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UBC Theses and Dissertations

A. -The selection of male-sterile lines in alfalfa. B. -The witches' broom virus disease of alfalfa in British Columbia Pettem, Frederick Douglas


In the hope of finding male-sterile alfalfa strains for use in the production of hybrid alfalfa, a microscopic study of the pollen produced by approximately 1000 lines of alfalfa grown at the University of British-Columbia was conducted. This study revealed 4 lines that consistently showed an absence of viable pollen under different environments. 2 of the male-sterile lines produced no seed on selfing and 2 produced very small quantities of seed. In plants grown from open-pollinated seed of the 4 male-sterile lines, the F₁ segregation for male-sterile to male-fertile were as follows: 1:7.9, 1:7.3, 1:6.8, and 1:8.3, for an incomplete count. These ratios suggest that the male-sterility is controlled by cytoplasmic factors in addition to recessive genes. However, the mode of inheritance will only be factorially interpreted by F₂ and backcross data. It appears that the sterility is caused by a breakdown in meiosis, as the pollen sacs are full of an amorphous material suggestive of arrested development of the pollen grains. The male-sterility should eventually prove of great economic value in the production of hybrid alfalfa seed. Male-sterility has not previously been isolated in alfalfa, although it is common in the plant kingdom. Over the past 10 years the Witches' Broom virus disease of alfalfa has developed into serious proportions in the Interior of B.C. - this was first pointed out by quadrat results obtained in 1942-44. The disease is known to be distributed throughout the low rainfall areas of the province where it is widespread although sporadic. The disease is shown to be the same disease that occurs in Washington and in Australia. Witches' Broom of Alfalfa causes severe dwarfing of the affected plants and a decimation of alfalfa stands. Drastic shortening of internodes and reduction in size of leaves is accompanied by proliferation of crowns and nodes. Up to 3000 thin spindly shoots are commonly produced by a single diseased crown. Typically, leaves are marginally chlorotic; inflorescences are reduced to 3-4 florets per raceme; and, very little, if any, seed is produced. Crowns and roots are symptomless until late stages of the disease are reached, when they show severe rotting. Affected plants gradually die in a period of 3 months to 2 years. However, 2 plants were shown to apparently recover from the disease when brought into the U.B.C. greenhouse from the Nicola Valley. A histological comparison of the healthy and diseased plants showed gummosis of the xylem vessels, a breakdown and degeneration of the chloropasts of the affected leaves, and a mechanical breakdown of the palisade layer in the outer edge of diseased leaves. Storage of starch in the crowns of plants was found to be depleted according to the stage of the disease, with no storage starch present in severely diseased crowns. However, sucrose was found to be present in storage in diseased crowns but not in healthy crowns. The disease was found to be easily transmitted by crown grafts. Out of 142 attempted grafts, union of scion and stock was achieved in 31 cases with positive transmission in 27 cases. Seed transmission and inoculations by expressed crown juice have given negative results to date. Quadrats were plotted in 1950 in the interior of B.C. to further study the disease. Twinning experiments were set up to study natural resistance of the members of the genus Medicago. Results from both of these studies will not be ready for publication for at least 2 more years. A nursery plot was established at U.B.C. and a replicate plot at Kamloops, B.C. One year after planting the Kamloops plot, several of the alfalfa plants were found producing symptoms of the disease, and to be badly diseased in 15 months time. None of the plants at U.B.C. showed any signs of the disease. From a potted plant yield trial conducted at U.B.C. the diseased plants were shown to have a statistically significant reduction in yields as compared to healthy plants.

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