter, while the six light-crop trees were apparently undam-aged. Havis and Lewis (45) compared the effects of a light and heavy fruit-thinning of peach trees on the subsequent winter injury to buds and found that the heavily thinned trees maintained a higher bud survival rate than unthinned trees. Crane (28) has noted that the effect of producing a crop of fruit is to reduce the hardiness of the tree tissues. He attributes this reduction in hardiness to a removal of food materials which are not replaced before leaf f a l l . Levitt (54) goes further and suggests that a heavy crop reduces the colloidal content of the plant cells, giving rise to large vacuoles within those cells. Such a cellular condition has already been shown to bring about cold temperature injury. (E) PRUNING PRACTICES Most investigators agree that f a l l or early winter pruning - 34 -of fruit trees must be considered a hazard insofar as the susceptibility to winter injury i s concerned. Havis and Lewis (45), working in Ohio, found that fruit trees pruned, even moderately, before the severe temp-eratures of January were usually injured more than those pruned later or not at a l l . Wherever early winter pruning had been severe, injury was also severe. In one orchard of grafted trees, the grower had cut back the sucker growth on some trees to let the grafts come. Other trees were not pruned. The pruned trees a l l died the following summer while those that were unpruned were not injured. Although young trees were usually injured less than bearing trees, they too showed the disastrous effects of early pruning. Many young trees showed injured areas around and just below the wound left by the removal of the limb. In many cases this injured area extended down the trunk to the point where the entire trunk and crotches were injured. Such trees did not survive. Then again, Crane (28), working in West Virginia, found that both summer and early dormant pruning decreased the hardiness of fruit buds. This held true regardless of the fertilizer treatment applied. Fraser (39) suggests that pruning trees to form strong, widely diverging frameworks is good insurance, since narrow crotches tend to be slow to ripen off, and, once injured, are very difficult to heal. Hedrick (47) found that low-headed trees suffered less winter injury in both trunks and branches than high-headed trees. He suggests that the former is true because the wood loses less moisture from desiccating winds than do high-headed trees. In addition, he feels the trunks of low-headed trees are well protected from sun-scald. However, i t i s generally agreed that no special system of pruning trees will help to offset the deleterious effects of pruning too early, and growers who choose late f a l l or early winter pruning appear to be asking for trouble. (F) CONDITION OF TREES One of the most important factors affecting the winter hardiness of fruit trees is maturity of the various tissues. The maturity of a plant determines i t s cold resistance during the dormant season. Tissues of the trunk and scaffold are notably slow to mature in the f a l l and hence are often injured by low temperatures when other portions of the tree are uninjured. Anthony, Sudds and Clarke (4), in a survey of winter injury in Pennsylvania, report that severe injury occurred where trees were subjected to an unseasonal October freeze which delayed maturation processes. This freeze, in itself, did not cause the injury but i t delayed maturity to such an extent that a sudden drop in temperature during January caused great damage. These same trees had withstood much lower temperatures in previous winters when maturation had not been delayed. Further observations indicated that trees of both extremes of vigour were damaged, while trees in moderate vigour were more resistant. Gourley and Howlett (42) report a Canadian experiment involving the testing of thousands of native and foreign species of plants for hardiness. Plants which were most damaged under the conditions of this experiment were those native to a region with a longer growing season than that found in Canada. They were incapable of maturing their wood in the relatively short growing period. Brierly (12) reports an unusual case of winter injury which appears to be directly related to the maturity of trees. In this case, the Haralson apple, which is very hardy in the Eastern United States, was subjected to a mid-November blizzard. Shoulder-deep snow and near-zero temperatures accompanied i t . As the snow settled i t began to smash branches and some were lifted from the snow, shaken and left exposed to the air. Others were not removed from the snow. Those that were removed - 3 6 -were subjected to a drop in temperature within two or three hours from about 25° F. in the snow to a near-zero air temperature. Observations the following summer indicated that the branches which had been lifted from the snow were completely killed. Both the snow-covered branches and those above the original snow-line developed good foliage and set a fairly heavy crop. Brierly concluded that the snow cover had interfered with the nor-mal maturation process to the extent that when the unhardened branches were suddenly exposed to low temperatures, they were severely damaged. Further evidence of the relationship of maturity to cold injury i s afforded by the fact that damage to the leaves of a tree, which doubtless interferes with the hardening process, often leads to serious killing of the wood. Similarly, the inner surfaces of branches, which usually possess the least foliage, are nearly always more tender than the exposed surfaces. And finally, young trees in good vigour, which have a dense and broadly distributed leaf surface which brings about early maturation of woody tissues, are often injured less than older trees whose hardening is delayed by lack of foliage. (G) DEFOLIATION Premature defoliation of deciduous fruit trees has been shown to increase their susceptibility to winter injury. To this end, Crane (28) reports that partial summer defoliation of peach trees resulted in proportionately greater winter injury both to fruit buds and limbs. In addition, Kennard (50) studied the effect on cold injury of defoliation by erroneous fertilizer treatments, drought, insect injury, disease injury and spray injury. He found that complete defoliation increased suscepti-b i l i t y of Montmorency sour cherry trees to low temperature injury and delayed blossoming the following spring. Trees completely defoliated by August 10 were more severely injured than those completely defoliated by - 37 -September 1. Gourley and Hewlett (42) report that a partial defoliation predetermines the amount of cold injury in direct proportion to the amount of defoliation. They report that the injury to flower buds was greatest on those branches having the smallest leaf-fruit ratios during the previous glimmer. Thus premature defoliation from any cause i s an important factor affecting the winter injury complex. The effect of defoliation no doubt is to decrease carbohydrate formation in the storage cells of the tree since the manufacture of food ceases when the leaves are removed from the tree. Cells low in colloidal materials but high in free water content are known to be readily injured. (H) OTHER FACTORS The time and extent of thinning a fruit crop are known to have a definite relationship to the cold resistance of the various tree parts. The earlier the fruits are thinned the greater i s the length of time left for the buildup of food materials in the woody cells. The food manu-factured in the leaves will be supplied to the growing fruits i f they are left on the tree, and this will be done in the case of a heavy set of fruit, at the expense of building up a reserve of food in the storage tissue of the plant. If, however, some of the fruits are removed in the spring or early summer, the leaves will supply some of the food to the remaining fruit and some for storage in the woody tissues. Removal of excess fruits at the earliest possible time would seem, therefore, to be a desirable practice from the point of view of cold hardiness. Experiments dealing with the effect of thinning tree fruits upon the hardiness of various parts of the tree have been conducted by Knowlton and Dorsey (52), Edgerton (33) and Foot (38). Foot, working with Rome Beauty and Jonathan in the Okanagan Valley, found that trees which had - 38 -been thinned at the f u l l bloom stage by chemical means withstood temper-atures of 30° F. without apparent injury, while adjacent trees of the same variety which had been hand thinned at a later date were severely damaged by the same low temperature. Similarly, results obtained by Edgerton, as shown in Table 3, indicate that effective blossom thinning of peach trees, which otherwise would set excessively increases the hardiness which the fruit buds on those trees may attain during the follow-ing winter. TABLE 3 Blossom Thinning Treatments Hay 9,1947 Effect of Blossom Thinning on the Survival of Elberta Fruit Buds Frozen under Orchard Conditions (-16° F.) Average Percent Fruit Set Yield Fruit Buds No. of % in Alive on Trees July 8,1947 Bu./tree Apr.1,1948 No. of Average Live Buds Fruit Bud Per Foot Set Apr.1,1948 Elgetol, 1| pt./ioo gal. 12 13.5 DN #1, W 100 gal. 9 16.9 Check 5 35.5 3.0 3.2 3.4 88.5 87.5 67.9 18.7 18.4 10.5 16.5 16.1 7.1 The exposure of trees to cold drying winds has, in many cases, contributed to the death or injury of trees. Such injury is usually character-ized by dead patches on the trunk and framework on that side of the tree adja-cent to the prevailing winds. The damage has been shown to result from several effects of the wind. In the f i r s t place, the wind causes dehydration of cells on the windward side of the tree. Such dehydration, i f carried to extremes may cause death of the cells. Secondly, the stretching of frozen cells on the windward side caused by the bending of the tree has been known to rupture the cells, resulting in rapid dehydration and death. And finally, the cooling - 39 -of wood has been shown to be greatly accelerated by wind in comparison with wood protected from the same wind. This accelerated cooling could cause injury in itself, especially i f the rate of cooling is rapid enough to overtake the natural ability of the woody cells to gain cold resistance. (VII) RELATIVE HARDINESS OF SPECIFIC STONE-FRUITS Not a l l kinds and varieties of fruit exhibit the same degree of resistance to low temperature. Nor can any specific temperature be said to be " k i l l i ng" since the susceptibility of any kind or variety is closely related to its stage of development at the time when the freeze occurs. Since, however, there are variations in hardiness between kinds and varieties, an analysis of these variations appears pertinent to this study. (A) CHERRY Most investigators agree that sweet charries on Mahaleb rootstocks are more cold hardy than those on Mazzard rootstocks, both in the nursery and in the orchard. This difference in hardiness is attributed to the fact that cherry wood on Mahaleb ripens earlier in the f a l l than does the same wood on Mazzard. Coe (25) found a block of 60,000 nursery trees on Mazzard which had been killed by low temperatures, and found that adjacent blocks on Mahaleb showed l i t t l e injury. He attributed the difference in survival directly to differences in maturity. In spite of i t s superiority over Mazzard in cold hardiness, however, Mahaleb is not now widely used as a sweet cherry rootstock. It has been found to be fairly short-lived and somewhat dwarfing in habit when compared with Mazzard. Carrich (41) in laboratory tests with these rootstocks found that Mazzard roots were killed by a temperature of 12 to 14° F., whereas Mahaleb roots were not killed until 5° F. was reached. Palmer (66) reports that seedlings of Gold cherry have shown - 40 -promise as sweet cherry rootstocks and are at least as hardy as Mahaleb. Gold is a vigorous growing seedling which is compatible with many of the sweet cherry varieties grown in the Pacific Northwest. Coe (25) mentions the use of American Morello rootstocks in areas where a high degree of hardiness is essential for growing cherries. He notes, however, that this rootstock tends to sucker badly. Hedrick (46) reports the use of Prunus pennsylvanica (Bird cherry) as a hardy stock for sweet cherry. However, i t dwarfs most standard cherry varieties and suckers badly as well. Sweet cherry varieties which are grown in the Okanagan Valley have been tested for hardiness under widely varying cultural con-ditions by Mann and Keane (56) at the Experimental Station, Summerland. Following is a summary of their observations* Lambert - Among the hardiest of commercial sweet cherries. Trees less than ten years old suffered l i t t l e damage. Those over twenty years old suffered injury in trunk and scaffold but appeared hardier than Bing, Deacon or Royal Ann in this respect. It was hardier in bud than Bing, Black Republican or Deacon and about the same as Royal Ann, Star and Van. Bing - Less hardy than Lambert, but a l i t t l e more so than Black Republican, Deacon or Royal Ann. Bing is less hardy in bud than Lambert, Royal Ann, Star and Van, about equal to Deacon and hardier than Black Republican. Deacon - Less hardy than Lambert and Bing and about equal to Royal Ann, In bud-hardiness, Deacon appears less hardy than Lambert, Royal Ann, Star or Van, about equal to Bing and hardier than Black Republican, Royal Ann - Less hardy than Bing and Lambert and about equal to Deacon. Hardier in bud than Bing, Black Republican and Deacon and about equal to Lambert, Star and Van. Van - A new variety which was introduced in 1944. The original tree, - 41 -planted in 1939, survived two severe Okanagan winters with l i t t l e injury. Star - A new variety which was introduced in 1949. The original tree, planted in 1939, survived two severe winters with slight injury rating in this respect slightly below Lambert and Van, but slightly above Bing. In bud-hardiness, Star is about equal to Royal Ann, Lambert and Van, hardier than Bing and Deacon and much more so than Black Republican. Windsor - Hardier than a l l other cherries grown in the Hudson River Valley (Anderson (3)). There are no records available for Okanagan conditions. (B) PEACH The danger of low winter temperatures is probably the greatest single limiting factor in the commercial production of peaches in the Okanagan Valley. There is no particular temperature at which winter injury can be said to occur, since the degree of winter injury in a peach tree appears to be directly related to the stage of development of that tree and to other factors of the winter injury complex. Campbell (17) reports the survival of thirty varieties of peach at a temperature of•"52° F. Brown (16), on the other hand, reports killing and severe injury to several varieties of peaches at-18° F. The fruit buds of peach are generally more tender to cold than other tissues, although Knowlton and Dorsey (52) report that the reverse has been true in certain cases. The degree of tenderness of peach fruit buds is usually closely associated with the progress of the rest period. Trees which enter dormancy early in the f a l l have usually completed their rest period by mid-winter and will commence growth activity during the f i r s t warm spell which occurs. If such a warm spell is followed by cold weather, the buds are often unable to regain sufficient cold-hardiness to offset the cold, and are therefore killed at temperatures considerably higher than those - 42 -which would have killed them a few weeks earlier. Cull in an and Weinberger (29) report that Elberta is a very-tender variety in late winter, since i t develops very rapidly after the break in its rest period and at the f i r s t warm weather. During one winter, they found that only a small percentage of fruit buds in one Elberta orchard survived a temperature of-10° F., when al l buds of the same variety in another orchard 15 miles south were killed at a temperature of -7° F. Knowlton and Dorsey (52) and Crane (28) report that the fruit buds of peach develop at vastly different rates. They indicate that the buds located on the base of a terminal shoot tend to develop late in the summer and that their rest period is not broken t i l l late winter. This feature makes the basal fruit buds hardier than those borne on the outer portions of terminal shoots. Chaplin (21) reports that peach buds exhibit high resistance to cold during the early autumn, even before leaf f a l l , although the woody tissues of the tree are very tender at this time. The bud hardiness increases up to a point in mid-winter which appears to co-incide with the breaking of the rest period, and the hardiness then commences to decrease and the peach buds lose hardiness with every warm spell which occurs during the dormant period. Observations made in Okanagan peach orchards during the winter of 1949-50 indicate that there is l i t t l e difference in hardiness among the various peach rootstocks now in use. In general, seedling root-stocks of Muir, Lovell, and Veteran exhibit approximately the same degree of cold resistance. Winklepleck and McClintock (83) found "that peach rootstocks are much less resistant to cold than other Prunus rootstocks. They noted that seedlings of Elberta peach were especially susceptible to cold injury and would, therefore make a poor rootstock for peaches. Prunus - 43 -Davidiana, on the other hand, showed almost as much cold resistance as Myrobalan plum rootstocks and showed promise as a rootstock for peaches. The difference in hardiness between peach varieties is not as consistent as desired. The difference varies greatly throughout the year and from year to year, depending upon environmental factors. But the relative position of varieties as to hardiness appears to remain fairly constant. The margin of difference between the most and the least hardy varieties is narrow, and consequently does not show up in years when the temperature drops so low as to nullify the differential. Whenever the low temperatures come within the range of this differential, the more hardy varieties will come through with a crop while most other varieties will lose their crops. It is erroneous to think that early-ripening varieties of peach commence growth activity during warm spells in winter, before late peach varieties do. According to Chandler (18), some of the very early , varieties of the Chinese Cling group are the most slowly started into growth in early winter and bloom as late as any of the varieties. Mann and Keane (58) have compared cold hardiness of a l l the peach varieties commonly grown in the Okanagan Valley. The following i s a "breakdown" of their findings: J.H. Hale - very tender both in wood and fruit bud. Not likely to withstand-15° F. without killing of buds, nor-20° F. without injury to wood. Elberta - moderately tender in both wood and fruit bud. Somewhat hardier than J.H. Hale but less hardy than Valiant, Vedette or Veteran. Veteran - one of the five hardiest varieties under test. Slightly hardier in wood than Valiant or Vedettej much hardier than Elberta, Golden Jubilee, J.H. Hale or Rochester. Less bud hardy than Vedette, slightly less than Rochester, about equal to Elberta and J.H. Hale and slightly hardier than Valiant. - 44 -Vedette - in tree-hardiness, slightly less hardy than Veteran, about equal to Valiant, hardier than Elberta, Golden Jubilee, J.H. Hale or Rochester. Greater bud-hardiness than a l l other commercial varieties, including Valiant and Veteran. Valiant - in tree-hardiness, less hardy than Veteran, and about equal to Vedettej hardier than Elberta, Golden Jubilee, J.H. Hale or Rochester. Less bud-hardy than Vedette, slightly less than Veteran and other commer-cial varieties. Rochester - less hardy than Veteran, Valiant and Vedette both in wood and bud, Hardier than Elberta, J.H. Hale, and Golden Jubilee in both. Superior - very tender in wood, probably ranking with J.H. Hale. Slightly greater bud-hardiness than most other varieties and about equal to Vedette in this respect. Red Haven - new variety which exhibits satisfactory hardiness in wood and bud in seven-year-old trees. Showed no fruit bud injury when J.H. Hale showed considerable injury. Halehaven - twelve-year-old trees killed during severe winter along with other varieties of same age. Young trees appear comparable in tree-hardiness to Valiant, Vedette and Veteran and, in bud-hardiness to Vedette. Fisher - equal in hardiness to Veteran and slightly hardier than Valiant and Vedette. Hardier in bud than J.H. Hale and Elberta. Spotlight - young trees only, hardier in bud than J.H. Hale and Elberta. Tree hardiness appears equal that of young trees of other varieties. Solo - greater bud hardiness than J.H. Hale and Elberta. Comparable with Valiant in tree and bud hardiness. (C) APRICOT Under Okanagan conditions apricot trees have shown themselves to be at least as hardy as peach trees. The limiting factor in apricot - 45 -production is the early blooming tendency of the trees. Most apricot varieties bloom before the danger of late spring frost is over, and con-sequently the crop is frequently lost or seriously reduced. Since most apricots are budded on peach seedling roots, they exhibit about the same root hardiness as do peaches. Recently, emphasis has been placed on the breeding of later-blooming and hardier varieties from certain Russian and Manchurian strains of apricot. These strains are known to be hardy in tree and bud, and are late blooming, but the fruits are small and of poor quality. The following summary (Mann & Keane (58)) based on obser-vations made following the most severe winter on record, indicates the relative tree and bud hardiness of the apricot varieties most commonly grown in the Okanagan Valley. Riland - among the hardiest of the commonly grown varieties. Rates equally with Kaleden and Wenatchee Moorpark in this respect. Intermediate in bud hardiness between Tilton and Wenatchee Moorpark. Kaleden - hardy in tree, about equal to Wenatchee Moorpark and Riland. Lacks bud hardiness and in this respect rates lower than any other variety. Tilton - in tree hardiness rates lower than Kaleden, Perfection, Riland, or Wenatchee Moorpark but hardier than Blenheim or Old Moorpark. Among the most bud-hardy of a l l commercial varieties, rating equally with Reliable in this respect. Blenheim - lacks tree hardiness, rating lower than Kaleden, Perfection, Riland, Tilton and Wenatchee Moorpark. Less bud-hardy than Tilton, about the same as Riland, somewhat hardier than Perfection and Wenatchee Moorpark and considerably hardier than Kaleden. Royal - similar to Blenheim in respect to tree and bud-hardiness. Wenatchee Moorpark - consistently tree hardy, rating equal to Kaleden - 4 6 -and Riland, and superior to Blenheim, Perfection and Tilton. Lacks bud-hardiness, rating lower than Riland and Tilton, but hardier than Kaleden, Old Moorpark - lacks tree hardiness, rating with Blenheim in this respect. Fairly bud hardy. Perfection - slightly less tree-hardy than Kaleden, Riland and Wenatchee Moorpark, but more so than Blenheim and Tilton. Far less bud-hardy than Tilton, less bud-hardy than Blenheim and Riland, about equal to Wenatchee Moorpark and more so than Kaleden, Reliable - in tree-hardiness, about equal to Perfection, less hardy than Riland or Wenatchee Moorpark, but hardier than Tilton or Blenheim, Very hardy in bud, being equal to Tilton in this respect. Rose - in tree-hardiness about equal to Perfection, less than Riland or Wenatchee Moorpark, but hardier than Tilton or Blenheim, Slightly less bud-hardy than Tilton, (D) PRUNE AND PLUM Prunes and plums have exhibited greater hardiness under Okanagan conditions than any others of the stone-fruits. Prunes and plums are usually grown on Myrobalan Plum rootstocks, which are also quite hardy. In some orchards, especially where the soil is light, prunes and plums have been grown on peach seedling rootstocks, in which case the hardiness of the prune or plum top is limited by the degree of hardiness found in the rootstock. Plums are of relatively small importance in the Okanagan Valley when compared with the other stone-fruits, In general, however, most varieties are quite hardy both in tree and bud. The hardiness of plums in the Okanagan follows quite closely the findings of Brierly and McCartney (14), In general, varieties of the i n s i t i t i a type were hardier than those of the domestic a type. The one outstanding exception to this - 47 -statement is the Stanley plum, which appears to be consistently hardy and has withstood-30° F. Prunes, during the most severe winter on record, appeared to be hardy both in tree and bud in most districts of the South Okanagan where the soil was considered suitable for prunes (Mann (58)). Prunes lacked tree hardiness - however, in most sections of the North Okanagan between Kelowna and Kamloops. This held true for a l l three strains of Italian prunes which are commonly grown in the Okanagan, namely Italian (De Maris), Italian (Greata) and Italian. The Italian (Richards) strain has not yet been assessed for hardiness. (VHI) THE 1949-50 WINTER IN THE OKANAGAN VALLEY (A) NATURE OF DAMAGE "Okanagan Firewood Worth $1000 a Cord" - This headline appeared in a British Columbia newspaper (71) following one of the coldest winters in the history of the Okanagan fruit industry. The headline i s based on the fact that an acre of mature peach trees, worth about §2000 before the freeze, yields about two cords of firewood, In a l l , the Okanagan Valley cut about 2,100 cords of peach firewood in 1950, to say nothing of the amount of cordwood from injured apple, pear, apricot and other kinds of fruit trees. Devastation in al l kinds of tree-fruits grown in the Okanagan Valley was widespread following the 1949-50 winter. No particular pattern of damage followed the severe temperatures, orchards in some areas suffering far greater damage than those in other areas. Typical of the most severe damage found in orchards of the Osoyoos area, on the 49th parallel, was the following count of dead and living trees in one five-acre orchard: - 340 dead peaches, 7 alive; 70 dead cherries, 2 alivej 11 dead Nature of Damage The above-ground p o r t i o n s o f t h i s prune t r e s were k i l l e d o u t r i g h t . There was no apparent i n j u r y to the r o o t system. Note v i g o r o u s peach shoot a r i s i n g from the r o o t S3rstem* Same t r e e as above showing the v i g o r o u s OHyi shoot a r i s i n g from an a p p a r e n t l y un-i n j u r e d peach r o o t system. - 48 -apricots, 4 alive; 50 dead prunes, 1 alive; 25 dead pears, 70 alivej 13 dead plums, none alive. A survey (7) of 2,249 orchards throughout the Okanagan Valley indicated a total of 336,610 dead trees out of a total of 1,664,037 trees in the valley. This represented an average loss of 20% of the trees in each orchard. However, average losses are not representative of the intense damage in certain areas and the relatively light damage in other areas. A breakdown of the damage according to kind of fruit tree and according to district is shown in Tables 4 and 5. Figures in this table are compiled from figures presented in Appendix A. In the ninety-one orchards surveyed in the Lytton-Kami oops-Chase district, 54?? of the total trees were killed. In the Salmon Arm-Sorrento district, 31?? of the trees were killed. Vernon district suffered a 25?? loss while losses in other districts to the south ranged from six per cent to thirty per cent. One hundred per cent losses were suffered by many growers in different dis-tricts. Table 6 outlines the percentage of trees killed by size groups. In addition to the trees killed outright by the 1949-50 frosts, there was also very heavy injury to fruit spurs and wood of other trees, resulting in a very small crop of some fruits for the 1950 season. Figures in Appendix A show a loss of about 111,000 peach trees out of a total of 343,500. But the peach crop following the freeze was only 160,000 boxes in 1950 as compared with 2,003,732 boxes harvested the previous season. Similarly, the 1950 apricot crop was 29,303 lugs, as compared with 603,339 lugs in 1949; the 1950 cherry crop totalled only 115,805 lugs compared with 520,431 lugs in 1949. Table 7 shows the total reduction in crop by district and kind of fruit. A brief (68) prepared by the British Columbia Fruit Growers' Association for presentation to the Dominion and Provincial governments, TABLE 4 Kind of Tree Under 2" Winter Injury Survey - Okanagan - Mainline - Grand Forks Area British Columbia 1949-50 2"-5" Number of Trees Killed 5"-7" 7"-12" Over 12" Total Total Trees as at Jan. 1,1950 % of Total Killed Apple Peach Apricot Cherry Pear 5,729 6,645 14,712 46,521 33,421 107,028 1,117,215 31.8 9,859 24,285 39,108 30,774 7,287 111,313 343,534 9,940 6,348 3,215 2,524 693 22,720 333,807 33.0 14,335 14,108 5,515 3,233 790 37,981 186,364 11.3 2,312 2,618 1,850 4,678 6,988 18,446 99,316 5.5 6.8 Plum 301 1,002 1,626 1,723 497 5,149 16,149 1.5 Prune 3,105 6,907 12,102 9,620 2,239 33,973 200,200 10.1 TOTAL 45,581 61,913 78,128 99,073 51,915 336,610 2,296,585 100.0 - 50 -TABLE 5 Winter Injury Survey by Districts (British Columbia 1949-50) Total Total Trees Number Average % Dead as at Orchards Loss Per District Trees Jan. 1,1950 Reported Orchard Lytton - Chase 24,629 45,435 91 54,0 Salmon Arm - Sorrento 24,956 80,404 160 31.0 Armstrong 2,919 7,871 22 37.1 Vernon 53,619 212,750 252 25.2 Oyama, Winfield, Okanagan Centre 16,144 117,832 175 13.7 Kelowna 50,536 354,977 395 14.2 Westbank 12,368 79,890 78 15.5 Peachland 8,423 45,885 68 18.3 Summerland 13,739 150,726 243 9.1 Penticton 5,915 89,007 139 6.7 Naramata 2,665 36,190 60 7.3 Kaleden 3,235 27,231 32 11.9 Keremeos - Caws ton 19,112 81,465 108 23.5 Oliver - Osoyoos 94,826 339,434 398 27.9 Grand Forks 3,524 14,939 28 23.6 Total 336,610 1,684,037 2,249 19.9 2,249 orchards represents S3% of a l l orchards in the Okanagan Valley of British Columbia. - 51 -TABLE 6 Percentage of Trees Killed by Size Groups Under Over Kind of Tree 2" 2"-5B 5n-7n 7"-12" 12" Total Apples 5.35 6.21 13.75 43.47 31.22 100.00?? Peaches 8.86 21.82 35.13 27.65 6.54 100.00?5 Apricots 37.74 37.15 14.52 8.51 2.08 100.00?? Cherries 12.53 34.19 10.03 25.36 37.89 100.00?? Pears 43.75 27.94 14.15 11.11 3.05 100.00?? Plums 5.85 19.46 31.58 33.46 9.65 100.00?? Prunes 9.14 20.33 35.62 28.32 6.59 100.00?? TOTALS 13.54 18.40 23.21 29.43 15.42 100.00?? Note: A l l tree trunks measured at a point six inches above ground level. - 52 -TABLE 7 Reduction in Crop Following 1949-50 Winter Number of Packages Average Total Crop Crop Reduction Per Cent District Kind 1946-49 1950 in Crop Reduction rPeach 1,819,986 206,346 1,613,640 87 Al l Districts i 'Apricot 372,060 46,132 325,928 88 1 Cherry 339,137 139,939 199,198 59 Teach 395 40 355 90 Lytton-Chase i Apricot 307 306 1 3 Cherry 277 251 26 9 Peach 50 0 50 100 Salmon Arm - Apricot 40 0 40 100 Sorrento .Cherry 6,350 156 6,194 98 Peach 0 0 0 0 Armstrong Apricot 23 0 23 100 Cherry 478 0 478 100 Peach 3,204 0 3,204 100 Vernon i Apricot 725 0 725 100 Cherry 2,612 320 2,292 88 Peach 34,039 0 34,039 100 Oyama-Winfield Apricot 8,131 0 8,131 100 Okanagan Centre Cherry 17,872 711 17,161 96 Peach 67,976 0 67,976 100 Kelowna Apricot 12,338 0 12,338 100 .Cherry 82,756 382 82,374 99 Peach 58,956 0 58,956 100 Westbank Apricot 2,487 0 2,487 100 Cherry 12,890 2 12,888 100 Peach 175,992 7,494 169,303 96 Peachland Apricot 6,689 396 6,293 94 Cherry 13,716 2,270 11,446 83 I Teach 285,836 30,753 255,083 89 Summerland Apricot 95,844 655 95,189 99 Cherry 46,759 20,047 26,712 57 1 Teach 315,582 81,481 234,101 74 Pen tic ton Apricot 50,421 8,494 41,927 83 .Cherry 47,306 39,405 7,901 17 Peach 67,646 16,805 50,841 75 Naramata « Apricot 39,191 4,838 34,353 88 Cherry 24,794 16,643 8,151 33 Peach 98,147 15,807 82,340 84 Kaleden Apricot 26,835 545 26,290 98 Cherry 9,951 7,460 2,491 25 Teach 674,250 53,700 620,550 92 Oliver-Osoyoo s Apricot 124,955 30,949 94,006 75 i .Cherry 71,606 52,270 19,336 27 'Peach 37,913 0 37,913 100 Keremeos-C awston Apricot 4,074 4 4,070 100 .Cherry 1,770 233 1,537 87 - 5 3 -outlined the loss in value of the soft fruit crop for 1950 in comparison with the 1949 crop, as shown in Table 7 A. TABLE 7 A Sale Value of Crop 1949 1950 Peaches §2,432,850 | 327,926 Cherries 1,651,765 577,371 Apricots 825,826 92,555 Pears 1,608,688 1,455,748 Plums 180,756 146,661 Prunes 784,989 496,135 17,484,874 $3,096,396 1950 Crop Loss - $4,388,478 The B.C. Fruit Growers1 Association requested partial com-pensation for growers on the basis shown in Tables 8 and 9. TABLE 8 Compensation for Apple, Cherry and Pear Trees a) For removing trees and preparing land Trees under 2" diam. Nil Trees 2"-5" diam. $2 Trees 5"-7" diam. $4 Trees 7"-12" diam. $6 Trees over 12" diam. $8 b) For replanting to orchard Per tree $2 c) For equipping for use other than orchard Per acre $100 - 54 -TABLE 9 Compensation for Peach, Apricot, Plum and Prune Trees a) For removing trees and preparing land Trees under 2" diam. Nil Trees 2"-5" diam. $2 Trees 5"-7" diam. $4 Trees 7"-12" diam. §5 Trees over 12" diam. $6 b) For replanting to orchard Per tree §2 c) For equipping for use other than orchard Per acre $100 On the basis of the above figures, the recommended compen-sation would be: Apples $618,632 Peaches 602,594 Apricots 71,181 Cherries 96, 608 Pears 4 6 , 2 4 4 Plums 20,105 Prunes 123,756 Total $1,379,120 Removing trees and preparing land - $1,379,120 Rehabilitation at §2 per tree 673,220 Total $2,052,340 (Comment: When, in 1951, growers were compensated on the basis of the above figures, the amount actually paid out by the federal and provincial govern-ments totalled $250,000, a very disappointing amount which worked a real hardship on the tree fruit industry.) - 55 -A survey of Okanagan orchards during the spring and summer of 1950 indicated that winter injury was present in practically every known form. The most common form was injury to the trunk and crotches of bearing trees. This was manifested by the appearance of frost cankers on the injured areas. A line of demarcation soon appeared between the dead and living tissues. In late summer the dead bark began to crack and slough off from tiie tree and rot fungi were often present. In some trees, there appeared a distinct line of demarcation between living and dead tissue at exactly the level on the trunk where the snow-line had been. In such cases the entire top of the tree was killed while the lower portions were unhurt. Large watersprouts often arose below the snow-line and many growers used these to form new trees. Nearly al l stone-fruit trees showed some injury in the trunk and framework, as was evidenced by the appearance of varying shades of brown in the cambium layers. Where this colour was a deep walnut shade, the trees generally did not recover. But trees in which the cambium layer was less discoloured, exhibited varying degrees of recovery during the summer of 1950. Blackheart was evident in most trees examined, both in the spur wood and in the main limb structure. This injury has become increasingly evident with the passing of time, and i t is safe to say that the lives of the trees injured in this manner have been greatly reduced. Extensive splitting and breaking of limbs of blackhearted trees has occurred since the 1949-50 winter. Vertical splits in the trunks of certain trees, especially cherry trees, were also in evidence after the freeze. Such splitting was probably the result of contraction of the medullary rays of the wood. These splits invariably led to drying out, and sometimes death, of the trunk tissue. Another type of injury showed up in the form of dead areas on the trunk and scaffold, especially on the north and south sides of the tree. - 5 6 -This injury appeared to result from the evaporation caused by drying winds from the north and south and from the tearing of trunk tissue due to bending of the tree when the bark was under tension. This injury was not confined to large wood, but was present in spur and twig growth as well and doubtless accounted for many of the dead spurs and twigs. It is probable that at least a part of the injury on the south side of trees could be attributed to winter sun-scald. Fruit buds of stone-fruit trees were generally killed or badly injured. This injury can be attributed to the desiccating effects of both the very low temperatures and highly drying winter winds. Fruit buds in protected positions appeared to survive better than those on the periphery of the tree. This may have been due to their earlier formation and hence better maturity, and to the partial protection afforded them. Leaf buds were usually not injured unless the entire twig or spur had been killed or injured. Root damage appeared not to be the primary cause of injury in the 1949-50 winter. Host orchards were well blanketed with snow which afforded good protection. However, the winter was characterized by strong, drying winds, which blew much of the snow away from the exposed areas of certain orchards. Wherever this happened and root injury occurred, the trees usually produced their normal blossoms and small leaves in the spring, as described by Brown (16). At the end of the blossoming period, after the fruit had set, definite injury began to show. The small leaves shrivelled and died, the limbs and small branches appeared dry and the bark shrunken, and the trees soon died. They had probably functioned normally on the food stored in the surrounding tissues until this was exhausted. When they came to depend upon food from the roots, the demand was greater than the roots could supply. More trees died with the advent of each hot spell. Some trees matured their crops before dying; others lingered on until the end of the summer and s t i l l - 57 -others are dying slower deaths and may struggle along for many years. Abnormalities in the few fruits which appeared in 1950 were abundant. Peaches were running heavily to "split-stones1*, a term used to describe fruits in which the pits have split in half so that each half adheres to the fleshy part of the fruit. This condition may have resulted from the fact that there were only a few fruits on each tree and their growth was forced to such an extent that the pits did not harden properly. But the likelihood is that the blossoms were injured and so did not have the ability to set normal fruits. Prunes were characterized by the presence of numerous checks in their skins. From the checks there exuded small masses of clear gum, which hardened on the surface. Beneath the checks, there appeared dis-coloured cavities containing callous tissue. These blemishes made the prunes very unacceptable on the commercial market. Many cherries showed abnormally deep sutures, and pears and apples often appeared "slab-sided", indicating that injury to some of the floral parts had probably occurred during the winter. The extent of injury from orchard to orchard varied con-siderably, and, from surface observations, no single system of orchard culture could be said to be superior from the point of view of tree sur-vival. It is true that, in some cases at least, the cause of severe injury was obvious, as in the case where f i f t y cherry trees were killed outright after having been heavily pruned and grafted the previous spring. However, in many cases, the cause of the damage from the point of view of orchard management, at least, was obscure. (B) RECORD OF TEMPERATURE AND PRECIPITATION The winter of 1949-50 in the Okanagan Valley was characterized by a prolonged period of extremely low temperatures, accompanied by continual F r u i t Malformations. Peaches ha r v e s t e d d u r i n g 1950 e x h i b i t e d a h i g h i n c i d e n c e o f " s p l i t - s t o n e " malformation. Such " s p l i t - s t o n e s " are n o t p e r m i t t e d i n the commercial pack. Prunes from i n j u r e d t r e e s e x h i b i t e d severe gummo d u r i n g 1950. Such f r u i t s are c l a s s e d as " c u l l s " . - 58 -high, drying winds. (See Fig. 1 and 2) That the winter was most unlike the usual Okanagan winter is evidenced by a quotation of Palmer (64) in an annual report written in Summerland, the heart of the Okanagan Valley, some years earlier. "Extremes of temperature are never experienced either in summer or winter. During -the twenty year period prior to 1936, the maximum temperature recorded was 103° F., the 100° F. mark having been reached only ten times. There was an average of eleven days each year when the temper-ature reached 90° F. or over. On the other hand, the coldest temperature recorded was -16° F. Most winters periods of zero temperatures are exper-ienced, but these are of only very short duration." In late December of 1949, the temperature dropped suddenly to figures ranging from 0° F. to -10° F. in various parts of the Okanagan. These temperatures dropped much lower during January and February as shown in Fig. 1 and 2. Throughout the twenty-four hours on some days temperatures remained well below zero, and in the Oliver and Osoyoos districts reached the extreme low of -23° F. during both January and February. The Okanagan, like most arid regions, is subject to intense radiation freezing, as described by Day (30), Shankland (72) and Comford (26). Gold daytime winds and clear skies were characteristic of the 1949-50 winter. They usually died down towards evening, giving way to intense radiation of heat under clear skies. Thermometer readings taken at night in various orchards at different levels in the valley indicated that on some radiation nights, the lowest places were coldest and temperatures increased with altitude. But on other nights in the same places when temperatures were a l l at or below zero, the lowest place was the warmest and the temper-ature decreased with altitude. Thus, during most nights, there existed a mosaic of cold and warm air patches. Temperatures within these patches differed by as much as 7° F., even within a one-half mile radius. Since DNLY RECORD O F MAXIMUM ANP MINIMUM TEMPERATURES — D £ C . f'frK-MAfi./WQ. PENVCTOH B.C.L A . '4.r>^t S|)fc»< iKtt 7 . f 1.1 »-/ 7.0 L ^ u i j W j . J r t $3</*» S3yz</* SSfM/j"- J u / i * MAXIMUM P A l L Y TCMPERATliRE MINIMUM DAILY TEMPERATURE - 3 6 D..e9 *V7 - 61 -there was never any constant difference in temperature between any two areas, i t is quite understandable that the winter injury was not confined to any particular altitude or region. Winter precipitation in the Okanagan Valley during 1949-50 was well above the average as i s evidenced by the Precipitation figures for the Oliver district which are presented in Table 10, TABLE 10 Precipitation for the Winter 1949-50 and Average Precipitation for the period shown, (inches) Nov, Dec. Jan. Feb. Mar. Apr^ Oliver 1.73 1.52 1.07 .99 2.01 1.22 Average .95 1.07 .79 .81 .65 .67 The snow, driven by high winds, drifted into most orchards, thereby providing good frost protection for the roots of orchard trees. However, i t was not deep enough to provide much protection for trunks and lower scaffolds. (IX) MATERIALS AND METHODS Early in the spring of 1950, when i t f i r s t became apparent that low winter temperatures had caused serious damage to orchard trees, a detailed questionnaire (See Fig. 3) was prepared for mailing to commercial fruit growers in the South Okanagan Valley. Seven hundred of these ques-tionnaires were distributed in June, 1950, and growers were requested to answer the questions and return the completed forms as soon as possible. By July, 1950, only thirty completed questionnaires had been forthcoming, and i t was decided that a contact survey was the only feasible means of recording the desired information. Between July 7 and September 18, - 62 -more than four hundred orchards were visited. The survey was confined to the area between Penticton and the International Border at Osoyoos. Peach, cherry, and apricot trees only were included in the survey, since previous injury records on these kinds of fruits indicate that they have received very scant attention from the point of view of winter injury under Okanagan conditions. During the survey, questionnaire forms were used to record the information and were supplemented with more detailed information when-ever the need arose. A record of the actual number of trees of each variety killed or severely damaged was made and was calculated as a percentage of the total number of trees of that variety grown in any given orchard. Orchards visited were a l l selected at random to ensure that representative samples were being taken. A l l data were later sorted for each factor under consideration and were separated to provide replication. Each set of data was statisti-cally analyzed by Fisher's Analysis of Variance technique and, where necess-ary, variances were tested for significance by the Single Degree of Freedom technique as described by Snedecor (1946). FIGURE 5 Questionnaire Used in A Survey of Winter-Injured Stone-Fruit Trees in the Okanagan Valley  1. General Information Name of grower ' Loc ation of orchard Orchard exposure Soil type Orchard elevation Air drainage - 63 -Distance from nearest large body of water Windbreaks - windward or leeward side? Depth of snow cover during January and February 1950 2. Specific Information A. Kind of Fruit (Insert here "Cherry1*, "Peach", or "Apricot". If you have inserted "Cherry" in the above space, answer -the following questions for cherry only.) When was the last complete irrigation applied during 1949? System of irrigation used Did the trees produce normal vigorous growth during 1949? , Were trees well hardened off in f a l l of 1949? Were trees damaged by low temperatures of 1948-49? Was 1949 crop heavy, moderate or light? Did trees suffer any insect, rodent or spray damage during 1949? Specify. What fertilizer was applied during 1949? At what rate? What cultivation do you practise? What cultivation was done in 1949? In 1948-49 were trees short-pruned or long-pruned? Were trees pruned before or after January 1,1950? - 64 -B. Variety of Fruit Please supply the following information for each variety of each kind of fruit: Variety Number of Trees Under 10 yrs. old Over 10 yrs. old Damaged Undamaged Damaged Undamaged Fbr the above variety, indicate the nature of the injury: Bud injury Killing of terminal growth Crotch injury Dead areas on trunks and branches Bark splitting Root injury Trunk injury (X) RESULTS AND DISCUSSION The following results, which are presented in tabular form, are based upon figures which are presented in Table 1 of Appendix B of this report. The values recorded in each table in the Appendix represent average percentages of the damaged trees in as many orchards as could be found for the factor under consideration. (A) EFFECT OF DISTRICT, SOIL TYPE, AND KIND OF FRUIT The analysis below represents figures compiled from nearly four hundred orchards. Analysis of Variance Factor SS Degrees Freedom Variance F • Sig. —IX)5 .01 Total 11027 35 Treatment 9446 11 858 57.2" 2.25 3.18 Soil 1253 2 627 41.8" 3.43 5.69 Error 328 22 15 Effect of District - Single Degree of Freedom Analysis Oliver Osoyoos Okanagan Falls Penticton Factor 198 395 142 51 Variance Oliver vs. (197)2» 2156.0n Osoyoos - 4- 2x9 Oliver vs. (56) 2 - 174.2" Ok an. Falls + - 2x9 Oliver vs. (147)2- 1200.5" Penticton • - 2x9 Osoyoos vs. (253)2» 3556.0" Ok an. Falls • - 2x9 Osoyoos vs. (344)2- 6574.2" Penticton 4- - 2x9 Okan. Falls (91) 2 =» 460.0" vs. Penticton + - 2x9 Required F Value N]_ - 1 ) .05 .01 N2 = 22 ) 4.30 7 # 9 4 Error Variance a 15 Value Required for Significance: .05 = 15 x 4.30 =. 64.5 .01 - 15 x 7.94 = 119.1 - 66 -Effect of Soil Type - Single Degree of Freedom Analysis Factor Gravel & Sand vs. Sandy Loam Gravel & Sand vs. S i l t loam & clay Sandy loam vs. Silt loam & clay Gravel Sand 362 Sandy Loam 216 Silt Loam Clay 208 Variance (146)2 = 888.1 2x12 (154)2 = 988.1 ** 2x12 (8)' 2.7 2x12 Required F Value Nj_ = 1 \ .05 .01 N2 = 22 5 4.30 7.94 Error Variance =15.0 Value Required for Significance: .05 = 15 x 4.30 - 64.5 .01 = 15 x 7.94 = 119.1 Effect of Kind of Fruit - Single Degree of Freedom Analysis Factor Cherry vs. Apricot Cherry vs. Peach Apricot vs. Peach Cherry 187 Apricot 216 Peach 383 Variance (29)2- 35.0 2xH~ -ft (I96r = 1600.6 2x12 (167)2 = 1162.0** 2x32 Required F Value N]_ - i ) * ° 5 « 0 1 N2 = 22 ) 4.30 7.94 Error Variance - 15.0 Value Required for Significance: .05 = 15 x 4.30 .01 = 15 x 7.94 64.5 119.1 DISCUSSION Among the districts of the South Okanagan, the Osoyoos district suffered more damage to its stone-fruit trees than did any other district. The analysis indicates that damage in the Osoyoos district was highly significant at the 1% level over al l other districts. Similarly, Oliver district encountered heavier damage than a l l other districts except Osoyoos, while Okanagan Palls had less damage than either Oliver or Osoyoos but significantly more damage than Penticton. It wil l be noted that trees in the Penticton district were damaged less than those in any other district and produced partial crops of stone-fruits in 1950. The results of this analysis appear to be in direct relation-ship to the extent of the various soil types in each district. Osoyoos soils are predominantly light, with only small areas of loamy soils dis-tributed among the coarser sandy and gravelly soils. The Oliver district i s characterized by sandy soils with somewhat larger areas of loamy soils interspersed than in the Osoyoos area. This lineal tendency towards larger areas of the heavier soils from south to north in the valley holds true for both Okanagan Falls and Penticton. The Okanagan Falls soils are generally heavier than those in Oliver and much heavier than those in Osoyoos. Penticton soils are predominantly heavy, having large areas of clays, clay loams and s i l t loams with relatively small areas of sandy soils. The analysis on soil types indicates that trees growing on the lighter soils were severely injured in comparison with those planted on the'heavier loams and clays and i t is suggested that the significance between districts results directly from the difference in soil types in the respective districts. In what manner the soil types induced variable amounts of injury is difficult to say. It i s known, however, that trees growing on - 68 -light soils tend to drop their leaves and harden off earlier in the f a l l than do trees growing on heavy soils. This early hardening off, in turn, induces an early breaking of the rest period following the f i r s t warm spell in late winter. It seems entirely feasible that trees growing on light soils commenced growth activity at the f i r s t sign of warm weather following the extreme cold. Trees on heavy soils, having entered dormancy later, probably had less tendency to break their rest period at this time. Scrutiny of the temperature charts for February and March 1950 indicates that there was a severe temperature drop on March 12 following a long period of relatively warm weather. It is suggested that this temperature drop may have been responsible for the severe injury sustained by trees growing on light soils. Results of the analysis of cherry, peach and apricot tree hardiness indicate that the peach was least hardy of a l l the stone-fruits during the winter of 1949-50, This lack of hardiness may have resulted from the failure of peach trees to ripen their wood following the heavy 1949 crop of peaches. Peach trees were conspicuously late in dropping their leaves and entering rest period and i t i s probable that their woody tissues never did attain maximum cold resistance before the onset of cold weather. There appeared to be no significant difference in hardiness between cherry and apricot trees at the 5% level but the trend in damage indicated that apricot trees were more heavily damaged than cherry trees. (B) RELATIVE COLD INJURY TO SEVEN VARIETIES OF APRICOT The following analysis may be considered to be a hardiness rating for the seven most commonly grown apricot varieties. The table on which this analysis is based appears as Table 2 of Appendix B. - 69 -Analysis of Variance Factor SS Total 1,423 Variety 1,042 Replic ation s 14 Error 367 Degrees of Freedom 20 Variance 12 7.0 30.6 Sig. .05 .01 173.7 5.68** 3.00 4.82 .23 3.88 6.93 2.179 3.055 Apricot Varieties in Order of Magnitude of Injury; Wenatchee Moorpark Reliable Perfection Tilton Riland Blenheim Kaleden 33.00?? 30.677? 22.337? 20.337? 19.677? 17.337? 11.007? Minimum Significant Difference Between Varieties; .05 level .01 level 3 0'^ x 2 x 2.179 9.837? ? u« b x * x 3.055 = 13.80?? DISCUSSION The fact that Wenatchee Moorpark trees proved to be the least hardy of a l l Apricot varieties studied in this survey is surprising. This variety has been rated among the most hardy by Mann and Keane (58). - 70 -It should be noted, however, that the Wenatchee Moorpark i s among the oldest of the apricot varieties grown in the Okanagan, and at the time of the freeze, there were many old Wenatchee Moorpark trees in the orchards surveyed. Since old trees, regardless of variety, were usually more seriously injured that were young trees, i t is probable that the high per-centage of injury recorded for Wenatchee Moorpark, as a variety, was weighted by the severe injury to nearly a l l old trees of this variety. On the other hand, injury to the Reliable apricot was not significantly less than injury to the Wenatchee Moorpark. The Reliable apricot i s a relatively new variety and no trees more than ten years old were included in the survey. Ever since this variety was introduced commercially on peach roots, growers have experienced difficulty with breakage of the young trees at the bud union. This frequent breakage suggests an incompatibility between stock and scion, and this one fact alone may have been partially responsible for the high percentage of winter injury recorded for Reliable apricot. Perfection and Tilton appeared moderately tree hardy in this survey, being significantly hardier than Wenatchee Moorpark, but significantly less hardy than Kaleden. Kaleden apricot appeared more tree-hardy than a l l other apricot varieties, but was not significantly hardier than Riland and Blenheim at the 5% level. (C) RELATIVE COLD INJURY TO SIX VARIETIES OF CHERRY The following analysis may be considered to be a hardiness rating for the most commonly grown cherry varieties. The table on which this analysis is based appears as Table 3 of Appendix B» - 71 -Analysis of Variance Factor SS Degrees Freedom Variance Total 14578 Variety 12785 Replications 24 Error 1769 17 5 2 10 Sig. .05 .01 2557 12 176.9 14.45** 3.53 5.64 .07 4.10 7.56 2.228 3.169 Cherry Varieties in Order of Magnitude of Injury; Royal Ann 85.00?? Carnival 41.67?? Bing 19.00?? Windsor 15.33?? Deacon 13.67?? Lambert 7 •( Minimum Significant Difference Between Varieties: .05 level 176.9 x 2 x 2.228 = 24.20?? .01 level 176.9 x 2 x 3 # 1 6 9 = 3 4 > 4 1 ? j 3 DISCUSSION Royal Ann was more severely injured than any other cherry variety recorded in the survey. The injury to this variety was highly significant at the 1?? level, and was most pronounced in trees over the age of 15 years. Carnival, a variety which is not widely grown in the Okanagan, was also severely damaged, and in tree hardiness, rated significantly lower -12 -than Windsor, Deacon and Lambert. Bing appeared intermediate in tree hardiness and rated approximately equal to Windsor and Deacon in this respect. Lambert, al-though appearing to be the least injured of the cherry varieties, was not significantly less damaged than Bing. In general, the results of this survey of cherry varieties appears to be in accord with the findings of Mann and Keane (58), with the exception of the Deacon variety, which appeared hardier than Royal Ann in this survey. (D) RELATIVE COLD INJURY TO EIGHT VARIETIES OF PEACH The following analysis may be considered to be a hardiness rating for the most commonly grown ohorry varieties. See Table 4 of Appendix B for information on which this analysis is based. Analysis of Variance Factor SS Degrees Freedom Variance Sig, .05 .01 Total 9342 Variety 8550 Repli c ations 198 Error 594 23 7 2 14 1221 99 42 ft* 290.7 2.77 2.35 3.74 4.28 6.51 2.145 2.997 Peach Varieties in Order of Magnitude of Injury: J. H. Hale 80.335? Golden Jubilee 66.00?? Rochester 62.33?? Elberta 44.33?? Veteran 42.67?? - 73 -39.00?? 28.337? 20.337? Minimum Significant Difference Between Varieties; Vedette Valiant Red Haven 4 2 * 2 x 2.145 = 11.357? 42 x 2 x 2 > g 7 7 = 15.757? DISCUSSION The three peach varieties which appeared to be most severely injured, J.H. Hale, Golden Jubilee, and Rochester, are no longer recommended for commercial planting in the Okanagan. J.H. Hale was least hardy of a l l the peach varieties. Golden Jubilee and Rochester rated significantly less tree-hardy than a l l other peach varieties with the exception of J.H. Hale. Intermediate in tree hardiness were Elberta, Veteran and Vedette. Among these three varieties there was no significant difference. Valiant and Red Haven exhibited greater tree hardiness than a l l other varieties surveyed. The hardiness rating for Red Haven, however, may not be entirely representative for that variety, since i t was intro-duced only a few years ago, and there are no commercial plantings over ten years of age. In general, the results of this hardiness rating for peach varieties are in accord with the findings of Mann and Keane (58). (E) EFFECT OF LATE IRRIGATION The following analysis, based on Table 5 in Appendix B, represents comparative damage to trees in orchards which received late .05 level .01 level - 74 -irrigation (after October 15) and those which received no late irrigation. Factor Total Treatment Replications Error Analysis of Variance SS Degrees Freedom Variance F Sig 4247 3386 265 596 17 5 2 10 677 132.5 59.6 11.3 2.22 .05 .01 3.33 5.64 4.10 7.56 Effect of Late Irrigation - Single Degree of Freedom Analysis Factor Late Irrigation No Late Irrigation Variance Peach Cherry Apricot Peach Cherry Apricot Late Irrigation vs. No Late Irrigation  Late Irrigation vs. No Late Irrigation  Peach 100 45 52 170 88 106 ( l f r ) 2 &3 " 1549.4 (70r = 816.7 2x3 Cherry (43)' - 308.2 Apricot (54) = 486.01 2x3 .05 .01 Required F Value ^ = 1 ) N2 = 10) 4 ' 9 6 1 0 - ° 4 Error Variance -59.6 Value Required for Significancet .05 level 59.6 x 4.96 a 295.6 .01 level 59.6x10.04 - 598.4 - 75 -DISCUSSION Within the past few years, there has been a tendency on the part of fruit growers to apply a late irrigation to their orchards. Pre-viously, the trees had been irrigated up to the time when the crop was harvested, after which the water was turned off. The use of a late i r r i -gation sometime after October 15 and following the time when trees have begun to harden off, has become increasingly popular. The results of this survey indicate that late irrigation of stone-fruit orchards is a desirable practice from the point of view of reducing winter injury. The percentage of trees injured in late-irrigated orchards was significantly lower than that in orchards not late-irrigated. This held true for a l l kinds of fruit surveyed and was highly significant in the case of peach orchards. These findings are in accord with the relationship of soil moisture to dormant trees. A moderately high level of soil moisture during the winter months apparently replenishes the water lost from the tree top due to the desiccation of drying winds. This movement of water probably occurs at any time during the winter when the vascular system is not frozen, and serves to prevent excess moisture depletion from cells of the woody tissues. (F) EFFECT OF AGE OF TREE The analysis of injury to trees according to their age is based upon two age groups, those trees under ten years of age and those over ten. The percentage of injury recorded within the two age groups i s presented in Table 6 of Appendix B. Since the age of a tree is difficult to judge precisely, the two age groups used in this survey were based entirely on trunk diameters. - 76 -Analysis of Variance Factor Total Treatment Replications Error SS Degrees Freedom Variance F Sig. 5930 4090 435 1405 .05 .01 17 5 2 10 818.0 217.5 140.5 5.82 1.54 3.33 5.64 4.10 7.56 Factor Effect of Age - Single Degree of Freedom Analysis 1-10 yrs Over 10 yrs. Peach Apricot Cherry Peach Apricot Cherry Variance 97 5 6 35 179 79 89 1-10 yrs. vs. Over 10 yrs. 1-10 yrs. vs, Over 10 yrs. Peach Apricot Cherry (82)* "2x3" (25) 2 -2XT (54) 2 ~2zr .05 .01 = 1120.7 - 88.1 = 486.0 Required F Value N-j_ = 1 ) N2 =10 ) 4.96 10.04 Error Variance = 140.5 Value Required for Significancet 5% level 140.5 x 4.96 o 696.9 1% level 140.5 x 10.04= 1410.6 DISCUSSION Results of this survey indicate that trees over ten years of - 77 -age were more severely injured than trees under ten. This held true for a l l trees as a group, but further analysis indicates that there was no signif-icant difference between the two age groups of cherries and apricots. Injury to peach trees of the older age group, however, was significantly greater than the injury shown by younger trees. These findings may relate directly to the.time of hardening off of the various kinds of fruits. Peaches were notably late in dropping their leaves following the 1949 harvest, and may have lacked tissue maturity at the onset of winter. Apricots followed the normal pattern of hardening off in the f a l l of 1949, while cherries were very nearly normal in their hardening-off activities. It would therefore appear that the age factor may not have come into play for apricots and cherries owing to the probability that both young and old trees had gained normal cold resistance before the arrival of cold weather. At any rate, age did not appear to be an important factor in the case of cherry and apricot. (G) EFFECT OF PRUNING TECHNIQUE Both long and short methods of pruning are used in the Okanagan Valley. The following analysis i s designed to evaluate each method in rela-tion to i t s influence upon winter injury. This analysis has been made for peaches and apricots only, since pruning of cherries is not an annual practice and since cherries are seldom short-pruned even when the pruning operation i s carried out. See Table 7 in Appendix B. Analysis of Variance Factor SS Degrees Freedom Variance F ^ S*05 .01 Total 2885 11 Treatment 2038 3 679.0 7.42* 4.76 9.78 Replications 298 2 149.0 1.63 5.14 10.92 Error 549 6 91.5 - 78 -Effect of Pruning Technique - Single Degree of Freedom Analysis  Long-Pruned Short-Pruned Factor Long-Pruned vs. Short-Pruned Long-Pruned vs. Short-Pruned Peach Apricot Peach Apricot Peach Apricot 87 55 162 91 Variance < y i > = 1026.8 * 4x3 (75) 2 = 937.5 * 2x3 ( 3 6 ) 2 - 216.0 2x3 Required F Value N-_ - 1 ) N2 = 6 ) Error Variance - 91 .5 Value Required for Significance: .05 .01 5.99 13.74 t level 91.5 x 5.99 = 548.08 t level 91.5 x 13.74 =1257.2 DISCUSSION The above analysis of the effect of pruning technique on cold temperature injury is based on the style of pruning carried out during the 1948-49 winter. Short-pruned trees as a group suffered more injury than long-pruned trees. This finding appears to bear out the suggestion that removal of large quantities of leaf surface by heavy pruning reduces the ability of the tree to build up adequate food reserves for normal tissue hardening. Once again, however, peaches suffered heavily from short-pruning while the difference in injury to short and long-pruned apricots was not significant. The reason for this lack of significance may l i e in the degree of short-pruning carried out on apricots. The term "short-pruning" i s , at best, - 79 -only a relative term, and while the short-pruned trees surveyed in this project were definitely pruned heavily, few of them could be considered to have been pruned as heavily as were the short-pruned peaches. In spite of the unexpected results with apricots, however, i t would appear that short-pruning is an unwise and unprofitable practice and should be discontinued, at least in the case of peaches. (H) EFFECT 0? VIGOUR During the planning of the survey, i t was found difficult to establish tangible descriptions of vigour. Since, however, the degree of vigour inherent in a tree is chiefly manifested by its terminal growth, "good" vigour was classified as meaning "terminal growth 10" - 18" long", while "poor" vigour meant "terminal growth less than 8"." The terminal growth measured in each case was the 1949 growth. (See Table 8 in Appendix B.) Factor SS Total 2166 Treatment 1792 Replications 7 2 3.49 .09 4.10 7.56 Analysis of Variance Degrees Freedom Variance F ^ e*05 .01 17 5 358.4 9.8** 3.33 5.64 Error 10 - 80 -Effect of Vigour - Single Degree of Freedom Analysis Factor Good Vigour Peach Apricot Cherry 68 48 45 Poor Vigour Peach Apricot Cherry 133 94 75 Variance Good Vigour vs. Poor Vigour Good Vigour vs. Poor Vigour Peach Apricot Cherry (141)2 = 1104.5** 6x3 (65)' : 704.1 (46)* = 352.7 : 150.0 2x3 "2xj" ( 3 0 ) ' ~ 2 x T Required F Value N-j_ = 1 ) .05 .01 4.96 10.04 I 2 : 10 ) Error Variance = 36.7 Value Required for Significance; .05 level 36.7 x 4.96 .01 level 36.7 xl0.04 182.0 368.5 DISCUSSION Vigour or the lack of i t appeared to be of utmost importance in determining the extent of winter injury of stone-fruits. Trees in poor vigour were badly damaged when compared with trees in normal vigour. This held true for a l l stone-fruit trees as a group, showing high significance at the 1$ level. A separate analysis for each kind of fruit indicated that the relationship held true for both peaches and apricots but did not follow through for cherries. Some factor other than vigour appears to be more important to the cold resistance of sweet cherry trees. - 81 -The explanation for heavy damage in trees of poor vigour probably relates directly to the lack of carbohydrate build-up in the cells of woody tissues. Trees showing lack of growth during 1949 usually bore heavy crops of fruit and were lacking in leaf surface. It i s probable that the scanty leaf surface was taxed to the limit in sizing the fruit and was not able to manufacture a sufficient reserve of food materials to offset the low winter temperatures. (I) EFFECT OF CULTIVATION Two basic cultural systems are practised in the Okanagan. The fi r s t involves a permanent cover of some kind, either a legume cover crop or grass sod. The second involves either continuous clean cultivation or inter-mittent clean cultivation. The following analysis strives to evaluate the effect of each system in relation to i t s influence on winter injury. See Table 9, Appendix B. Analysis of Variance Factor SS Total 1072 Treatment 225 Replications 23 Error 824 Degrees Freedom 17 5 2 10 Variance 45.0 11.5 82.4 Sig. F .05 .01 .55 .14 3.33 5.64 4.10 7.56 No significance from "Treatments" DISCUSSION The results obtained in this analysis do nothing to support either the permanent cover or cultivation systems of orchard culture. There was no significant difference recorded between the two types of culture. - 82 -It is probable that any effects resulting from one or other system of culture would be felt chiefly by the root systems of the trees. Since snow cover was generally heavy and frost penetration of the ground slight, the root systems were not usually injured. Whether or not the heavy snow cover was responsible for nullifying any possible effect from cultural practices, the analysis seems to indicate that cultivation or the lack of i t was not a contributing factor to winter injury during the winter of 1949-50. (J) EFFECT OF SNOW MULCH During the progress of this survey, an effort was made to determine the effect of a snow mulch in reducing winter injury. Accurate measurements of snow depth, however, had not been made by most growers and the meagre data available did not justify an analysis. At the f i r s t onset of snow in December, the cover was fairly uniform throughout -the Southern Okanagan Valley. But as the winter pro-gressed, the snow drifted heavily in most areas, leaving the ground almost bare in some places. Snow cover varied from 0" to 40" in some orchards, the high knolls and ridges often showing bare ground. Trees growing on knolls and ridges were usually killed out-right, indicating that the roots had been frozen to death. Trees growing nearby, but with a snow mulch covering the roots, were often killed as well, but differed from the others in that they often exhibited a vigorous shoot growth from the base of the trunk. This shoot growth appeared to coincide closely with the height of the snow line, and indicated that roots had not been damaged by the low temperatures. Many growers elected to retain the old roots and removed the original tree top, training one of the vigorous young shoots to form a new tree. - 83 -(K) EFFECT OF PREVIOUS CROP Since nearly a l l stone-fruit crops had been heavy in 1949, there appeared to be no basis of comparison for the effect of the previous crop upon inducing winter injury. A statistical analysis for this factor was therefore impossible. It seems reasonable to suppose, however, that the extremely heavy crop borne by stone-fruit trees in 1949 was a major contributing factor to winter injury. This crop apparently depleted food reserves to the point where vigour was at a generally lower level than that desired for profitable stone-fruit production. This resulted in a high degree of susceptibility of the woody tissues to low temperature injury. (XI) MEANS OF OFFSETTING- WINTER INJURY Results of this survey and a close scrutiny of the voluminous literature dealing with winter injury to stone-fruit trees, indicate that there are certain practices which growers might well follow in an attempt to minimize winter injury in their orchards. A summary of these practices follows. Healthy trees will survive low temperatures better than trees which have been weakened by overcropping, drought, wet feet, spray burn, insects, disease and other factors. Assuming, however, that trees are in a healthy condition when they enter the dormant period, one may adopt certain techniques to minimize injury during a winter which would ordinarily damage even the healthiest of trees. (A) SITE During every severe winter, some orchards always fare better than others from the point of view of cold injury. These differences are not just a matter of good luck for one orchardist and i l l luck for another. Means of Reducing '."/inter Injury-Trunk of cherry tree protected from "south-west" i n j u r y by presence of V-shaped board placed against the tree. Trees showed no i n j u r y where such protection was provided. This protector was cut too short and injury has occurred above the board. - 84 -They are the result of certain patterns of planning and procedure, some of which lead to severe tree injury and others to less injury or perhaps none at a l l . Often the orchards which survive well in low temperatures do so more by good luck than good management. Nevertheless, the reasons for good survival are there and are well worth observing. Possibilities of the occurrence of winter injury in any proposed orchard area should be considered before any actual planting is undertaken. This is best done by a careful study of weather records as far back as such records have been kept. Factors such as frost free periods, the frequency of temperatures low enough to cause injury to orchard trees, and the duration of these low temperature periods are of utmost importance. In any given area which appears suitable, the selection of a site which i s close to a large, deep body of water is desirable, since a body of water is known to exert a modifying influence upon temperature in the nearby orchards. In addition, the ground selected for planting should be "frost free", or situated on ground which is subject to good air drainage. The use of depressions in the land, or areas located in the midst of heavily-wooded country i s to be avoided, since air drainage in such locations is usually poor. (B) SOIL Soil type, too, has a definite bearing on the ability of trees to withstand cold. A deep, well-drained soil which is capable of holding adequate moisture for tree growth is ideal. Light sandy soils generally freeze deeper than loams and are thus apt to experience deeper frost pene-tration. Roots in such soils are therefore often injured. Shallow, sandy soils which are underlain with gravel should be avoided since trees on such soils tend to be shallow rooted and hence are susceptible to cold injury. Trees growing on poorly drained soils are also very susceptible to winter - 85 -injury and should never be planted on such soils until adequate drainage has been provided. Even some of the heavy clays and silt s , which are other-wise suitable as orchard soils, are better with a drainage system installed. Such heavy soils tend to provide available moisture, such that they promote prolonged tree growth, beyond the time when normal f a l l hardening-off should take place. Delayed hardening-off is a common cause of cold injury. (C) HARDY ROOTSTOCKS, FRAMEWORKS AND VARIETIES Assuming that a suitable site and soil have been selected, one of the best forms of insurance against cold injury is the choice of suitable kinds and varieties of fruits for the particular area in question. Stone fruits, for instance, should ordinarily not be planted in any area where temperatures can regularly be expected to go below about -10° F. Apples, on the other hand, generally have a somewhat greater tolerance to low temperatures and might safely be planted in areas which, from the stand- , point of winter temperatures, are questionable for stone fruits. Within any given kind of fruit there occurs a wide range of varieties. Some of these varieties inherently have more cold resistance than others. In planting consideration should be given to the cold hardiness ratings of the varieties selected. Orchardists should always adhere to the kinds and varieties of fruit recommended by authorities for use in their particular districts. The use of hardy varieties in cold districts i s a useless procedure unless some attention is given to the use of hardy rootstocks and hardy frameworks. Unfortunately, too much use is made of seedling root-stocks of unknown cold-hardiness, and too l i t t l e use made of clonal rootstocks of known hardiness. Growers may often wonder why three or four trees in a row of some fifteen to twenty survive the winter while the rest are damaged beyond recovery. Without doubt, there are instances when this hit-and-miss •type of injury can be attributed to seedling rootstocks, each one having a - 86 -somewhat different degree of cold hardiness. Certain semi-tender varieties of fruit may be adapted to cold areas through the use of hardy framework stocks. Red Delicious apple, for instance, a variety ordinarily too tender in trunk and scaffold to be grown in the Kelowna, B.C. district, may be adapted to this district by topworking Red Delicious on Mcintosh. Some combinations in fldouble-workedn trees are commonly used in the Okanagan Valley of British Columbia as a safeguard against winter injury. (D) HANDLING YOUNG TREES TO PREVENT SUNSCALD Reduction of the incidence of winter sunscald or south-west injury may be assisted by certain techniques used in the handling of young trees. In areas where sunscald i s a familiar type of winter injury, young trees should be planted so that they lean slightly to the south-west. This procedure helps to reduce the angle at which the sun's rays strike the trunk of the tree. In addition, pruning of young trees should be carried out in the late winter or early spring, and the pruner should strive to leave a low Hmh on the south-west side of the tree to help break the direct contact of the sun's rays with the trunk. Some growers make a practice of loosely wrapping the trunks of young trees with waterproof paper. Where this procedure is followed, the paper should be put on late in the f a l l and removed early in the spring. (E) COVER CROPS OR MULCHES VS. CULTIVATION It is well known that a mulch of hay, straw or sawdust, or a heavy cover crop will tend to delay the deep ground penetration of frost. Such ground covers thus permit root activity during cold weather and enable the tree to replace moisture lost by evaporation through winter desiccation. From the point of view of winter injury, then, the practice of f a l l cultivation Means o f Reducing Winter I n j u r y Young Red D e l i c i o u s apple t r e e s showing use o f aluminum p a i n t on trunks t o reduce s u s c e p t i b i l i t y to w i n t e r s u n - s c a l d . - 87 appears to be unwise. A deep snow mulch wil l retard frost penetration in the same way as vegetation mulches and offers good winter protection both to roots and to trunks. Snow is known to drift away from certain exposed areas in the orchard, and where this occurs, growers would do well to erect snow fences in an effort to hold the snow in the exposed orchard area. Good as snow may be, sufficient quantities of i t to be helpful against cold cannot be relied upon from year to year in most districts, and i t i s for this reason that f a l l cover cropping and mulching are reco-mmended as being sound orchard practices. The use of fall-sown cover crops probably has an advantage over the use of mulches, in that a late-sown crop tends to absorb any surplus nitrogen at the expense of the tree and thus tend to stop tree growth at an early f a l l date. Such early hardening-off is known to be desirable. (F) FERTILIZER PRACTICES Trees which are showing moderate vigour are more tolerant of low temperatures than are those in high or low vigour. Thus a bearing apple tree should be fertilized only to the extent that will promote approximately 12 to 15 inches of terminal growth each year. In general, mature stone-fruit trees should be fertilized to promote 18 to 24 inches of terminal growth each year. This terminal growth should occur early in the growing season and should not continue much beyond mid-summer. Termin-ation of this growth will promote early hardening-off of tissue in prepar-ation for the colder months to come. This early growth is best obtained in the Okanagan Valley by fertilizer applications during the very late f a l l on heavy soils and very early spring on the light soils. Summer or early f a l l applications of fertilizer must be avoided. Failure to apply adequate quantities of fertilizer will undoubtedly lead to low vigour, which in turn will often - 88 -lead to winter injury. In general, i t appears that the lack of any essential food element will upset the physiological balance of the trees and might well lead to winter injury. (G) PRUNING PRACTICES Pruning practices should always be patterned to minimize the effects of low temperatures on the trees. If a normal winter has been experienced, orchards should be pruned in the late winter or early spring. If a severely cold winter has been experienced, l i t t l e or no pruning should be done. These latter two statements apply particularly in the case of soft fruits and to a lesser extent in the case of apples. Late pruning also affords the tree better opportunity to start healing its wounds promptly. It also reduces the possibility of die-back from the pruning cuts and the possibility of fungous invasion at the wound. Even when there has occurred no particularly cold weather prior to the pruning season, only a moderate pruning of limbs should be practised. Heavy pruning reduces the leaf surface of the trees during the following growing season and so limits the ability of those trees to build up adequate food reserves in the storage cells. This lack of sufficient food naturally weakens the tree and renders i t susceptible to injury during the following winter. For this reason, too, grafting of trees should be delayed until early spring. Where risk of cold damage to late f a l l or early winter pruned trees exists, orchardists would be wiser to hire additional help to prune the orchard at a later date than to run the risk of winter injury by pruning their acreages over a longer period of time. In general, the most tender fruits should be pruned last, and within any given kind, the most tender varieties should be pruned last. (H) THINNING THE CROP Overcropping of trees tends to deplete food reserves to the point where woody tissues are unable to withstand low temperatures. Thus trees should be adequately blossom-thinned or fruit-thinned in rela-tion to their leaf surface. The thinning operation should be carried out at the earliest possible date, since the longer the excess fruits are left on the tree, the greater will be the tendency to reduce the food storage processes within the woody tissues. To this end, the blossom thinning of tree fruits appears to be a promising method of reducing the set of fruit at the earliest possible date, so that the trees will develop maximum food reserves during the growing season. Edgerton (33), using dinitro-ortho-cresol materials for blossom thinning increased fruit bud hardiness of peaches as noted in Table 3. No doubt such treatment also increases the hardiness of the woody tissues. (I) IRRIGATION PRACTICES Both excess and lack of water appear to enhance the pro-bability of winter injury. An excess of water may cause growth to continue too late into the f a l l months, or, i f carried to the extreme where trees have wet feet, may cause a decided lack of vigour. Both results of this excess moisture will render the trees susceptible to winter injury. Similarly, a lack of water induces low vigour in trees with resulting poor hardiness characteristics. On the basis of these observations, growers should irrigate their trees to promote adequate growth during the early part of the season and to size the crop prior to harvest. Following the harvest, irrigation should be reduced or perhaps stopped entirely to encourage cessation of growth and to hasten the ripening of woody tissues. When the trees have commenced to harden off, they may then require, depending on the season, a late irrigation to ensure that the soil moisture is adequate - 90 -to meet tree requirements during the winter months. (J) OTHER PRACTICES The practice of whitewashing tree trunks to offset winter sunscald may have some virtue with certain varieties of fruit which can tolerate heating of the cambium layer on the south-west side of the trunk up to but not past a certain temperature. Aluminum paint used for the same purpose may offset the danger of sunscald to an even greater extent, since aluminum is known to exhibit remarkable heat-reflecting properties. The use of aluminum paint as a trunk treatment, however, is in the experimental stage only and more knowledge of its abilities i s necessary before i t can be recommended. The practice of hoeing around trees very late in the f a l l cannot be recommended since i t tends to expose the crowns of the trees to the ravages of low temperature. Where the presence of crown rot or mouse infestation demands the removal of debris from the tender crowns late in the f a l l the debris should be replaced by coarse ashes or pea gravel. A much better practice, however, is to remove the debris early enough in the f a l l to permit hardening-off of the crown tissues. The use of windbreaks in orchard areas is a commendable practice only when the windbreaks do not prevent good air drainage through the orchard. Too often, they are so dense and so poorly placed that they create frost pockets which are most detrimental to the orchards. Wind-breaks should be used to protect exposed areas of orchard only and should be so placed that they act as wind deflectors. Over long periods of time, most fruit growers find i t impossible to prevent entirely a l l winter injury. Sooner or later, some or a l l of their trees are affected. It i s therefore wise for a grower to diversify his plantings as much as good economy will permit. He should - 91 -maintain an acreage of each kind of fruit and might well grow several varieties of each kind. Finally, growers should try to carry out such a program of constant orchard renewal, that they will have plantings of trees of varying ages and of varying susceptibility to winter injury. Some such plan would tend to minimize disastrous tree or crop losses in any one season, such as occurred in 1949-50. (XII) CARE OF TREES AFTER INJURY Trees which are suspected of having been injured during the winter should receive very special attention. It is often impossible to forecast by an examination of the tree early in the spring the extent or severity of the injury. Very often, trees which appear dark brown in the cambium layer recover to the extent that they once again become profitable entities in the orchard. Hasty decisions to pull out appar-ently injured trees should therefore be discouraged. Injured trees should receive their regular dormant sprays to reduce the possibility of an infestation of insects and diseases which could weaken the trees. No pruning should be done to an injured tree, since i t is difficult to judge the extent of the injury and there is danger of removing sound wood and leaving injured wood. Furthermore, any removal of potential leaf surface would minimize the chances of tree recovery. Mulching of the trees with some moisture-holding material would be a wise practice, since the moisture relations in the soil beneath an injured tree are most important to i t s welfare. Moderate fertilizer applications with ammonium nitrate, i f none has been applied the previous f a l l , would tend to assist the tree in developing a maximum amount of new leaf surface. Irrigation practices should be geared to prevent any over Treatment o f I n j u r e d Trees M R I H B a • c* i< ' H P * f^mW ' • ^ * " w^, 7* " -W. i ^ Jfa' T h i s s i x - y e a r - o l d peach t r e e was s e r i o u s l y weakened by trunk and s c a f f o l d i n j u r y , c a u s i n g i t to break down under o n l y a moderate crop. F i v e - y e a r - o l d a p r i c o t t r e e which was b a d l y i n j u r e d i n c o l d w i n t e r . Owner e l e c t e d to c u t o f f the e n t i r e top o f the t r e e . Photo taken i n 1954 i n d i c a t e s almost complete r e c o v e r y , w i t h o n l y one s m a l l p r u n i n g wound showing on the trun. E n t i r e orchard was treate< the same way with complete su c c e s s . - 92 -supply of water, especially where there is injury to the tree roots. Wherever clean cultivation is to be practised only very shallow cul-tivation i s recommended to remove weed growth. During the growing season following the severe winter, the orchardist should study the injured trees and decide which are to be kept, which are worth repairing, and which should be removed or replaced. This cannot be done by rule-of-thumb. Each tree wil l be a problem in itself and wil l call for the exercise of careful judgment. Trees having the trunk and a considerable portion of the scaffold damaged, should probably be removed. If there has been no root injury, however, i t may be possible to renew badly damaged trees by cutting them off near the ground at a decided slant, saving a new sprout at the upper edge of the cut. In this case, the large, uninjured root system may soon grow a new top and a profitable tree be formed much sooner than as i f the old tree were replaced with a new nursery tree. This practice of renewal should not be attempted with trees over ten years of age, however, since the wound would be so large that i t is doubtful i f i t would heal before decay would set in. The renewal sprout should be staked to prevent i t breaking away from the old stump. Trees which the grower elects to keep should be examined for loose bark. Such loose bark should be tacked in place with roofing nails at an early stage of growth to prevent the drying out of tissues beneath the bark and thereby to encourage rapid healing. The practice of tacking cherry bark with nails is to be discouraged, since the tree will often bleed through the punctures. Binding the trunks with cloth strips is the desirable practice on injured cherry trees. After the healing process has continued into late summer, the loose sections of bark should be removed to reveal the extent of healing. Wherever the new tissue has - 9 3 -not covered the old heart^wood, this wood should be treated with a good, prepared tree emulsion, or white lead paint containing mercuric b i -chloride. Such treatment will reduce the possibility of entrance of fungi and insects. Painting, however, should always be delayed until the newly-formed tissue i s calloused sufficiently to offset possible burning or injury to the cambium layer. If an apple or a pear tree appears to have a fair chance of recovery, the dead limbs should be removed flush with the trunk or • large limb from which they extend. The winter injury cankers around the base of dead limbs should be removed neatly by scraping the dead bark away with a sharp knife or chisel. The extremities of the wounds should be lef t pointed to hasten healing and the exposed wood painted with a suitable protectant. Such treatment does not apply to peach and apricot, where the procedure should be to remove dead limbs and paint large pruning wounds. In cases where the injury is known to be localized in the trunk or a s m a l l portion of main scaffold branches, the damaged areas may be inarched or bridge-grafted in the same manner as would be done for mouse damage. Care should be used in selecting the scions for this purpose so that injured shoots are not used as grafting material. Whether or not injured trees recover depends largely on the weather conditions that follow the injury. If the spring and summer follow-ing the injury are cool and growing conditions are good, the trees may make a remarkable recovery. Hot, dry weather following the freeze, however, is most unfavourable to recovery. Under such weather conditions, trees which appeared to suffer only slight winter injury may succumb entirely by the end of summer. Whatever the extent of recovery, growers must expect their trees to become rather brittle in the years following a freeze, and must - 94 therefore be prepared to brace the scaffold limbs and prop heavily when the trees are in crop. This brittleness results from the failure of resins and gums to infiltrate the young sapwood which has been frozen to death. The young sapwood therefore never develops the same strength that heartwood ordinarily would. (XTTT) SUMMARY A winter injury survey of several hundred stone-fruit orchards in the South Okanagan Valley of British Columbia following the severe winter of 1949-50 indicated that numerous factors contributed to the widespread damage. Sudden temperature declines following periods of relatively mild weather were undoubtedly responsible for a major portion of the damage. Notwithstanding these temperature declines, however, statistical analyses indicate that certain factors, both controllable and uncontrollable, affected the degree of severity of injury. Soil type bore directly on the amount of injury, the lighter soils being responsible for greater injury than were the heavier soils. The intensity of injury in the various districts surveyed appeared to be directly associated with the soil type most predominant in each district. Of the various kinds of fruit included in the survey, peach trees were injured more than apricot trees and apricot trees more than cherry trees. Within each kind of fruit, varieties exhibited variable degrees of hardiness, some varieties showing almost 100$ injury, others less than 10% injury. Irrigation practices appeared also to have a direct bearing on the extent of winter injury. Trees which were irrigated later than October 15, after f i r s t having been allowed to commence hardening-off, - 95 -came through the winter with less injury than trees which were not late-irrigated. The age of trees was of significant importance to a l l trees when trees over ten years of age were compared with trees under ten years of age. This fact was particularly evident in the case of peach trees - the older age group showing most injury. Then again, the pruning technique used on trees, regardless of age, appeared to be a factor involved in the extent of injury sus-tained. Short-pruned trees, especially in the case of peaches, suffered more winter injury than long-pruned trees. Vigour of the trees was also an important contributing factor to the amount of injury sustained. Trees in low vigour, regardless of variety or kind, exhibited more cold injury than trees in good vigour. This factor was especially important in the case of peaches and apricots. Orchard cultivation or lack of cultivation did not appear to be an important factor bearing on the extent of winter injury. There was no statistical difference between trees growing in cultivated orchards and those growing in orchards having permanent cover crops. This similar-ity in response probably resulted from the heavy snow cover in a l l orchards, nullifying any possible root damage attributable to one treatment or the other. Root damage did not appear to be an important form of winter injury in 1949-50. An analysis of the effect of snow mulch in reducing injury was impossible since snow cover was general and comparisons were lacking. Reliable data on actual depth of snow cover was also lacking. Similarly the effect of previous crops could not be analysed since nearly a l l trees bore a heavy crop of fruit in 1949, Interactions between the various factors could not be - 96 -analysed, even though interactions are recognized to be of extreme importance in any study of winter injury. In a survey of this kind, however, setting up data to facilitate analysis of interactions did not seem feasible, since the variables from orchard to orchard were not controlled. The results of this survey and the difficulties encountered in tabulating data from such a vast number of orchards, points out the desirability and necessity of setting up controlled experiments dealing with the various factors associated with the winter injury complex. The results indicate, too, the desirability of analysing one small phase of the complex at a time, so that the results of a l l these small phases may ultimately be related to each other to present a clear picture of the factors responsible for winter injury. - 97 -BIBLIOGRAPHY 1. AMERICAN FRUIT GROWER. 1946 Infra-red ray frost fighter. Vol. 67:23, Dec. 1946. 2. AMERICAN FRUIT GROWER. 1947 New developments on the frost front. Vol. 67:29, Mar. 1947. 3. ANDERSON, L.C. 1936 A survey of the behaviour of cherry trees in the Hudson River  Valley with particular reference to losses from winter k i l l i n g  and other causes. Agr. Ex. Stn. Bull. 653, Geneva, N.Y. 4. ANTHONY, R.D., SUDDS, R.H., and CLARKE, W.S. 1936 Low temperature injuries to orchards in Pennsylvania and  adjoining states in the f a l l and winter of 1935-36. Proc. Am. Soc. Hort. Sc., Vol. 34, 1936, p. 33. 5. AUCHTER, E.C. and KNAPP, H.B. 1945 Orchard and Small Fruit Culture. John Wiley and Sons Inc., London, pp. 467-480. 6. BARNETT, R.J. 1944 Effect of ground cover on the freezing and thawing of orchard soils. Proc. Am. Soc. Hort, Sc., Vol. 44, 1944. p. 57. 7. B.C. DEPARTMENT OF AGRICULTURE. 1950 Annual Report. Queen's Printer, Victoria, B.C., 1951. 8. B.C. DEPARTMENT OF AGRICULTURE. 1950 Unpublished correspondence. Kelowna, B.C. 98 9. BLAKE, M.A. 1938 Hardy rootstocks for the peach should extend well above the  surface of the so i l . Proc. Am. Soc. Hort. Sc. Vol. 36, 1938j p. 138. 10. BLAKE, M.A. 1946 Some problems in securing an accurate measure of the cold  resistance of dormant buds of different varieties of peaches. Proc. Am. Soc. Hort Sc., Vol. 48, 1946, p. 89. 11. BRADFORD, E.C. 1922 Observations on winter injury. Miss. Agr. Exp. Stn., Res. Bull. 56, 1922. 12. BHIERLT, W.G. 1942 A note on an unusual case of cold injury to the Haralson  apple. Proc. Am. Soc. Hort Sc., Vol. 40, 1942, p. 236. 13. BRIERLT, W.G. 1947 The winter hardiness complex in deciduous woody plants. Proc. Am. Soc. Hort Sc., Vol. 50, 1947, p. 10. 14. BRIEHLT, W.G. and McCARTNET, J.S. 1950 A study of the cold resistance of European plums. Proc. Am. Soc. Hort. Sc., Vol. 55, 1950, p. 254. 15. BROOKS, F.A. 1947 Action of wind machines in frost protection. American Fruit Grower, 67, March, 1947, p. 15. - 99 -16. BROWN, D.S. 1943 A report of injury by cold weather to peach trees in  Illinois during the winter 1941-42. Proc. Am. Soc. Hort. Sc., Vol. 42, 1943, p. 298. 17. CAMPBELL, R.W. 1948 More than thirty peach varieties survived minus thirty-two  degrees F. Proc. Am. Soc. Hort Sc., Vol. 52, 1948, p. 117. 18. CHANDLER, W.H. 1913 The killing of plant tissue by low temperature. Mo. Agr. Exp. Stn., Res. Bull. No. 8, Dec., 1913. 19. CHANDLER, W.H. 1942 Deciduous Fruits. Lea and Felinger, Philadelphia, U.S.A., 1942, pp. 116-131. 20. CHANDLER, W.H. 1942 Deciduous Fruits. Lea and Felinger, Philadelphia, 1942, p. 147. 21. CHAPLIN, C.E. 1948 Some art i f i c i a l freezing tests of peach fruit buds. Proc. Am. Soc. Hort Sc., Vol. 52, 1948, p. 121. 22. CHUDERS, N.F. 1949 Fruit Science. Ed. R.W. Gregory, J.B. Lippincott Co., N.T., p.279. 23. CLIMATE OF BRITISH COLUMBIA. 1949 Tables of temperature, precipitation and sunshine. B.C. Dept. Agr. Report for 1949, Queen's Printer, Victoria, B.C. - 100 -24. CLIMATE OF BRITISH COLUMBIA. 1950 Tables of temperature, precipitation and sunshine. B.C. Dept. Agr. Report for 1950, Queen's Printer, Victoria, B.C. 25. COE, F.M. 1945 Cherry rootstocks. N.T. Agr. Exp. Stn., Bull. 319, May, 1945. 26. COMFORD, C.E. 1939 Researches into the cause and prevention of frost damage. Sc. Hort., Vol. 7, 1939. 27. COWART, F.F. and SAVAGE, E.F. 1941 Important factors affecting peach tree longevity in Georgia. Proc. Am. Soc. Hort. Sc., Vol. 39, 1941, p. 348. 28. CRAKE, H.L. 1930 Physiological investigations on the resistance of peach buds  to freezing temperatures. W. Va. Agr. Exp. Stn., Bull. 236, Aug., 1930. 29. CULLINAN, F.P. and WEINBERGER, J.H. 1934 Studies on the resistance of peach buds to injury at low  temperatures. Proc. Am. Soc. Hort Sc., Vol. 32, 1934, p. 244. 30. DAT, P.C. 1915 Frost data of the United States. U.S.D.A. Weather Bureau, Bull. No. 5, 1915, p. 4. 31. DICKSON, G.H. 1945 A study of the extent to which apple orchard cultivation may  be reduced. Sc. Agr. Vol. 17, No. 11, 1945, p. 670. - 101 -32. DORSET, M.J. 1934 Ice formation in the fruit bud of the peach. Proc. Am. Soc. Hort. Sc., Vol. 31, 1934, pp. 22-27. 33. EDGERTON, L.J. 1948 Peach fruit bud hardiness aa affected by blossom thinning  treatments. Proc. Am. Soc. Hort. Sc., Vol. 52, 1948, p. 112, 34. EDGERTON, L.J. and HARRIS, R.W. 1950 Effect of nitrogen and cultural treatment on Elberta Peach  fruit bud hardiness. Proc. Am. Soc. Hort. Sc., Vol. 55, 1950, p. 51. 35. EGGERT, R. 1944 Cambium temperatures of peach and apple trees in winter. Proc. Am. Soc. Hort. Sc., Vol. 45, 1944, pp. 33-36. 36. FENSKA, R.R. 1945 The winter care of trees. Horticultural Vol. 23, Dec. 1945, pp. 533-34. 37. FISHER, R.A. 1934 Statistical methods for research workers. Oliver and Boyd, Edinburgh, 5th ed. 1934. 38. FOOT, E.J. 1950 Effect of blossom thinning sprays on winter hardiness of apple trees. Unpublished correspondence. 1950. 39. FRASER, S. 1924 American Fruits. Orange Judd Pub. Co., New York, 1924. - 102 -40. GONZALES OBSERVATORY. 1954 Weather records. Personal correspondence. 41. GODRLEY, J.H. 1917 Some observations on the growth of apple trees. N.H. Agr. Exp. Stn. Tech. Bui. 12, 1917. 42. GOURLEY, J.H. and HOWLETT, F.S. 1941 Modern Fruit Production. The Macmillan Company, N.Y., 1941. 43. HARGRAVE, P.D. 1949 The physiological aspect of cold and winter killin g . Joint review on hardiness by the Fruit Committee of the A.C.S.H., 1949. 44. HASSLER, F., HANSEN, C.L., and FARRALL, A.W. 1947 Protection of crops from frost damage by use of radiant  energy. Quarterly Bui. Mich. State Coll., Vol. 30, Pt. 2, No. 1., Aug. 1947, p. 21. 45. HAVIS, L. and LEWIS, I.P. 1938 Winter injury of fruit trees in Ohio. Ohio Agr. Exp. Stn., Bui. 596, Nov. 1938. 46. HEDRICK, V.P. 1915 Cherries of New York. J.B. Lyon and Co., N.Y., 1915. 47. HEDRICK, V.P. 1917 Peaches of New York. J.B. Lyon and Co., N.Y., 1917. 48. HIGGINS, B.B., WALTON, G.P., and SKINNER, G.P. 1943 The effect of nitrogen fertilizers on cold injury of peach trees. Ga. Agr. Exp. Stn., Bui. 226, July, 1943. - 103 -49. KELLET, V.W. and McMUNN, R.L. 1942 November, 1940, cold damage to fruit plants in Illinois. Proc. Am. Soc. Hort. Sc., Vol. 40, 1942, p. 220. 50. KENNARD, W.C. 1949 Defoliation of Montmorency sour cherry trees in relation to  winter hardiness. Proc. Am. Soc. Hort Sc., Vol. 53, 1949, p. 129. 51. KNIGHT, A.T. 1942 The influence of porosity of some orchard soils upon root  behaviour. Proc. Am. Soc. Hort. Sc., Vol. 40, 1942, p. 23. 52. KNOWLTON, H.E., and DORSEY, M.J. 1927 A study of the hardiness of the fruit buds of the peach. N.Y. Agr. Exp. Stn., Bui. 21, Dec. 1927. 53. LEVITT, J. and SCARTH, G.W. 1936 Frost hardiness studies with living cells. Can. Jour. Res. Vol. 34, Sec. C, No. 8, Aug., 1936. 54. LEVITT, J. 1948 Frost killing and hardiness of plants. Burgess Pub. Co., Minneapolis, Minn., 1948. 55. MACODN, W.T. 1919 Winter injury in Canada. Proc. Am. Soc. Hort. Sc., Vol. 15, 1919, p. 13. 56. MANEY, T.J. 1942 Fruit tree injury resulting from the midwest blizzard of  November, 1940. Proc. Am. Soc. Hort. Sc., Vol. 40, 1942, p. 217. - 104 -57. MANN, A.J. 1940 Hardy stocks for apple trees. Summerland Exp. Stn. Hort. Circ. 99, 1940. 58. MANN, A.J. and KEANE, F.W.L. 1950-1951. Varietal descriptions. Summerland Exp. Stn.,Pomology Bui. Nos. 1-65, 1950-51. 59. MANN, A.J. and PALMER, R.C. 1942 Fruit tree understock problems in British Columbia. Summerland Exp. Stn. Hort. Circ. 210, 1942. 60. MEADER, E.M., DAVIDSON, O.W. and BLAKE, M.A. 1945 A method for determining the relative cold hardiness of dormant peach fruit buds. Jour. Agr. Res., Vol. 70, No. 9, May 1, 1945. 61. McMUNN, R.L. and DORSET, M.J. 1934 Seven years results of the hardiness of Elberta fruit buds in  a fertilizer experiment. Proc. Am. Soc. Hort. Sc., Vol. 32, 1934, p. 239. 62. MIX, A.J. 1916 Sunscald of fruit trees a type of winter injury. Cornell University Agr. Exp. Stn., Bui. 382, Oct. 1916. 63. OSKAMP, J. 1918 Winter injury of fruit trees. Purdue Univ. Agr. Exp. Stn. Circ. 87, Nov. lglS. - 105 -64. PALMER, R.C. 1932-36 Weather records. Dom. Exp. Stn. Summerland, B.C. Results of Experiments 1932-36, p. 5. 65. PALMER, R.C. 1944 Low temperature injuries. Dom. Exp. Stn. Bui., Summerland, B.C., Hort. Circ. 406, Jan. 1944. 66. PALMER, R.C. 1950 Sweet cherry rootstocks. Unpublished correspondence, 1950. 67. PATTERSON, D.D. 1939 Statistical technique in agricultural research. McGraw-Hill Book Co., New York and London, 1939. 68. PENTICTON HERALD. 1950 Brief documenting winter damage to valley orchards presented  to cabinet. Nov. 16, 1950. 69. PUTNAM, D.F., AND CHAPMAN, L.J. 1938 The climate of southern Ontario. Sc. Agr. Vol. 18, No. 8, Apr., 1938, p. 488. 70. ROBERTS, R.H. 1922 The development and winter injury of cherry blossom buds. Wis. Agr. Exp. Stn. Res. Bui. 52, July, 1922. 71. ROSE, W. 1950 Okanagan firewood worth $1,000 cord. Vancouver Sun, Nov. 20,1950. 72. SHANKLAND, R.S. 1947 Cold - what is it? American Fruit Grower, Vol. 67, March, 1947, p . l l . - 106 -73. SNEDECOR, G.W. 1937 Statistical Methods. Collegiate Press, Iowa, 1937. 74. STEINMETZ, F.H. and HILBORN, M.T. 1937. A histological evaluation of low temperature injury of apple  trees. Maine Agr. Exp. Stn. Bui. 386, 1937. 75. STEWART, N.W. 1937 Cold hardiness of some apple understocks and the reciprocal  influence of stock and scion on hardiness. Proc. Am. Soc. Hort. Sc., Vol. 35, 1937, p. 386. 76. SOTJDS, R.H. and MARSH, R.S. 1943 Winter injury to trunks of young bearing apple trees in West  Virginia following a f a l l application of nitrate of soda. Proc. Am. Soc. Hort. Sc., Vol. 42, 1943, p. 293. 77. SWINGLE, C.F. 1932 The exosmosis method of determining injury as applied to apple  rootstock hardiness studies. Proc. Am. Soc. Hort. Sc. Vol, 29, 1932, p. 380. 78. THOMAS, H.E. and McDANTELS, L.H. 1933 Freezing injury to the roots and crowns of apple trees. Cornell Univ. Bui. 556, 1933. 79. THOMAS, J.J. 1914 The American Fruit Cultorist. Orange Judd Co., New lork, 1914. 80. TINGLEY, M.A. 1936 Frost rings in hardy varieties. Proc. Am. Soc. Hort Sc., Vol. 34, 1936, p. 57. - 107 81. TINGLE Y, M.A., SMITH. W.W., PHILLIPS, T.G. and POTTER, G.F. 1938 Experimental production of winter injury to the trunks of apple trees by applying nitrogenous fertilizers in the autumn. Proc. Am. Soc. Hort. Sc., Vol. 36, 1938, p. 177. 82. TDKEY, H.B. and BRASE, K.D. 1935 Random notes on fruit tree rootstocks and plant propagation H. N.Y. State Agr. Exp. Stn. Bui. 657, October, 1935. 83. WXNKLEPLECE, R.L. and McCLINTOCK, J.A. 1939 The relative cold resistance of some species of Prunus used as stocks. Proc. Am. Soc. Hort. Sc., Vol. 37, 1939, p.324. 84. YOUNG, F.D. 1929 Frost and the prevention of frost damage. U.S.D.A. Farmers' Bui. 1588, April, 1929. - 108 -APPENDIX A TABLE 1 Record of Winter-Injured Trees  Totals - A l l Interior B.C. Orchards Reported (2,249 orchards) N u m b e r o f T r e e s Total Kind of Under Over A l l % Tree 2 n 2 n-5 n 5B-7" 7"-12n 12" Sizes Killed Apples 5,729 6,645 14,712 46,521 53,421 107,028 31.8 Peaches 9,859 24,285 39,108 30,774 7,287 111,313 33.0 Apricots 14,335 14,108 5,515 3,233 790 37,981 11.3 Cherries 2,312 2,618 1,850 4,678 6,988 18,446 5.5 Pears 9,940 6,348 3,215 2,524 693 22,720 6.8 Plums 301 1,002 1,626 1,723 497 5,149 1.5 Prunes 3,105 6,907 12,102 9,620 2,239 33,973 10.1 Totals 45,581 61,913 78,128 99,073 51,915 336,610 19.9 Total Trees in Orchard 1,684,037 Average Loss per Orchard 19.9/2 - 109 -APPENDIX A TABLE 2 Record of Winter-Injured Trees  Lytton - Chase  (91 orchards) Kind of Under Over Tree 2" 2 n-5 w 5u-7n 7"-12" 12" Apple 459 1,246 3,088 8,408 6,982 Peach 29 324 161 30 Apricot 7 122 86 14 11 Cherry 7 301 22 78 13 Pear 156 351 64 129 1 Plum 7 84 123 33 2 Prune 31 581 1,198 476 5 Total dead trees 24,629 Total trees in orchard 45,435 Average loss per orchard 54% - 110 -APPENDIX A TABLE 3 Record of Winter-Injured Trees  S a l m o n Arm - Sorrento (160 orchards) Kind of Under Over Tree 2" 2"-5w 5 "-7" 7"-12" 12" Apple 531 635 2,691 8,301 7,175 Peach 11 90 57 122 1 Apricot 25 32 124 2 -Cherry 175 85 254 332 364 Pear 79 236 509 858 218 Plum 9 71 129 119 76 Prune 227 183 564 425 246 Total dead trees Total trees in orchard Average loss per orchard 24,956 80,405 31# - I l l -APPENDIX A TABLE 4 Record of Winter-Injured Trees  Armstrong  (22 orchards) Kind of Under Over Tree 2" 2"-5n 5"-7" 7"-12" 12" Apple 49 69 84 539 382 Peach 7 20 19 5 3 Apricot 3 10 6 1 1 Cherry 80 38 28 72 45 Pear 38 36 143 100 17 Plum - 21 51 105 69 Prune 9 111 383 332 43 Total dead trees 2,919 Total trees in orchard 7,871 Average loss per orchard 71% - 112 -APPENDIX A TABLE 5 Record of Winter-Injured Trees  Vernon  (252 orchards) Kind of Under Over Tree 2" 2"-5" 5"-7" 7 "-12" 12" Apple 847 2,253 5,053 13,147 6,498 Peach 362 1,451 1,000 235 18 Apricot 252 1,870 124 94 7 Cherry 75 214 88 193 72 Pear 514 1,581 823 606 64 Plum 80 346 470 254 95 Prune 576 2,789 6,311 4,295 962 Total dead trees 53,619 Total trees in orchard 212,750 Average loss per orchard 25.2% - 113 -APPENDIX A TABLE 6 Record of Winter-Injured Trees Oyama, Winfield - Okanagan Centre  (175 orchards) Kind of Under Over Tree 2^ 2n-5" 5"-7n 7"-12" 12" Apple 281 261 541 2,848 2,783 Peach 199 957 1,308 1,1B5 281 Apricot 377 565 213 141 45 Cherry 146 162 107 416 765 Pear 310 247 144 77 135 Plum 5 11 45 88 36 Prune 195 76 227 641 326 Total dead trees 16,144 Total trees in orchard 117,832 Average loss per orchard. 13*1% - 114 -APPENDIX A TABLE 7 Record of Winter-Injured Trees  Kelowna  (395 orchards) Kind of Under Over Tree 2" 2"-5n 5a-7tt 7 n_l2 n 12" Apple 602 1,000 1,972 10,987 7,542 Peach 551 1,645 2,785 1,618 243 Apricot 803 1,590 916 373 82 Cherry 548 658 429 1,861 3,146 Pear 1,202 2,096 834 397 145 Plum 52 106 317 602 51 Prune 314 885 1,362 2,372 450 Total dead trees 50,536 Total trees i n orchard 354,977 Average loss per orchard 14.2% - 115 -APPENDIX A TABLE 8 Record of Winter-Injured Trees  Westbank  (78 orchards) Kind of Under Over Tree 2" 2w-5" 5"-7" 7"-12" 12" Apple 1 296 405 647 385 Peach 229 1,655 2,569 2,064 137 Apricot 126 683 200 101 20 Cherry 63 99 7 9 229 316 Pear 1,020 87 63 19 1 Plum 7 86 219 187 39 Prune 42 86 117 25 66 Total dead trees 12,368 Total trees in orchard 79,890 Average loss per orchard 15»5% - 116 -APPENDIX A TABLE 9 Record of Winter-Injured Trees  Peachland (68 orchards) Kind of Under Over Tree 2" 2"-5w 5 "-7" 7"-12" 12" Apple 22 6 65 46 Peach 217 892 3,262 2,016 966 Apricot 72 59 42 56 19 Cherry 60 15 12 94 289 Pear 63 1 11 10 13 Plum - 17 8 15 18 Prune 1 4 43 7 2 Total dead trees 8,423 Total trees in orchard 45,885 Average loss per orchard 13,3% - 117 -APPENDIX A TABLE 10 Record of Winter-Injured Trees  Summerland  (243 orchards) Kind of Under Over Tree 2" 2"-5" 5"-7" 7"-12" 12° Apple 107 86 45 234 246 Peach 133 848 2,411 4,281 758 Apricot 550 915 480 454 89 Cherry 37 119 38 182 822 Pear 223 30 65 12 21 Plum 1 15 66 131 50 Prune 114 59 51 61 5 Total dead trees 13,739 Total trees in orchard 150,726 Average loss per orchard 9»1# - 118 -APPENDIX A TABLE 11 Record of Winter-Injured Trees  Penticton  (139 orchards) Kind of Under Over Tree 2" 2W-5W 5n-7a 7"-12" 12" Apple 3 3 4 19 49 Peach 307 590 697 1,925 731 Apricot 203 339 93 271 38 Cherry 13 15 2 72 324 Pear 41 18 17 10 1 Plum - 2 5 37 13 Prune 10 23 4 15 21 Total dead trees 5,915 Total trees in orchard 89,007 Average loss per orchard 6,7% - 119 -APPENDIX A TABLE 12 Record of Winter-Injured Trees  Naramata  (60 orchards) Kind of Under Over Tree 2 W 2"-5" 5"-7" 7"-12" 12" Apple 32 2 11 38 44 Peach 87 302 397 359 354 Apricot 356 60 56 75 70 Cherry 2 12 11 35 257 Pear 30 2 - Q Plum - 8 - 26 4 Prune 6 5 12 6 Total dead trees 2,665 Total trees in orchard 36,190 Average loss per orchard 7.3% - 120 -APPENDIX A TABLE 13 Record of Winter-Injured Trees  Kaleden  (32 orchards) Kind of Under Over Tree 2" 2n-5n 5W-7W 7"-.12n 12" Apple 6 9 1 28 166 Peach 78 35 494 530 97 Apricot 1,105 227 50 81 37 Cherry 34 10 2 10 25 Pear 88 3 2 Plum 7 - - 1 1 Prune 29 22 47 8 2 Total dead trees 3,235 Total trees in orchard 27,231 Average loss per orchard 11.9$ - 1 2 1 -APPENDIX A TABLE 1 4 Record of Winter-Injured Trees  Keremeos - Cawston  ( 1 0 8 orchards) Kind of Under Over Tree 2 N 2 " - 5 " - 5 " - 7 " Y " - 1 2 " 1 2 " Apple 1 , 4 7 0 1 0 2 1 4 7 3 8 7 4 6 7 Peach 2 , 7 9 2 2 , 2 1 0 1 , 8 1 9 9 0 9 6 2 8 Apricot 2 , 5 9 1 1 , 6 0 3 2 4 7 8 5 3 8 Cherry 4 4 3 3 4 2 0 7 6 7 1 Pear 2 , 0 9 4 1 3 1 5 0 3 2 3 8 Plum 6 3 7 1 1 2 1 0 Prune 1 6 4 2 1 7 1 5 5 9 1 7 Total dead trees 1 9 , 1 1 2 Total trees in orchard 8 1 , 4 6 5 Average loss per orchard 2 3 * 5 $ - 122 -APPENDIX A TABLE 15 Record of Winter-Injured Trees  Oliver - Osoyoos  (598 orchards) Kind of Under Over Tree 2" 2 w-5 n 5 g-7 n 7 "-12" 12° Apple 1,250 410 111 222 116 Peach 4,845 13,254 22,123 15,495 3,070 Apricot 7,863 6,030 2,874 1,483 332 Cherry 629 855 743 1,032 437 Pear 4,060 1,500 473 253 33 Plum 102 108 170 109 28 Prune 1,377 1,760 999 619 81 Total dead trees 94,826 Total trees in orchard 339,434 Average loss per orchard 27«9?« - 123 -APPENDIX A TABLE 16 Record, of Winter-Injured Trees  Grand Forks  (28 orchards) Kind of Under Over Tree 2" 2"-5" 5"-7" 7"-12" 12" Apple 69 273 553 651 540 Peach 12 12 6 Apricot 2 3 4 2 1 Cherry - 1 15 16 42 Pear 22 29 17 15 6 Plum 25 90 . 22 4 5 Prune 10 106 629 329 13 Total dead trees 3524 Total trees i n orchard 14939 Average loss per orchard 23.6% - 124 -APPENDIX B TABLE 1 Effect of Soil Type Per Cent Damaged Trees Sand & Sandy Silt Loam District Kind Gravel Loam & Clay Total Cherry 26 11 10 47 Oliver Apricot 28 16 9 53 Peach 43 27 28 98 Total 97 54 47 198 Cherry 42 32 30 104 Osoyoos Apricot 49 35 28 112 Peach 66 48 65 179 Total 157 115 123 395 Cherry 15 8 2 25 Okanagan Apricot 18 8 6 32 Falls Peach 41 22 22 85 Total 74 38 30 142 Cherry 8 2 1 11 Penticton Apricot 12 3 4 19 Peach 14 4 3 21 Total 34 9 8 51 Grand Totals 362 216 208 786 - 125 -APPENDIX B TABLE 2 Relative Cold Injury of Seven Varieties of Apricot Variety Per Cent Damaged Trees Total Wenatchee 31 39 29 99 Moorpark Perfection 22 26 19 67 Riland 16 27 16 59 Tilton 20 17 24 61 Blenheim 18 14 20 52 Reliable 26 36 30 92 Kaleden U 2 17 33 Total 147 161 155 463 - 126 -APPENDIX B TABLE 3 Relative Gold Injury of Six Varieties of Cherry Variety Per Cent Damaged Trees Total Bing 24 19 14 57 Lambert 9 12 2 23 Royal Ann 95 60 100 255 Deacon 12 20 9 41 Camival 22 57 46 125 Windsor 12 14 20 46 Total 174 182 191 547 - 127 -APPENDIX B TABLE 4 Relative Cold Injury of Eight Varieties of Peach Variety Per Cent Damaged Trees Total Veteran 36 50 42 128 Vedetta 38 42 37 117 Valiant 24 33 28 85 J. H. Hale 78 78 85 241 Elberta 46 39 48 133 Golden Jubilee 60 72 66 198 Rochester 50 79 58 187 Red Haven 25 20 16 61 Total 357 413 380 1,150 - 128 -TABLE 5 APPENDIX B Effect of Late Irrigation Irrigation Practice Kind Per Cent Damaged Trees Total Late Irrigation (After Oct.15) Peach Cherry Apricot 32 38 17 19 9 23 30 20 100 45 52 Total 58 80 59 197 Peach 59 68 43 170 No Late Irrigation Cherry 24 32 32 88 Apricot 22 38 46 106 Total 105 138 121 364 Grand Total 163 218 180 561 - 129 -TABLE 6 APPENDIX B Effect of Age of Tree Age Kind Per Cent Damaged Trees Total Peach 33 35 2 9 9 7 Under 1 0 Years Apricot 2 0 18 18 56 Cherry 1 2 1 4 35 Total 62 65 61 188 Peach 48 39 92 179 Over 1 0 Years Apricot 3 2 18 2 9 7 9 Cherry 24 28 3 7 89 Total 1 0 4 85 158 347 Grand Total 166 150 219 535 - 130 -APPENDIX B TABLE 7 Effect of Pruning Technique Type of Pruning Kind Per Cent Damaged Trees Total Peach 26 32 2 9 87 Long Apricot 16 2 1 18 55 Total 42 53 47 U2 Peach 46 40 76 162 Short Apricot 24- 31 36 91 Total 70 71 112 253 Grand Total 112 124 159 395 TABLE 8 Good 10"-18" Poor <8" - 131 -APPENDIX B Effect of Vigour Vigour K i n d Per Cent Damaged Trees Peach 28 24 16 Apricot 19 14 15 Cherry 8 15 22 Total 55 53 53 Peach 41 42 50 Apricot 31 37 26 Cherry 27 18 30 Total 99 97 106 Grand Total 154 150 159 132 -TABLE 9 APPENDIX B Effect of Cultivation Culture Kind Per Cent Damaged Trees Total Peach 32 37 28 97 Permanent Cover Apricot 22 19 22 63 Cherry 36 UO a 117 Total 90 96 91 277 Cultivated (Either Clean or Turned Under in the Pall) Peach Apricot Cherry 29 U0 ZU 26 30 21 99 71 39 36 LA 119 Total 92 102 95 289 Grand Total 182 198 186 566 

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