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Western gall rust (Endocronartium harknessii (Moore) Hirat.) on lodgepole pine (Pinus contorta dougl.) in British Columbia - a study of variation and inheritance of resistance in a natural pathosystem Kojwang, Harrison Ochieng


Clones, open-pollinated, and full-sib families of lodgepole pine were inoculated with various spore collections of western gall rust to assess and describe variation in early symptom development, host resistance, and rust virulence and to determine the mode of inheritance of resistance. In addition, studies of the cytology of immature, mature and germinating aeciospores and one-dimensional SDS-PAGE (with silver stain) of total spore protein were undertaken. The frequency of some early symptoms varied significantly between open-pollinated families under some inoculation conditions, but was not related to the susceptibility of these families. In addition, the proportion of symptomatic seedlings that became galled was only slightly greater than that of asymptomatic seedlings. Early symptoms were not reliable indicators of successful infection. The frequency of uninucleate cells (58%) did not vary between the youngest and the oldest cells in immature spore chains. In mature spores, 57.5, 41.0 and 1.5 percent were uni-, bi- and trinucleate respectively. The number of nuclei in spores and germtubes increased gradually following germination up to an average of 5.6 (range of 2-9) at 34 hours. At no stage during the development and germination of aeciospores was there evidence of karyogamy in the form of a reduction in the number of nuclei per spore. Karyogamy and meiosis do not occur at spore germination in the coastal rust population sampled. Silver stained SDS-PAGE gels showed some variation among single gall spore sources. The approach has potential as a technique for distinguishing among spore sources. Sixteen grafted clones inoculated with four single-gall spore sources showed a significant interaction between clone and spore source. There were also large differences in relative susceptibility among pine clones and smaller differences among spore sources with respect to the average infection levels of pine clones. The infection levels of clones was considered to provide a better measure of the genetically determined resistance of parent trees than the degree of infection of those trees in the field. Forty open-pollinated pine families inoculated with coastal and interior spore collections showed significant spore-family interactions attributable to six pine families that showed equal susceptibility to both spore sources. The coastal spore source caused much higher infection than the interior source on the other families. Estimates of narrow sense heritability h² were as follows; h²[sub Indiv] = 0.21 ± 0.10, h²[sub Family] = 0.51 ± 0.16. Regressions of the infection levels of offspring on those of their female parents were not significant. Hence selection of superior individuals requires progeny testing. A 4 by 4 diallel showed significant GCA effects and barely detectable SCA effects. The SCA component was about one third of the GCA component, indicating that inheritance of resistance is largely additive. Reciprocal and maternal effects were not significant. Stability in the pathosystem was attributed to the wide variation in host resistance and some degree of differential interactions between pine and spore genotypes. The highly variable host populations interact with much less variable pathogen populations; the latter possibly caused by the lack of sexual reproduction. As a result, the rate of selection for greater virulence may be matched by the rate of selection for resistance in spite of the much shorter life cycle of the pathogen.

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