The stacking of multiple resistance genes with different mechanisms is a proven breeding strategy to improve the durability and the level of resistance. This strategy requires identification of multiple sources of resistance, functional characterization of resistance mechanisms to prioritize optimal genetic combinations, and genetic markers that allow the different resistances to be monitored through crosses among themselves and with elite cultivars. This study used and developed genetic markers to eight grape powdery mildew resistance sources: Muscadinia rotundifolia, the Chinese species Vitis piasezkii and V. romanetii, and an array of V. vinifera cultivars and V. sylvestris accessions from central Asia. Multiple steps are required to combine resistance from two or more genetic backgrounds while maintaining high fruit quality attributes and other important horticultural traits. First, a resistance locus is introgressed into a high-quality variety. Superior individuals with the resistance gene are then selected and selfed to develop homozygous resistant lines for each resistance source. Progeny are selected from these homozygous resistant lines with the goal of combining multiple resistance forms via marker-assisted breeding. At all stages of the breeding program, seedlings and plants are tested for genetic markers and powdery mildew resistance is confirmed by careful evaluation for disease development in field, greenhouse, and in vitro assays. The stacking of resistance from three sources into one line and the development of a fourth source for later introgression are presented.

Funding Support: We gratefully acknowledge support for Daniel Pap from Corvinus University of Budapest, Hungary, and research funds from the Louise Rossi Endowed Chair and the Louis P. Martini Endowed Chair