Even though ‘Chardonnay’ disease severity was similar to that of V. cinerea B9 in both the greenhouse and field, and to ‘Horizon’ in the field, we were not able to identify resistance loci from ‘Chardonnay’. If ‘Chardonnay’ resistance is quantitative, our experiment may not have had enough statistical power to detect minor effect QTL. Among ‘Chardonnay’ × V. cinerea B9 F1 progeny, segregation of ‘Chardonnay’ loci can only be observed among progeny with Rda1 susceptible alleles, reducing the effective size of the population to fewer than 100 individuals. While ‘Chardonnay’ resistance could instead be recessively inherited, the rare presence of extremely susceptible phenotypes in related V. vinifera cultivars argues against that possibility.