Submitted to: American Phytopathological Society Abstracts
Publication Type: Abstract Only
Publication Acceptance Date: 5/1/2008
Publication Date: N/A
Technical Abstract: Over the past 70 years, at least ten screens for resistance to grape downy mildew have been conducted around the world using uncharacterized populations of the pathogen Plasmopara viticola. Summarizing the results into three categories (resistant, moderate, susceptible) for shared host genotypes, only 32% of the ratings corresponded, due to variable environment, pathogen source, or mislabeling of host genotype. To establish a baseline for controlled inoculation studies, a single isolate of P. viticola was inoculated in 2006 onto 883 genotypes of wild Vitis spp. and named cultivars. A second isolate was used in 2007, and ratings were consistent for 95% of genotypes across the two years. For the 89 shared host genotypes screened in this and previous studies, only 37% of the ratings corresponded. Resistant and susceptible genotypes were identified in every well-represented species except V. vinifera and V. acerifolia, which had only susceptible genotypes. Of the varieties previously screened in the U.S., only V. hybrid cvs. ‘Yates’ and ‘Concord’ were resistant or moderately resistant at all locations including this study. Grape breeders should test germplasm for downy mildew resistance in diverse environments to know the spectrum of resistance efficacy. Future screens should consider intraspecies variation in resistance, report identities for wild and cultivated genotypes, and carefully consider the genetics of the pathogen.