Segregation ratios observed in the three hybrid families suggested the presence of one major dominant locus in ‘Chardonnay’ × V. cinerea B9 (1 resistant:1 susceptible) and at least two major dominant loci in ‘Horizon’ × V. cinerea B9 (3 resistant:1 susceptible) and ‘Horizon’ × Illinois 547-1 (V. rupestris B38 × V. cinerea B9). This observation was corroborated by QTL mapping, where the loci Rda1 and Rda2 were found in V. cinerea B9 and ‘Horizon’, respectively. Co-localization of loci obtained from cane and cluster phenotypes suggests that resistance in both tissues was due to the same loci. Therefore, we used the Rda1 and Rda2 designations for both phenotypes.