RESUMO
Fungus-resistant grapevine cultivars, so called PIWIs, are characterized by increased resistance to powdery mildew and downy mildew. However, in order to maintain the durability of resistance in these new grape cultivars, targeted fungicide treatments are recommended. For ideal schedule of these treatments, it is necessary to recognize the most sensitive organs of the grape. This study introduces a method for phenotypic evaluation of Plasmopara viticola resistance in grape clusters under controlled and standardized conditions during phenological development over the entire season. The approach was validated with the traditional cultivar Pinot Noir and the PIWIs Cabernet Cortis (Rpv3.3, Rpv10), Solaris (Rpv3.3, Rpv10) and Souvignier Gris (Rpv3.2). All cultivars were susceptible during the early stages of development up to flowering, and resistance levels increased as phenological development progressed. Cabernet Cortis and Solaris clusters were susceptible until fruit development (BBCH 71-73) when they became almost completely resistant. No differences between Souvignier Gris and Pinot Noir were detected until berries were pea-sized (BBCH 75) when P. viticola resistance of Souvignier Gris clusters increased significantly. Ontogenetic resistance in Pinot Noir was detected at berry touch (BBCH 77-79) and clusters of this cultivar were almost completely resistant at the beginning of ripening (BBCH 81-83). These results indicate that the approach presented is suitable for determining the resistance of grape cultivars at different stages of development. Consequently, in the future, fungicide applications can be adjusted more precisely to the resistance level of a grape cultivar during the growing season.