ABSTRACT
To complement existing control strategies, grape growers in humid climates desire cultivars with resistance to downy mildew caused by Plasmopara viticola. Numerous disease resistance screens of diverse Vitis germplasm have been conducted previously to identify downy mildew resistance; however, ratings of named cultivars were inconsistent and identities of resistant individuals in wild species were not typically provided. Inconsistencies among previous studies could be due to race-specific resistance. In the current study, controlled inoculations of two single isolates onto two leaf ages of 883 Vitis accessions were used and these results compared with natural infection in a fivefold replicated vineyard of 80 Vitis accessions in 2006 and 2007. Of the accessions rated in both assays, 16.2% were resistant to a single isolate but susceptible in the vineyard. Otherwise, there was good correlation of ratings between the field assay and the rating of older leaves (r = 0.62 to 0.71). Five accessions from Vitis cinerea, V. labrusca, and Vitis × champinii averaged zero severity in both vineyard years, yet some individuals of V. cinerea and V. labrusca were moderately or highly susceptible in the field. Similarly, although significant differences in mean severity separated V. vinifera, Vitis hybrid, V. riparia, and V. labrusca for single-isolate inoculations (from susceptible to resistant), notable intraspecies variation was identified for all well-represented species. Resistant individuals were identified in most species with the prominent exceptions of V. vinifera and V. acerifolia. Single-isolate, detached-leaf resistance ratings in 2006 corresponded well (94.6%) to 2007 ratings using a separate isolate collected from the same vineyard. Categorizing the ratings for this and previous studies, ratings infrequently corresponded among previous studies (31.9%) as well as between previous studies and the current single-isolate (34.9%) or vineyard (46.4%) ratings. These results highlight important factors for downy mildew resistance screens: leaf age, pathogen genotype, and host species and accession. The results further underscore the importance to breeders of uniform testing in multiple environments.
