Genetics of Downy Mildew Resistance in Two Interspecific Hybrid Grapevine Families

Due to lack of co-evolution with Plasmopara viticola, the causal pathogen of grapevine downy mildew, nearly all cultivated grapevines are susceptible to downy mildew, whereas their wild relatives are frequently resistant. In order to find QTL for downy mildew resistance and susceptibility, we performed leaf disc assays on two families of grapevine (V. rupestris B38 x ‘Horizon’ [3 experiments; n = 159] and ‘Horizon’ x V. cinerea B9 [2 experiments; n = 152]) and evaluated disease using a visual rating system that measured hypersensitive response (HR) at 2 days post-inoculation (dpi) and sporulation at 3, 4, 5, 6, and 7 dpi. QTL analysis was performed using R/qtl, and a QTL was considered reproducible if it was significant in at least two experiments. Two reproducible QTL were found in V. rupestris, but none was found in V. cinerea. Four reproducible QTL were found in ‘Horizon’, two of which were significant in both families. Out of the six QTL, four affected sporulation without a corresponding change in HR, and the other two involved increased HR. The QTL found in these experiments can be introgressed into grapevines with a V. vinifera background using marker-assisted selection, eventually leading to grapevine cultivars resistant to downy mildew.