To complement existing control strategies, grape growers desire cultivars with resistance to powdery mildew caused by Erysiphe necator. Numerous disease resistance screens of diverse Vitis germplasm have been conducted previously to identify powdery mildew resistance but ratings of named cultivars were inconsistent and identities of resistant individuals in wild species were not typically provided. In the current study, controlled inoculations of a single isolate were made onto detached leaves from 1,025 Vitis accessions. The results were compared with natural epidemics in two vineyards: the cold-hardy Vitis spp. repository in Geneva, NY, in 2007–08, and a replicated vineyard of 89 Vitis accessions in Fredonia, NY in 2006–08. Of the genotypes screened using both natural infection and single-isolate inoculation, 33% were resistant to a single isolate but susceptible to diverse isolates in either or both vineyards, possibly due to race-specific resistance. This was exemplified by selection of E. necator genotypes virulent to Vitis labrusca in the Fredonia, NY vineyard, which is surrounded by production of the interspecific labrusca hybrids Concord and Niagara. Otherwise, there was good correlation of ratings between the vineyard and single-isolate ratings (r = 0.55 to 0.56) and between Geneva and Fredonia vineyard ratings (r = 0.75). No accession rated in all three screens was immune from infection. Although individual accessions of V. aestivalis, V. palmata, Vitis × doaniana, and Ampelopsis brevipedunculata were resistant in Geneva and Fredonia, each well-represented species had notable intraspecific variation in resistance. For 129 interspecific hybrids in this and previous studies, ratings infrequently corresponded among previous studies (39%) and between the current and previous studies (17 to 46%). However, three cultivars (Cayuga White, Diana, and Mars) were consistently rated as resistant across four independent studies. The results underscore the importance of uniform testing in multiple environments and the need for strategies for the development of cultivars with durable resistance.
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