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Studies on the host ranges of some facultative parasites Sivak, Bela 1964

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STUDIES ON THE HOST RANGES OF SOME FACULTATIVE PARASITES BY BELA SIVAK B.S.F., University of British Columbia, 1959 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF •THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in the Department of Biology and Botany •University of British Columbia ¥e accept this thesis as conforming to the "required standard THE UNIVERSITY OF BRITISH COLUMBIA •April, 1964 In presenting this thesis i n p a r t i a l fulfilment of the requirements for an advanced degree at the University of B r i t i s h Columbia, I agree that the Library shall make i t freely., available for reference and study* I further agree that per-mission for extensive copying of this thesis f o r scholarly purposes may be granted by the Head of my Department or by his representatives. I t i s understood that.copying or publi-cation of this thesis for f i n a n c i a l gain sh a l l not be allowed without my written permission-. Department of The University of B r i t i s h Columbia, Vancouver 8, Canada '4, Date i ABSTRACT Inoculation experiments were carried out to determine the relation between bark moisture level of certain host species and their suscept-i b i l i t y to facultative parasites. In these experiments, cuttings of 1-to 3-year old host material and the mycelial mat of the pathogens contained i n an agar cylinder were used. In the f i r s t instance, fungi that were known or found i n association with bark lesions were considered: these were Cryptodiaporthe s a l i c e l l a (Fr.) Petrak on Salix scouleriana Barratt (Scouler willow), Dactylosporium sp. and Fusarium sp. on Acer macrophyllum Pursh. (broadleaf maple, Liber-ie 11a sp. on Cornus stolonifera (Mchx.) var. occidentalis (T» and G.) C. L. Hitchc. (western dogwood), Melanconis sp. on Alnus rubra Bong, (red alder). The results demonstrated that fungi normally associated with lesions of l i v i n g host material proved to be pathogenic when the relative turgid-i t y of the host bark was lowered from the f i e l d level of above 80 per cent to the range of 69 to 77 per cent. Secondly, an attempt was made to determine i f correlation existed between bark moisture level and canker development by fungi not known, and not found to occur i n association with lesions of some hosts. The following fungi and hosts were considered: C. s a l i c e l l a on red alder (Alnus rubra Bong), trembling aspen (Populus tremuloides Michx.), bitter cherry (Prunus emarginata Dougl.), black cottonwood (Populus trichocarpa Torrey and Gray),^western dogwood (Cornus stolonifera var. occidentalis), and on broadleaf maple (Acer macrophyllum Pursh.); Fusarium sp. on red i i alder, bitter cherry, western dogwood, and on Scouler willow; Liber- t e l l a sp. on red alder, bitter cherry, broadleaf maple, and on Scouler willow; Melanconis sp. on bitter cherry, western dogwood, broadleaf maple and on Scouler willow. It was shown that a l l of these parasites extended their host ranges, to varying extent when the bark moisture level was reduced to levels within the range of 69 to 77 per cent, or i n some instances to the range of 41 to 67 per cent of saturation. Cuttings with as low bark moisture levels as 41 per cent appeared to be viable as indicated by the pro-duction of roots and (or) shoots. i i i ACMOWLEDGMENTS The writer expresses his appreciation to Dr. J. E. Bier, Professor of Forest Pathology, Department of Biology and Botany, for his guidance and help throughout this investigation. He would also like to acknowledge the assistance of Dr. R. J. Bandoni with various phases of this study. Thanks are extended to Dr. T. M. C. Taylor, Head of the Depart-ment, for his personal help and for the f a c i l i t i e s made available i n the Department of Biology and Botany. Dr. W. J. Bloomberg, Dr. A. Funk, Mr. J. W. Roff, Forest Pathol-ogists i n the Canada Department of Forestry, Miss G. D. Pentland, Research Associate, Department of Biology and Botany, Dr. G. E. Rouse, Dr. W. B. Schofield and Mr. J. M. Powell are kindly thanked for their helpful suggestions and advice i n writing this thesis. V CONTENTS INTRODUCTION 1 LITERATURE REVIEW 3 OBJECTIVES 5 METHODS 6 1) Seasonal moisture studies of tree bark 6 2) Isolation of fungi 7 3) Inoculation experiments 9. 4) Cultural studies 10 5) Cultural studies on sterilized sections of shoots 11 RESULTS 11 1) Seasonal changes i n the water content of the bark tissues.. 11 2) Canker development by C. s a l i c e l l a on inoculated cuttings of several tree species i n relation to the water content of the bark tissues 14 3) Cankers arising from latent infections of red alder, western dogwood and broadleaf maple 16 4) Relation of the bark moisture content to cankering susceptibility of red alder, western dogwood and broadleaf maple by facultative parasites that are known to occur on these hosts 17 5) New hosts for Melanconis sp., Liberte11a sp., and Fusarium sp 17 6) The growth of fungi at different temperatures on malt agar 21 7) Host preferences of the fungi 21 DISCUSSION 26 SUMMARY 30 BIBLIOGRAPHY 32 APPENDIX v i TABLES Table I. Monthly changes i n the relative turgidity of 1- to • 3-year-old bark of various tree species 13 Table I I . Relation of the relative turgidity per cent of bark i n i n i t i a l infections i n cuttings of various tree species inoculated with C. s a l i c e l l a 15 Table III. Canker development i n relation to the moisture content of the host bark 18 Table IV. Summary of inoculations and bark moisture studies on host material at room temperature (18-23°C) 20 Table V. The effect of temperature on the growth of fungi on malt agar over a period of eight days 22 Table VI. Growth of fungi on autoclaved sections of shoots related to their a b i l i t y to infect l i v i n g cuttings 24 Table VII. The extension of Cryptodiaporthe canker on cuttings of i t s nev; hosts and on those of i t s f i e l d host 25 v i i FIGURES Figure 1. The method of preparing wound inoculation with agar plug inoculum. Figure 2. The method of incubation. High humidity provided for inoculated cuttings i n jars by plastic bag cover. Figure 3. Method of achieving quick recovery of cankered cuttings. Plastic cover used to maintain high humidity around cuttings with continuous water supply. Figure 4. Cuttings from 1-year-old Scouler willow shoots inoculated with C. s a l i c e l l a . Column on the l e f t indicates dates at which relative turgidity of bark was determined for each cutting. a) water withheld from cuttings un t i l cankers formed. b) water continuously supplied to cuttings. Note absence of cankers. Figure 5. Cuttings from 1-year-old trembling aspen shoots inoculated with C. s a l i c e l l a . Column on the l e f t indicates dates at which relative turgidity of bark was determined for each cutting. a) water withheld from cuttings u n t i l cankers formed. b) water continuously supplied to cuttings. Note absence of cankers. Figure 6. Cuttings from 1-year-old black cottonwood shoots inoculated with C. s a l i c e l l a . Column on the l e f t indicates dates at which relative turgidity of bark was determined for each cutting. Water was withheld from cuttings u n t i l cankers formed. Figure 7. Cuttings from 3-year-old red alder branches inoculated with C. s a l i c e l l a . Column on the l e f t indicates dates at which relative turgidity of bark was determined for each cutting. Figure 8. Cuttings from 3-year-old bitter cherry branches cankered with C. s a l i c e l l a . Column on the l e f t indicates dates at which relative turgidity of bark was determined for each cutting. a) water withheld from cuttings u n t i l canker formed. b) water supplied to cuttings continuously. Note absence of cankers. Figure 9. Cuttings from 1-year-old broadleaf maple shoots cankered with C. s a l i c e l l a . Column on the l e f t indicates the dates at which viii (Figure 9) relative turgidity of bark was determined for each cutting. a) ^ater withheld from cuttings u n t i l cankers formed. b) water continuously supplied to cuttings. Note absence of cankers. Figure 10 Cuttings inoculated with and cankered by fungi. Water was to 20. withheld from cuttings u n t i l canker growth. Column on the l e f t indicates dates of inoculation, cankering and cessation of canker growth. Columns on the right show the relative turgidity level of the bark at each of these dates. Figure 10. Cuttings from 3-year-old red alder branches cankered by Melanconis sp. Figure 11. Cuttings from 3-year-old western dogwood branches cankered by Libertella sp. Figure 12. Cuttings from 1-year-old broadleaf maple shoots cankered by Dactylosporium sp. Figure 13. Cuttings from 1-year-old broadleaf maple shoots cankered by Fusarium sp. Figure 14. Cuttings from 1-year-old broadleaf maple shoots cankered by Melanconis sp. Figure 15. Cuttings from 1-year-old bitter cherry and broadleaf maple shoots cankered by Melanconis sp. Figure 16• Cuttings from 3-year-old western dogwood branches cankered by Melanconis sp. Figure 17. Cuttings from 1-year-old red alder shoots cankered by Fusarium sp. Figure 18. Cuttings from 3-year-old western dogwood branches cankered by Fusarium sp. Figure 19. Cuttings from 1-year-old bitter cherry shoots cankered by Fusarium sp. Figure 20. Cuttings from 1-year-old bitter cherry shoots cankered by Libertella sp. INTRODUCTION Two distinct approaches have been made i n studies on the epidemiology of canker diseases caused by facultative parasites. In the f i r s t , the incidence of the disease has been related to climatic and s o i l factors. Since these factors vary considerably i n different l o c a l i t i e s , there has been no basis for comparing the results of different studies of the same disease. Also, because of the interrelation of many climatic and s o i l factors, i t has been d i f f i c u l t to determine the relative significance of any one factor i n predisposing the trees to diseases. In the second approach, the emphasis has been placed on host factors as influenced by changes i n climate and s o i l . These investigators (Bier, 1959 a, b; Bloomberg, i960) were of the opinion that although there may be a large number of environmental factors involved, their combined action was evident through the determination of an index which w i l l express the degree of host vigor. From the results of these investigations i t appeared that the f i e l d moisture content of l i v i n g tissues expressed as percentage of the amount at saturation (relative turgidity) may be a useful index for the degree of host vigor. In the present investigation the second approach was adopted. Special reference was made to the relative turgidity l e v e l of one- to three-year-old bark tissues as a possible index of the degree of host vigor i n various species of hardwoods, and their vulnerability to attack by some facultative parasites, Under f i e l d conditions i n Vancouver, Cryptodiaporthe s a l i c e l l a (Fr.) Petrak occurred only on Salix sp., Dactylosporium sp. on broadleaf maple -2-(Acer macrophyllum Pursh.), Libertella sp. on western dogwood (Cornus  stolonifera (Mitchx.) var. occidentalis (T. & G.) C. L. Hitchock), Melanconis sp. on red alder (Alnus rubra Bong.), Under laboratory conditions, Fusarium sp. occurred only on broadleaf maple shoots. It seemed pertinent, therefore, to determine; 1) whether any correlation existed between bark moisture content and canker-ing by these organisms, and 2) whether the host range of each fungus could be enlarged by changing the moisture content of the l i v i n g bark tissues of a species not normally attacked. -3-LITERATURE REVIEW In forest pathology, facultative parasites are often considered to be saprophytic organisms with parasitic tendencies. As parasites, the organisms occur on plants adversely affected by f i r e , insects and unfavourable environ-mental conditions (Brooks, 1928; Boyce, 1933; Day, 1948; Mooi, 1948; Gaumann, 1950; Waterman, 1955). In some instances disease development occurred during the dormant season when the physiological activities of the host were lessened, while conditions remained favourable for the development of the pathogens (Brooks, 1928; Boyce, 1948). Undoubtedly there are many factors that are related to the establish-ment of the parasitic relationship. The water content of the host tissues, however, has been considered as one of the most important relationships.. As early as 1909, Munch indicated that Nectria cinnabarina Fr. was pathogenic on elm when the moisture content of bark tissues was at a low leve l . Accord-ing to Gaumann (1950), the increased water content may protect the tissues from facultative parasites by displacing the ai r necessary for the fungi and by increasing the v i t a l energy of the c e l l s . More recently, Butin (1955) found that a low water content of poplar cuttings favoured their colonization by Cytospora chrysosperma (Pers.) Fr. because of a decreased rate i n the production of secondary periderm. Gibbs (1957) suggested that short, mild periods i n the winter might thaw out the younger parts of trees while leaving the roots, s o i l and stem s t i l l frozen. This, he suggested, would result i n marked water losses from twigs, and the development of physiological stress. Weatherley (1949) expressed the water content of the leaves of cotton plants as a percentage of that at maximum turgor. This percentage reflected -4-the combined effects of the amount of s o i l moisture and level of atmospheric humidity on the plants.^ Using the same method for expressing the water content of sapwood of Douglas f i r and Sitka spruce, Chalk and Bigg (1956). found that the index, relative turgidity percent, varied i n trees on d i f -ferent sites, being higher i n trees on good sites. In a series of papers, Bier (1959 a,b,c,; I960 a,b) demonstrated that the relative turgidity level of young bark tissues was related to the degree of host vigor. It was demonstrated that low turgor levels i n the bark tissues, brought about by climatic changes, root competition and suppression, resulted i n the susceptibility of the trees to canker diseases caused by facultative parasites. According to Kramer and Kozlowski (i960), physiological processes and structure of a plant are closely related. Bloomberg (i960) argued that the lower relative turgidity of the bark of Populus trichocarpa Torrey and Gray than that of P. 'robusta' i n dormancy was related to anatomical character-i s t i c s that favoured slow uptake and poor retention of water i n the bark of this species. Bier (I96I) gave further evidence for the correlation of the relative turgidity of the host bark to canker resistance. He found that Populus  tremuloides Michx., P. 'robusta' and P. trichocarpa shoots i n the f i e l d with bark moisture content values higher than 80 per cent did not become infected with C. saliceLla. However, susceptibility occurred when the turgor level of the bark was lowered to levels below 80 per cent. Further, i t has been demonstrated (Bier, I96I b) that the bark tissues of P. tremuloides, P. trichocarpa, Salix sp. and Acer macrophyllum Pursh. were attacked by Hypoxylon pruinatum (Klotzsche) Cke. at higher bark moisture levels than by -5-Septoria musiva Pk., indicating that Hypoxylon was the more virulent pathogen on these hosts. Since these organisms did not occur on these hosts i n the Vancouver area, i t appeared that the host ranges of some facultative parasites might be extended i f the relative turgidity per cent of the bark was reduced to levels below those normally occurring i n the f i e l d . In addition to the moisture hypothesis, the importance of the nutrition of the facultative parasite on i t s parasitism of the host has also been emphasized (Brown, 1936; L i l l y and Barnett, 1951). These authors suggested that the chemical composition of the substrata may influence the enzyme systems of the fungi. Vasudeva (1930) demonstrated that a failure of Botrytis a l l i i Munn. to parasitize apple may have been due to the low amount of nitrogenous substances i n the host tissues. The addition of nitrogenous substances to spores stimulated the attack. Dufrenoy (1930) placed emphasis on the hysto-chemical reactions of the host to the parasite, indicating that the host tissues produced tannin bar-riers preventing the advance of the parasite. Bloomberg and Farris (I963) found a close correlation between the moisture content of the tissues and the manner and amount of deposition of tanniferous substances i n the poplar bark tissues. This was considered to be a defence reaction on the part of the host against disease attack. OBJECTIVES In pathogenicity tests on the host ranges of C. s a l i c e l l a , Fusarium sp., Liberte11a sp., and Melanconis sp. the following hypotheses were considered: 1) The host ranges of some facultative parasites may be extended to include tree species normally not cankered by them, when the degree -6-. of host vigor as indicated by the relative turgidity of young bark i s lowered to a value below a c r i t i c a l l e v e l , 2) The food material i n the host for the development of the pathogen may be of importance i n determining the establishment of the para-s i t i c relationship between the fungus and the host. The objectives of this investigation were as follows: 1) To determine the water regime of the bark tissues of various host species both i n dormancy and i n growth. 2) To relate the water content of the dormant bark tissues of various host species to their susceptibility to cankering by C. s a l i c e l l a . 3) To relate the moisture content of the bark tissues of various tree species with attack by canker organisms i n the f i e l d . 4) To relate the moisture content of the bark tissues of tree species to attack by fungi not known and not found i n association with their bark cankers, 5) To undertake (comparative) cultural studies on the temperature requirements of test fungi, and on their possible nutritional preferences as tested on autoclaved sections of shoots. METHODS 1) Seasonal moisture studies of tree bark. One-year-old coppice shoots of red alder (Alnus rubra Bong.), bitter cherry (Prunus emarginata Dougl.), broadleaf maple (Acer macrophyllum Pursh.), trembling aspen (Populus tremuloides Michx.), black cottonwood (Populus  trichocarpa Torrey and Gray), Scouler willow (Salix scouleriana Barratt) and three-year-old western dogwood trees (Cornus stolonifera (Mitchx.) var. occidentalis (T. & G.) C. L. Hitchock^ were collected on the University of British Columbia Endowment Lands, during the winter of 1959 and the spring of i960. A l l material collected had shown vigorous growth and was apparently free from disease. The excised shoots were wrapped i n poly-ethylene bags and harvested into cuttings—ten for each s p e c i e s — i n the laboratory. The cuttings were of uniform size, measuring 25 cm i n length with a mid-diameter of 1,0 cm. The relative turgidity of the bark tissues of each cutting was then determined. Because the bark did not separate easily from the wood along the cambium, the bark samples contained small amounts of secondary xylem. The fresh weight of the samples was determined to the nearest milligram. The samples were then saturated by allowing them to float on the surface of d i s t i l l e d water i n closed Petri dishes. After a 24-hour period samples were removed from the dishes, the excess water blotted off, and the weights determined. The samples were then ovenr-dried for 24 hours at 79 to 90°C and weighed again. From these three weighings the relative turgidity was determined as follows: Relative turgidity <*) = (Green weight - Dry weight) x 100 (Saturated weight - Dry weight) Bark moisture values were also expressed on a water/dry weight basis. 2) Isolation of Fungi, The source of C, s a l i c e l l a was a stock culture from the Forest Pathology Laboratory of the Department. Pure cultures of Dactylosporium sp., Fusarium sp, and Melanconis sp. were obtained by making isolations from diseased areas associated with fruiting bodies on the shoots. Cultures from single spores were also obtained for comparison and to identify the causal agency. The following techniques were used: -8-a) Tissue transplant: the surface contamination of the host material i n the v i c i n i t y of diseased areas was eliminated by swabbing with 70 per cent ethyl alcohol. Small pieces of diseased bark tissue were removed from the advancing margin of lesions beneath the epidermis using a ste r i l i z e d scalpel and transferred to malt agar plates. Pure cultures were obtained by making hyphal t i p isolations from the colonies developing from the pieces of diseased bark tissues. b) Single spore isolation: The spores of Dactylosporium sp,, Fusarium sp. and Liberte11a sp. were large enough for isolating single spores with a pointed needle using a dissecting microscope. Spore suspensions of Melanconis sp. were poured on malt agar. After the germ tubes had become visible under the dissecting microscope but before the spores lost their identity, they were removed and transferred singly to fresh plates. The imperfect fructifications produced on malt agar by colonies from tissue transplants and from single spores were identical for each fungus. Further, the fruiting bodies formed on the a r t i f i c i a l medium were identical to those occurring i n nature on the plants for each fungus. The fungus isolated from the diseased shoots of western dogwood was identified as Libertella sp. by Dr. A. Funk, Mycologist, Canada Department of Forestry, Victoria Laboratory, B. C. This was the f i r s t record of the occurrence of this fungus on western dogwood i n British Columbia. The pathogens on broadleaf maple shoots were identified as Dactylosporium sp. and Fusarium sp. and that on red alder shoots was identified as Melanconis sp. by the writer. -9-3) Inoculation experiments. During the dormant season of 1959-60 and 1960-61, cuttings were collected from red alder, bitter cherry, western dogwood, broadleaf maple, trembling aspen, black cottonwood and Scouler willow trees. These measured from 25.0 cm to 35.0 cm i n length with mid-diameters ranging from 0.5 to 2.0 cm. The red alder and bitter cherry cuttings were collected from three-and four-year-old lower branches of fifteen to twenty-year-old trees. The broadleaf maple cuttings were collected from minor and major shoots. The minor shoots were from four to six feet t a l l , one-year-old and pale green in colour. The major shoots were more than ten feet t a l l and were three-year-old with dark green colour. Cuttings of each species were harvested from the same root system i n order to avoid genetic differences i n the sample. The percentages for relative turgidity and water per/dry weight were then determined for each cutting. The bark of the cuttings was surface-sterilized with 70 per cent ethyl alcohol prior to making the inoculation wounds. One inoculation and one control wound were made on each cutting. The following types of wounds were employed: a) Burns were made using the t i p of a hot iron rod three mm. i n diameter. b) Disks of bark were removed with a heat-sterilized cork borer. c) The outer bark was incised with a heat-sterilized spear-headed needle. An agar-plug inoculum, consisting of an agar cylinder with the fungus ' cut out from the advancing margins of colonies was placed on the wound and covered with Scotch tape (Figure 1). The control wound was covered with Scotch tape, but without inoculum. -10-Ten of the twenty cuttings inoculated were kept i n a jar and under a polyethylene cover to maintain a moist atmosphere. No water was supplied to these cuttings. The remaining cuttings were identically treated except that the basal ends of the cuttings were provided with a continuous supply of d i s t i l l e d water (Figure 2). A l l jars were stored outdoors for ten days i n cold frames with polyethylene covers. After this period, the polyethylene bags were loosened on the jars. The water was changed weekly and fresh cuts were made on the basal ends of the cuttings at two-week intervals i n an effort to f a c i l i t a t e water movement into the cuttings. Bark moisture measurements were taken as soon as there was visible evidence of canker development. Bark moisture measurements were also made on the cuttings provided with a continuous supply of water. Following this the diseased cuttings were provided with a continuous supply of d i s t i l l e d water and a l l cuttings were placed i n a moist chamber i n the laboratory (Figure 3). Under these favourable moisture and temperature conditions, the cuttings broke dormancy and produced roots, shoots and secondary periderm i n the bark tissues at the margins of cankers. Cankered cuttings that fa i l e d to produce any of these signs of l i f e were discarded. The bark moisture content was determined again after the canker development had ceased and the best organisms were reisolated into pure culture from the diseased areas. 4) Cultural studies. The malt agar used i n these experiments was composed of 28 gm of Difco Malt Extract and 16 gm Bacto agar per l i t e r of d i s t i l l e d water. After the freshly poured plates had hardened, each was inoculated with agar-plug inoculum (4.0 mm i n diameter) of the test fungi. -11-After inoculation the plates were wrapped i n polyethylene bags to prevent rapid desiccation of the medium. Five plates of each organism were kept i n constant temperature units ranging from 5°C to 35°C at five degree intervals. At two-<iay intervals, the average diameters of the colonies were determined by measuring the colonies along two diameters at right angles to one another and the average rate of growth was calculated for each organism. 5) Cultural studies on sterilized sections of shoots. Fresh cuttings were collected from red alder, bitter cherry, western dogwood, broadleaf maple and Scouler willow and cut into sections 6 to 8 cm long. Each of these sections was then s p l i t into halves. A strip of the epidermis or of the periderm was removed along the length of each section f . c to expose the inner bark. The sections were supported by glass rods i n Petri plates containing 15 cc of d i s t i l l e d water. The plates were then autoclaved for seven minutes under 15 lbs. pressure. After s t e r i l i z a t i o n the sections were inoculated with agar-plug inoculum. Eight replicates were made for each fungus on each plant species. After inoculation the plates were wrapped i n polyethylene bags and stored at 20°C for a period of eight days. At the end of this period the extent of fungous growth was determined by measuring the colonies along the length of the section. The average growth was then calculated. RESULTS 1) Seasonal changes i n the water Content of the bark tissues. The seasonal variations i n the relative turgidity of bark of trembling aspen, black cottonwood, Scouler willow, red alder, bitter cherry, western -12-dogwood and broadleaf maple are presented i n Table I, During the dormant season of 1959 and the spring of i960, the relative turgidity of the bark tissues of a l l species tested except Scouler willow was above 80 per cent. In Scouler willow the relative turgidity dropped below 80 per cent i n December. Both trembling aspen and black cottonwood had higher relative turgidity values with less variation than that found i n Scouler willow. Red alder, western dogwood, and broadleaf maple had relative turgidity values of 85 to 90 per cent. Relative turgidity values sl i g h t l y above 80 per cent were obtained for the bark of bitter cherry. The relative turgidity of the bark of a l l trees studied was i n excess of 90 per cent during the growing season. On each collection date, observations were also made for the presence or absence of disease i n shoots other than those sampled. During the sampling period, infection was rarely found i n the f i e l d . Occasionally, i n broadleaf maple and Scouler willow cankers were found that were confined to the upper parts of the shoots. On red alder, cankers occurred on l i v i n g branches that were damaged. On western dogwood, small latent cankers were found i n the v i c i n i t y of lenticels. No infection was found on trembling aspen, bitter cherry and black cottonwood. TABLE I Monthly changes i n the relative turgidity of 1- to 3-yr-old bark of various species 1959 I960 Dec. 10 Jan. 5 Feb. 8 Mar. 14 June 8 July 11 Aug. 8 P 8 tremuloides 84 83 84 95 95 94 P. trichocarpa 84 83 81 82 96 — 95 scouleriana 79 85 80 84 95 94 93 A. macrophyllum 85 84 87 85 96 97 95 rubra 88 89 86 — 95 97 • 96 C. stolonifera var. occidentalis 86 90 91 — 96 95 97 emarginata 83 82 80 82 92 93 93 2) Canker development by C. s a l i c e l l a on inoculated cuttings of several  tree species i n relation to the water content of the bark tissues. From Table II i t would appear that Scouler willow, trembling aspen and black cottonwood were most susceptible to C. s a l i c e l l a as canker attack occurred at the highest bark moisture level among the species trees tested (Figures 4-6). Table II also shows that the bark of red alder, bi t t e r cherry, western dogwood and broadleaf maple became infected by C. s a l i c e l l a when the relative turgidity of the bark tissues ranged from 47 to 67 per cent (Figures 7-9). The susceptibility of red alder, and bitter cherry cuttings appeared to depend on the age of the host bark i n addition to the bark moisture content. This was indicated by the larger number of successful inoculations obtained on cuttings of three-year-old branches than i n those from one-year-old shoots. Also, the bark moisture level at which canker attack occurred was higher i n cuttings from three-year-old branches than i n those from one-year-old shoots. The data obtained from i n i t i a l infections of viable cuttings (Table II) shows three different bark moisture ranges at which canker attack occurred on the various tree species. These moisture ranges might indicate different degrees of host vigor thus different rates of susceptibility to the fungus. These moisture ranges were as follows: 1) In the bark moisture range of 70 to 76 per cent, trembling aspen, black cottonwood and Scouler willow cuttings were cankered. 2) In the bark moisture range of 52 to 67 per cent, cuttings of one-year-old broadleaf maple and those from three-year-old red alder, bitter cherry, and western dogwood were infected. 3) In the bark moisture range of 41 to 47 per cent, red alder and bitter cherry cuttings of one-year-old shoots were infected. (This low bark TABLE II Relation of the relative turgidity per cent of bark to i n i t i a l infection in cuttings of various tree species inoculated with C. s a l i c e l l a . -Tree species and age of Tot. No. of cuttings No. of cuttings Incubation period before Average r e l . turgidity % of host bark cutting wood Inoc. Cank. * • !> o o <9 i n i t i a l l y infected i n i t i a l infections days at the time of harvest at the time of i n i t . cankering S. scoulerana 1 20 18 18 4 23 80 76 P. tremuloides 1 20 19 17 5 20 87 77 P. trichocarpa 1 20 18 12 4 25 81 70 A. macrophyllum 1 40 25 15 4 - 16 84 67 A. rubra 1 40 12 4 3 40 89 41 3 40 25 15 4 22 83 65 C. stolonifera var. occidentalis 3 40 16 8 4 38 93 52 P. emarginata 1 40 10 3 3 33 85 47 3 40 20 13 5 15 82 62 # Recovered cuttings were those that remained viable after infection. -16-moisture level appeared to approach the lowest values that may be encountered i n l i v i n g cuttings.) The number of successful inoculations was found to be highest i n the f i r s t bark moisture range (70-76), i t was smaller i n the second one (52-67) and reached the minimum number i n the third one (i+1-47)-In general, cankers obtained on red alder, bitter cherry and western dogwood and broadleaf maple were of small size, only occasionally longer than 2.5 cm. They also expanded at a slower rate. On the other hand, cankers on cuttings of trembling aspen, black cottonwood and Scouler willow developed faster and extended to the total length of the cuttings i f moisture was not supplied to them. 3) Cankers arising from latent infections on red alder, western dogwood and  broadleaf maple. Rapidly expanding cankers developed on cuttings that were harvested and l e f t to dry gradually on the ground i n the v i c i n i t y of parent trees. These cankers originated at lenticels, buds and healed or partly healed wounds. On the cuttings of red alder, broadleaf maple and western dogwood, inoc-ulated with C. s a l i c e l l a , cankers of latent infections of Melanconis sp., Fusarium sp. and Libertella sp. developed before or concurrently with Cryptodiaporthe canker occasionally overgrowing the lesions caused by C. s a l i c e l l a . This phenomenon appeared to indicate higher susceptibility of host to the usually occurring pathogens. Reisolation experiments con-firmed that Melanconis sp. was consistently associated with lesions on red alder, Libertella sp. with lesions of western dogwood, Dactylosporium sp. with lesions of broadleaf maple and C. s a l i c e l l a was consistently -17-associated with lesions of Scouler willow. 4) Relation of the bark moisture content to canker susceptibility of red  alder, western dogwood, and broadleaf maple by facultative parasites that  are known to occur on these hosts. The f i e l d susceptibility of red alder, western dogwood and broadleaf maple to attack by some fungi offered; an opportunity to investigate a possible correlation between the relative turgidity per cent of the bark tissues and susceptibility to canker attack. Fungi were isolated into pure culture from the lesions on each tree species. They were used i n inocula-tion experiments on malt agar inoculum plugs. The results are given i n Table III and illustrated i n Figures 10, 11, 12, and 13. Canker development caused by fungi normally associated with bark .cankers of these tree species was more extensive and occurred at higher bark moisture levels than that associated with C. s a l i c e l l a which was not known to occur i n the bark of these tree species. 5) New hosts for Melanconis sp., Fusarium sp. and Libertella sp. The relationship between the moisture content of the bark tissues and canker susceptibility was further tested using the following test pathogens and hosts: Melanconis sp. on cuttings of bitter cherry, western dogwood, broadleaf maple and Scouler willow; Fusarium sp. on cuttings of red alder, bit t e r cherry, western dogwood and Scouler willow; Libertella sp. on cut-tings of red alder, bitter cherry, broadleaf maple and Scouler willow. Inoculation experiments were carried out with the same type of host material i n the manner outlined by Bier (1959) and used earlier i n this investigation. Malt agar plug inoculum consisting of an agar cylinder TABLE III Canker development i n relation to the moisture content of the host bark Tree species Inoculum Tot. No. of Incubation No. of Ave. relative turgidity and age of inoculations period before cuttings (%) of host bark  cutting wood successful i n i t . infection i n i t i a l l y at time of at time of inoculations* days infected harvest i n i t . infection. A. macro phyllum 1-yr-old A. rubra Dactylosporium sp. Fusarium sp. Melanconis sp. 3-yr-old C. stolonifera var. occidentalis Libertella sp. 3-yr-old 12/11 12/11 12/10 12/9 8 6 15 10 5 6 5 85 85 82 90 72 77 77 69 * Successful inoculations consisted of cuttings that remained viable after infection -19-and mycelial mat of the fungus was placed on the bark wounds and covered with Scotch tape. Cuttings were then incubated and allowed to dry gradually under polyethylene covers at room temperature. Water was withheld from the cut-tings u n t i l i n i t i a l canker development. The results of this inoculation experiment are shown i n Table IV and illustrated i n Figures 14, 15, 16, 17, 18, 19 and 20. It was found that although Melanconis sp. was associated with cankers only on red alder i n the f i e l d , i t readily cankered cuttings of a l l tree species tested i f the bark moisture content was reduced to levels between 54 and 75 per cent from the f i e l d values of above 80 per cent. Fusarium sp. also caused cankers on cuttings of tree species other than broadleaf maple from which i t was isolated. It was noted (Table IV) that bitter cherry and western dogwood were cankered when the moisture content of the bark was lowered to the range of 63 to 66 per cent. Although the pathogen produced small cankers (0.5 cm. long) on red alder cuttings when the relative turgidity per cent of the bark tissues was lowered to 77 per cent, the extension of these cankers was delayed u n t i l further reduction of bark moisture content took place (Figure 17). Libertella sp., normally pathogenic on western dogwood, produced cankers only on bitter cherry as a new host. On both hosts, however, canker develop-ment occurred only i f the bark moisture content was lowered to a certain level (Table IV and Figures 18 and 19). Red alder, broadleaf maple and Scouler willow cuttings were not infected by this fungus, although hyphae and conidia-bearing sporophores were present i n the inoculation wounds. TABLE IV Summary of inoculations and bark moisture studies on host material at room temperature (18-23°C) Fungi Tree Sp. Age of cutting wood -yrs. Tot. No, of inoc; No. of successful inoculations Incubation No. of Ave. relative turgidity period cuttings (%) of bark  days i n i t i a l l y at time of at time of infected harvest i n i t i a l infection Melanconis sp. Acer  Cornus  Prunus Salix 1 3 1 1 12/10 12/10 12/12 12/12 10 10 10 7 5 6 6 6 85 85 83 82 70 71 64 75 Fusarium sp. Alnus  Cornus  Prunus Salix 1 3 1 1 12/10 12/10 12/12 12/00 10 12 7 6 6 6 85 86 81 80 77 63 66 Libertella sp. Acer 1 Alnus 1 Prunus 1 Salix 1 12/00 12/00 12/12 12/00 15 85 85 83 80 60 -21-6) The growth of the fungi at different temperatures on malt agar. The effect of temperature on the growth of the fungi was determined on malt agar at temperatures ranging from 5°C to 35°C with 5 degree intervals. The average growth of the fungi over a period of 8 days i s shown i n Table V. Maximum growth of Fusarium sp. occurred between 20 and 25°C. At 30°C growth was s t i l l relatively good indicating the small retarding effect of high temperature on the mycelial growth of this fungus. At lower tempera-tures the size of the colonies was abruptly reduced. Libertella sp. produced the largest growth at 25°C whereas at 5°C and at 30°C the growth was very small. The maximum growth of C. s a l i c e l l a occurred at 20°C. The amount of mycelia produced at 25°C was very small, however growth at 10°C and 15°C was greater than at 25°C. Melanconis sp. had a wide temperature range for good growth, growth being relatively good at both 5°C (0.8 cm.) and at 30°C (0.9 cm.). The greatest growth was produced between 15°C and 20°G. 7) Host preferences of the fungi. The study of possible nutritional preferences of C. s a l i c e l l a . Fusarium sp., Libertella sp. and Melanconis sp. was carried out by inoculations of li v i n g cuttings from tree species other than their f i e l d host. In addition, sections of the same shoots from which cuttings were obtained were autoclaved and then inoculated to see the extent of mycelial growth when the resistance of the host attributed to l i v i n g tissues was excluded. It was realized that, besides excluding the functional resistance of hosts, changes i n their chemical constitution could have taken place as a result of the st e r i l i z a t i o n process. The average lengths of the mycelial TABLE V The effect of temperature on the growth of fungi on Malt agar over a period of eight days at: 5°C 10°C 15°C 20°C 25°C 30°C 35°C Width of mycelial mat cm. Cryptodiaporthe s a l i c e l l a 0.05 2.4 4.3 5.6 0.3 0.05 0.0 Fusarium sp. 0.05 2.5 4.3 9.0 9.0 4.2 0.0 Libertella sp. 0.05 0.65 1.2 1.6 1.8 0.2 0.0 Melanconis sp. 0.8 2.1 3.8 4.6 1.8 0.9 0.0 -23-mats produced by the fungi on the autoclaved bark of each host was calcu-lated at the end of an 8-day period. The extent of this mycelial growth then was compared with growth of fungi i n l i v i n g cuttings of each host producing or f a i l i n g to produce cankers. Fusarium sp., isolated from cuttings of broadleaf maple, produced the largest colonies on the autoclaved bark of this host (Table VI). Living cuttings of red alder, bitter cherry and western dogwood were readily can-kered. Although the mycelial growth of the fungus was evident i n the inoculation wounds of Scouler willow, canker development i n this species did not ensue. Libertella sp. produced the largest colonies on the autoclaved bark of i t s f i e l d host, western dogwood. On autoclaved bark of other hosts, the lengths of the mycelial mats were from 30-40$ smaller. Cankering occurred only on bitter cherry although mycelia and conidia bearing sporophores were present i n the inoculation wounds of red alder, broadleaf maple and Scouler willow. C. salicella» occurring as a pathogen on Scouler willow i n the Vancouver area, developed the largest colonies on the autoclaved bark of this host. At the same time i t produced small mycelial mats on the autoclaved bark of red alder, bitter cherry, western dogwood and broadleaf maple. Canker development took place i n l i v i n g cuttings of a l l species tested. The cankers produced on the new and on i t s f i e l d hosts, however, differed i n their growth rate. Although they grew nearly to the same length 15 days after inoculation on both the new hosts and the f i e l d host, two weeks later Cryptodiaporthe canker on i t s f i e l d host was about twice as long as on i t s new hosts (Table VII). -24-TABLE VI Growth of fungi on autoclaved sections of shoots related to their a b i l i t y to infect l i v i n g cuttings. Fungi Tree Age of Ave. length Cankering of sp. wood of 8 livin g cuttings yrs. colonies (+) successful (-) unsuccessful C. s a l i c e l l a Acer 1 0.7 + Alnus 3 0.3 + Cornus 3 0.3 + Prunus 3 0.8 + * Salix 1 2.5 + Fusarium sp. Acer 1 5.0 + Alnus 3 3.0 + Cornus 3 4.0 + Prunus 3 4.3 + Salix 1 3.0 -Libertella sp. Acer 1 2.0 Alnus 3 2.0 -* Cornus 3 3.2 + Prunus 3 2.1 + Salix 1 2.5 -Melanconis sp. Acer 1 2.5 + * Alnus 3 3.6 + Cornus 3 2.0 + Prunus 3 2.3 + Salix 1 2.5 + * f i e l d host for the pathogen. -25-TABLE VII The extension of Cryptodiaporthe canker on cuttings of and on those of i t s f i e l d host. i t s new hosts Tree species Age of wood No. of cuttings Ave. canker growth (cm) Rel.turgidity % yrs. 15 30 15 30 days after inoc. days after inoc. A. rubra * 3 6 1.1 2.0 64 42 c. V stolonifera . occidentalis. * 3 4 1.0 1.8 53 40 P. emarginata * 3 5 1.0 2.3 63 41 s. scouleriana 1 8 1.5 4.6 70 44 * new hosts for C. s a l i c e l l a -» f i e l d host for C. s a l i c e l l a DISCUSSION In determining the host range of certain facultative parasites the emphasis was placed on: 1) the degree of host vigour as measured by the relative turgidity per cent of the bark tissues, 2) the nutritional status of autoclaved bark tissues. For measurements of the seasonal moisture pattern of bark tissues 10 cuttings were collected monthly for each tree species. Because of the large number of cuttings required to be collected at a time i t was not possible to sample the same clone for each tree species throughout this study. It was thought, however, that the uniform size of cuttings, the healthy appearance of shoots and their similar bark texture would warrant comparison of moisture readings. In order to avoid the effect on the moisture reading the actual moisture content of the functional bark tissues was expressed as percentage of that required for their saturation. The evidence points to the likelihood that latent infections of li v i n g host material by facultative parasites occur i n the f i e l d on red alder, western dogwood, broadleaf maple and Scouler willow. The f i e l d temperatures over long periods during the dormant season, approximated the optimal temperature for the growth of these fungi i n culture. In the spring, how-ever, although temperature conditions were optimal, canker development did not occur. During this period the shoots had moisture contents of 80 to 90 per cent bark moisture content i f they were dormant shoots and higher than 90 per cent i f the shoots were actively growing. A r t i f i c i a l inoculations of cuttings revealed that cankering by C. s a l i c e l l a on Scouler willow, Melanconis sp. on red alder, Libertella sp. on -27-western dogwood and Dactylosporium sp. on broadleaf maple consistently occurred when the f i e l d values i n relative turgidity were reduced to between 69 and 77 per cent. Although these pathogens were observed to occur only on their f i e l d hosts i n the Vancouver area, further inoculation studies with C. s a l i c e l l a , Fusarium sp., Libertella and Melanconis sp. on new hosts demonstrated that the host ranges of these parasites were extended when the moisture level of the bark tissues was lowered to values below the 80 per cent lev e l . Infection of new hosts by Fusarium sp., Libertella sp. and Melanconis sp. occurred i n the moisture range of 60 to 77 per cent. C. s a l i c e l l a developed cankers on red alder, bitter cherry, western dogwood and broad-leaf maple when the bark moisture ranged between 41 and 68 per cent. The 41$ value appeared to approach the lowest value that could be encountered i n l i v i n g cuttings. The fluctuation i n bark moisture under f i e l d condition never approached this low value during the course of this study. The lowest value under f i e l d condition was 79$ obtained once i n Scouler willow. These results showed that the relative turgidity of the bark tissues may serve as an expression of the degree of host vigour and suggested that a high moisture content—above 80 per cent—of the bark of these tree species was a factor preventing host range expansion under f i e l d conditions. The practical use of relative turgidity of the bark as an index of host vigour i s that i t might indicate periods of increased susceptibility to facultative parasites. This would be of value i n nurseries and plantations. For example, according to the experimental results, Scouler willow becomes susceptible to both C. s a l i c e l l a and Melanconis sp. before the -28-other tree species tested, Scouler willow bark showed a tendency to decrease rapidly i n water content to a level where the tree would become susceptible to the pathogens. Under the same conditions with a higher bark moisture content i t would be resistant to both of these pathogens. Also, i t appears that C. s a l i c e l l a on red alder, bi t t e r cherry, western dogwood and broadleaf maple i s a minor threat because the very low bark moisture level required for predisposing the host rarely occurs under f i e l d condi-tions. Cankering of these trees by their usually occurring pathogens takes place long before the relative turgidity of the bark i s low enough to favour C. s a l i c e l l a . It was seen that Cryptodiaporthe canker on red alder, bitter cherry, western dogwood and broadleaf maple were small i n size and grew slowly. The sterilized sections of these hosts supported poor growth of C, s a l i c e l l a whereas their usually occurring parasites (Melanconis sp, on red alder, Libertella sp. on western dogwood, Fusarium sp. on broadleaf maple) developed profusely on the st e r i l i z e d host material. These factors indicated that the chemical constituents of the bark tissues of these trees proved to be r e l a -t i v e l y poor as nutrition for the growth of C. s a l i c e l l a . Although Fusarium sp. grew rapidly on sterilized willow bark, l i v i n g cuttings were not infected. In the inoculation wounds, however, the mycelium of the fungus was clearly v i s i b l e . Similarly, Libertella sp, grew abundantly on the autoclaved bark of red alder, broadleaf maple and Scouler willow canker development did not follow, although mycelia and sporophores of the fungus were present i n the inoculation wounds. It appeared that the l i v i n g tissues did not provide sufficient amounts of food material for the growth of the pathogens thus cankering did not take place. It i s also possible that the saprophyte fl o r a of the l i v i n g bark (Bier and Rowat, 1962a) could have suppressed the development of the a r t i f i c i a l l y introduced pathogens. Of the two factors underlying the parasitism of facultative parasites, bark moisture content of the host i s therefore considered to be most import-ant. The chemical constituents of the bark tissues as food for the fungi are considered as a secondary factor. Obviously, because of the probable changes i n the nutritional status of the bark tissues as a result of the s t e r i l i z a t i o n , the significance of this latter factor could not be f u l l y evaluated. Recently, discoveries of Bier and Rowat (I962 a, b) have indicated that the saprophytic f l o r a of the bark tissues has a definite role i n disease resistance of trees. This aspect i n general has been overlooked. Further, objections have been raised against the use of agar plug inocula (Bier I963). In view of these latest suggestions the role of different factors i n para-sitism of trees by facultative parasites w i l l have to be more intensively studied before the f u l l implications of bark moisture content and other factors w i l l be realized. -30-SUMMARY The occurrence of facultative parasites on different hosts was investigated to determine whether any correlation existed between bark moisture content and canker susceptibility. Cuttings of red alder, western dogwood, broadleaf maple and Scouler willow were inoculated with fungi that normally occurred on them in association with bark cankers. Each cutting with bark moisture content higher than 80 per cent of saturation was resistant to the pathogen normally encountered on that species. When bark moisture content of the cuttings was lowered within the range of 69 to 78 per cent, infection occurred i n the cuttings. Each of the pathogens studied could infect cuttings of hosts other than those on which they occurred under f i e l d conditions provided moisture contents were reduced below the level required for infection by their f i e l d hosts. Measurements of the f i e l d level i n the bark tissues of these trees indicated that i t remained over 80 per cent during the dormant season and in the succeeding spring and summer. On the basis of the inoculation studies i t was suggested that the high bark moisture content of bark of trees under f i e l d conditions could have played a major part i n preventing host range expansion i n the f i e l d . The cultivation of these parasites on the autoclaved bark tissues of a l l tree species indicated that they produced larger colonies on the s t e r i -lized bark of their f i e l d hosts than on similarly treated bark tissues of new hosts. The growth of the pathogen in l i v i n g tissues was examined only i n the case of C. s a l i c e l l a . The results showed that even i f favourable -31-moisture conditions were provided canker development on the f i e l d host reached a larger size i n the same length of time than on the cuttings of new hosts. From these results i t was suggested that, aside from the bark moisture content, the chemical constitution of the bark—as food for the pathogen—may play a role i n the establishment of cankers. / -32-BIBLIOGRAPHY 1) Anastasiou, C. J. 1952, The biology of Melanconis thelebola (Fr.) Sacc. and i t s associated canker on Alnus oregana Nutt. Essay for Bot. 517, Department of Biology and Botany, U. B. C., Vancouver. 2) Bastin, E. S. 1895. The structure of cherry barks. The American Journal of Pharmacy, 67: 435-477. 3) Bier, J. E. 1959a. The relation of bark moisture to the development of canker diseases caused by facultative parasites. I. Cryptodiaporthe canker on willow. Can. J. Botany, 37: 229-238. 4) Bier, J. E. 1959b. The relation of bark moisture to the development of canker diseases caused by native facultative parasites. II, Fusarium canker on black cottonwood. Can. J. Botany, 37: 781-788. 5) Bier, J. E. 1959c. The relation of bark moisture to the development of canker diseases caused by native facultative parasites. III, Cephalosporium canker on western hemlock. Can. J. Botany, 37: 1140-1142. 6) Bier, J. E. 196la, The relation of bark moisture to the development of canker diseases caused by facultative parasites. IV. Pathogen-i c i t y studies of Cryptodiaporthe s a l i c e l l a (Fr.) Petrak and Fusarium  lateritum Nees. on Populus trichocarpa Torrey and Gray, P. 'robusta 1, P. tremuloides Michx. and Salix sp. Can. J. Botany, 39: 139-144. 7) Bier, J. E. 196lb. The relation of bark moisture to the development of canker diseases caused by native facultative parasites, VI. Patho-genicity studies of Hypoxylon pruinatum (Klotzsch) Cke. and Septoria  musciva Pk. on species of Acer, Populus, and Salix. Can. J. Botany, 39: 1555-1561. 8) Bier, J. E. and M. H. Rowat, 1962a. The relation of bark moisture to the development of canker diseases caused by native facultative para-sites. VII. Some effects of the saprophytes of the bark of poplar and willow on the incidence of Hypoxylon canker. Can. J. Botany, 40: 61-69. 9) Bier, J. E. and M. H. Rowat, 1962b. The relation of bark moisture to the development of canker diseases caused by native facultative para-sites. VIII. Ascospore infection of Hypoxylon pruinatum (Klotzsch) Cke. Can. J. Botany, 40: 89-901. 10) Bier, J. E. I963. Unpublished. 11) Bloomberg, ¥. J. i960. A c r i t i c a l study of the cankering of certain poplars by Cytospora chrysosperma (Pers.) Fr. with special reference to the water relations of the host. Ph.D. Thesis i n the Department of Biology and Botany and Faculty of Forestry, U. B. C. i960. -33-12) Bloomberg, W, J, and S. H. Farris, 1963. Cytospora canker of poplars bark wounding i n relation to canker development. Can. J. Botany, 41: 303-312. 13) Boyce, J. S. 1933. A canker of Douglas f i r associated with Phomopsis lokoyae. Jour. Forestry, 31: 664-672. 14) Boyce, J. S. 1948. Forest Pathology. McGraw-Hill Book Co., New York, Toronto, London. 15) Brooks, F. T. 1928. Disease resistance i n plants. The New Phytologist, 27: No. 2, May 30. 16) Brown, M. 1936. The physiology of host-parasite relations. Bot. Rev. 2: 236-281. 17) Butin, H. 1955. Uber den Einfluss des Wassergehaltes der Papell auf ihre Resistenz gegenuber Cytospora chrysosperma (Pers.) Fr. Phyto. Path. Zeeitschrift, 24: 245-265 (English translation by Miss G. Bury). 18) Chalk, L. and J. M. Bigg, 1956. The distribution of moisture i n the li v i n g stem of sitka spruce and Douglas f i r . Forestry, 29: 5-21. 19) Day, W. R. 1931. The relationship between frost damage and larch canker. Forestry, 5: 41-56. 20) Dufrenoy, J. 1936. Cellular immunity. Am. J. Botany, 23: 70-79. 21) Esau, K. 1953. Plant Anatomy. John Wiley and Sons, Inc. New York, London. 22) Gaumann, E. 1950. Principles of plant infection. Hafner Publ. Co, New York. 23) Gibbs, R. D. 1957. Patterns i n the seasonal water content of trees. In the Physiology of Forest Trees, Ed, by K. V. Thimann. 24) Kramer, P. J. and T. T. Kozlowski, i960. Physiology of Trees. McGraw-H i l l Book Co., Inc., New York, Toronto, London. 25) Kramer, P. J. and T. T. Kozlowski, i960. Physiology of Trees. McGraw-H i l l Book Co., Inc., New York. Toronto, London. 26) L i l l y , V. G. and Vf, L. Barnett, 1951. Physiology of fungi. McGraw-Hill Book Co., Inc.., New York, Toronto, London. 27) Mooi, J. C. 1948. Kanker en takinsterving van de wilg veroorzaakt door Nectria galligena en Cryptodiaporthe salicina. Thesis, Gitgeverij and Drukkerij, Baarn, Hollandia. -34-28) Munch, E. 1909. Untersuchungen uber Immunitat and Krankheitsanfallig-heit der Holzpflanzen. Naturw. Z. Forst. i i . Landvfirtsch. 7: 54-75, 87-114, 129-160. 29) Vasudeva, R. S. 1930. Studies i n the Physiology of parasitism. XI. An analysis of the factors underlying parasitism with special reference to the fungi Botrytis a l l i i and Monilia fructigena Ann. Botany. 44: 469-493. 30) Waterman, A. 1955. The relation of Valsa kunzei to cankers on conifers. Phytopathology, 45: 686-692. 31) Vfeatherley, P. E, 1949. Studies i n the water relations of the cotton plant. I. The f i e l d measurements of water deficits i n leaves. New Phytologist, 49: 81-97. Figure 1. The method of preparing wound inoculation with agar plug inocula. i i Figure 1 Figure 2. The method of incubation. High humidity provided for inoculated cuttings i n jars covered by plastic bags. i v Figure 2 Figure 3 -Method of achieving quick recovery of cankered cuttings. Plastic cover used to maintain high humidity around cuttings with continuous water supply. v i Figure 3 Figure 4. Cuttings of 1-year-old Scouler willow shoots inoculated with C. s a l i c e l l a . Column on the l e f t indicates dates at which relative turgidity of bark was determined for each cutting. a) water withheld from cuttings u n t i l canker formed. b) water continuously supplied to cuttings. Note absence of cankers. Figure 5. Cuttings of 1-year-old trembling aspen shoots inoculated with C. s a l i c e l l a . Column on the l e f t indicates dates at which relative turgidity of bark was determined for each cutting. a) water withheld from cuttings u n t i l cankers formed. b) water continuously supplied to cuttings. Note absence of cankers. v i i i Figure 5a. Figure 5b. Figure 6. Cuttings of 1-year-old black cottonwood shoots inoculated with C. s a l i c e l l a . Column on the l e f t indicates dates at which relative turgidity of bark was determined for each cutting. Water was withheld from cutting until canker formed. Figure 7. Cuttings of 3-year-old red alder branches inoculated with C. s a l i c e l l a . Column on the l e f t indicates dates at which relative turgidity of bark was determined for each cutting. a) water withheld from cuttings u n t i l canker formed, b) water supplied to cuttings continuously. Note absence of cankers. X Figure 7a. Figure 7b. Figure 8. Cuttings of 3-year-old bitter cherry branches cankered with C, s a l i c e l l a . Column on the l e f t indicates dates at which relative turgidity of bark was determined for each cutting, a) water withheld from cuttings u n t i l canker formed. b) water supplied to cuttings continuously. Note absence of cankers. Figure 9. Cuttings of 1-year-old broadleaf maple shoots cankered with C. s a l i c e l l a . Column on the l e f t indicates the dates at which relative turgidity of bark was determined for each cutting. a) water withheld from cuttings u n t i l canker formed. b) water continuously supplied to cuttings. Note absence of cankers. X I 1 PRUNUS CRYPTO. 152 62 85 87 86 26 2 63 60 50 18 3 89 88 87 Figure 8a. Figure 8b. ACER CRYPTO, m 10:1 84 82 83 251 77 64 66 i 33 83 91 84 ACER CRYPTO 101 01 84 85 £ 25J 85 88 t Figure 9a. Figure 9b. x i i i Figure 10 Cuttings inoculated with and cankered by fungi. Water was to 20. withheld from cuttings u n t i l canker growth. Column on the l e f t indicates dates of inoculation, cankering and cessation of canker growth. Columns on the right show the relative turgidity level of the bark at each of these dates. Figure 10. Cuttings from 3-year-old red alder branches cankered by Melanconis sp. Figure 11. Cuttings from 3-year-old western dogwood branches cankered by Libertella sp. Figure 12. Cuttings from 1-year-old broadleaf maple shoots cankered by Dactylosporium sp. Figure 13. Cuttings from 1-year-old broadleaf maple shoots cankered by Fusarium sp. Figure 14. Cuttings from 1-year-old broadleaf maple shoots cankered by Melanconis sp. Figure 15. Cuttings from 1-year-old bitter cherry and broadleaf maple shoots cankered by Melanconis sp. Figure 16. Cuttings from 3-year-old western dogwood branches cankered by Melanconis sp. Figure 17. Cuttings from 1-year-old red alder shoots cankered by Fusarium sp. Figure 18. Cuttings from 3-year-old western dogwood branches cankered by Fusarium sp. Figure 19. Cuttings from 1-year-old bitter cherry shoots cankered by Fusarium sp. Figure 20. Cuttings from 1-year-old bitter cherry shoots cankered by Libertella sp. xiv Figure 10. Figure 12. Figure 13. X V Figure 16 Figure 17 xvi Figure 20. 

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