Evaluation of powdery mildew resistance of a diverse set of grape cultivars and testing the association between powdery mildew resistance and pr gene expression

Powdery mildew caused by the fungal pathogen Erysiphe necator is a global disease that reduces yield and quality in grapes (Vitis vinifera L.). Most grape cultivars are susceptible to this pathogen and thus the selection of powdery mildew resistant cultivars is a major objective of grape breeding programs. In this study, we evaluated powdery mildew resistance of a diverse set of 28 commonly cultivated local and international V. vinifera cultivars after artificial inoculations with Erysiphe necator. Of these, 7 cultivars were found to be resistant while 10 and 11, were found to be moderately resistant and susceptible to powdery mildew, respectively. We also analyzed pathogen-responsive expression profiles of two defense genes, VvPR1 and VvGLP3 encoding a pathogenesis-related protein 1 homolog and a Germin-Like protein, respectively, in the same cultivars by qRT-PCR to determine whether defense gene expression is correlated with powdery mildew resistance. There was no significant correlation between powdery mildew ratings and induction patterns of these two defense genes. However, a significant correlation was found between VvPR1 and VvGLP3 expression only in cultivars that showed increased powdery mildew resistance. In addition, the grape cultivars we analyzed differed by berry ripening times, were shown to influence disease resistance. However, no significant correlation was found between powdery mildew ratings and the ripening times. Although potential mechanisms of powdery mildew resistance or susceptibility remain unclear, powdery mildew resistant grape cultivars identified here will be useful for growers and breeders interested in using these lines as parents in grape breeding programs.