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Comparative Infectivity of Cronartium ribicola Aeciospores and Urediniospores in Genotypes of Ribes nigrum

Hosts for the fungus Cronartium ribicola, causal agent of white pine blister rust (WPBR), include five-needle pines as aecial hosts, and currants and gooseberries as uredinial/telial hosts. Aeciospores produced on diseased pine, and urediniospores produced on diseased Ribes plants, can infect Ribes foliage. Resistance and susceptibility for both spore types have been reported for Ribes; however, the comparative infectivity of these spore types on clonal Ribes nigrum genotypes is under-described. Immunity, resistance, and susceptibility to WPBR resides at a clonal level in Ribes. Previous studies have emphasized fungal diversity or forestry considerations, rather than horticultural aspects. The objective of this study was to determine if aeciospores and urediniospores were equally infective to specific genotypes of black currant, Ribes nigrum, with differential responses. A family of 51 black currant genotypes from a cross between a known-immune cultivar containing the Cr gene and a susceptible cultivar was examined. Single-leaf softwood cuttings of each of these genotypes were artificially inoculated with a spore solution and incubated in airtight plastic containers within a growth chamber. Inoculations were replicated three times for each spore type. Twenty-two of the F₁ genotypes did not develop uredia after artificial inoculation. These may be immune; 22 developed uredia after exposure to both types of inoculum, and were susceptible; whereas seven exhibited differential responses and may have some mechanism for resistance to WPBR other than the Cr gene. The infectivity of aeciospores and urediniospores was not significantly different on specific Ribes genotypes. Either spore type can therefore be considered equally effective as inoculum when screening for WPBR resistance or immunity in horticultural settings.