Durable powdery mildew resistance in grapevines: myth or reality

Grape powdery mildew (PM) is an economically important disease worldwide, and necessitates high input of fungicides for successful grape production. Development of cultivars that carry resistance from multiple backgrounds is an important strategy to obtain durable resistance in the field. This strategy requires a) the identification of sources of resistance and marker development to track those resistances, b) characterization of the mechanisms of resistance, c) marker-assisted breeding to introduce the selected loci into elite cultivars, and, finally, d) understanding of pathogen biology and responses to different resistance sources. We have identified and tagged novel sources of genetic resistance from eight genetic backgrounds, including Muscadinia rotundifolia, two different Chinese species (Vitis piasezkii and Vitis romanetii), a wide array of Vitis vinifera cultivars and wild forms of V. vinifera subsp. sylvestris from Asia. Molecular markers that are tightly linked to different loci are used at the early stage to select seedlings that carry resistance loci. PM resistance from two or more genetic backgrounds is combined in multiple steps, while maintaining high fruit-quality attributes and other important horticultural traits. Resistance is confirmed by careful evaluation for disease development in the field, the greenhouse or by in-vitro assay. Multipronged testing of seedling plants for resistance allows better assessment of pathogen biology and interaction with different resistance loci and allows us to make decisions about combining resistance loci. Results will be presented with elucidation of the mechanism of resistance from different backgrounds, specificity to different isolates and the effectiveness of molecular markers in the breeding program.