Variation in ontogenic resistance to Uncinula necator in the USDA-ARS, PGRU Vitis germplasm collection

Ontogenic resistance (OR) to grape powdery mildew (Uncinula necator) has been shown to confer near-immunity to infection by ca 20 days post anthesis in several cultivars of Vitis vinifera and V. labrusca. We quantified variation in the magnitude and temporal distribution of OR within a diverse collection of Vitis spp. Candidate genotypes were inoculated with U. necator in a time series beginning at bloom for two growing seasons. In year 1, forty genotypes were highly resistant at all phenological stages, 9 exhibited a statisticaily significant decrease in susceptibility, and 4 showed no change in OR within 30 days after bloom, and remained susceptible. Year 2 results will be discussed. Berries of selected genotypes were harvested for a biochemical time-course study of cuticle development, as it relates to changes in OR. These results will facilitate our subsequent studies on the genetics and biochemistry of OR in grapevine berries.