A technique for screening seedlings of Amelanchier alnifolia Nutt. for resistance to the rust pathogen Gymnosporangium nelsonii Arth.
Kabaluk, James Todd
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A technique for screening seedlings of Amelanchier alnifolia Nutt. (saskatoon berry) for resistance to the fungal pathogen Gymnosporangium nelsonii Arth. was developed. The rust species used in the experiments, G. nelsonii, has a heteroecious life cycle and commonly alternates between Juniperus horizontalis or J. scopulorum and A. alnifolia. A reliable procedure for generating basidiospores, the inoculum infectious to saskatoons, from telia, was developed. Conditions allowing the expression of susceptibility in saskatoon seedlings were investigated. Susceptibility was evaluated on the basis of pycnial lesion density on leaves. The average density of pycnial lesions on the youngest three leaves was used to score disease severity. High correlations were observed when this scoring system was compared with other scoring systems (average of youngest two leaves, r=0.962; average of 2nd and 3rd youngest leaves, r=0.760; average of youngest five leaves, r=0.943; maximum density on any leaf, r=0.933), showing that the use of the average density-of lesions on the youngest three leaves for describing the amount of disease of saskatoon seedlings was reasonable by comparison with the other scoring systems. Three and four week old seedlings exhibited the greatest disease severity when seedlings from one to seven weeks of age were tested. As leaves aged, their susceptibility to infection decreased. Leaves 2-5 days old were the most susceptible and leaves greater than 10 days old did not show substantial infection. The minimum leaf wetness period resulting in the greatest disease severity was 24 hours. Shorter leaf wetness periods resulted in fewer lesions while periods greater than 24 hours did not induce a greater number of lesions. The relationship between inoculum concentration and disease severity was linear (r2=0.975) over the range of concentrations tested. The inoculum suspension naturally ran off the leaves easily without the use of some method to allow the it to adhere to the leaf surface. After testing four commercial adjuvants (Biostick, Postick, Forevergreen, and Tween 20), molasses, surface rubbing, and a control, it was shown that rubbing the surface of the leaves or a 1-2% solution of molasses were effective in sticking the suspension to the leaves; this resulted in a high number of infections. The use of an adjuvant required considerably less labour than the surface rubbing treatment.