Inheritance of downy mildew resistance in table grapes

A Study was conducted to determine how resistance to downy mildew [Plasmopa ra viticola (Bert, & Curt.) Berl, & de Toni] is inherited in germplasm (Vit is vinifera L., V. labrusca L., V. rupestris Scheele, and V. riparia Michx, ) used for breeding table grapes. Crosses, including reciprocals, among par ents Possessing different levels of downy mildew resistance were evaluated in 1994 and 1995, The proportion of foliar tissue with sporulation, chloros is, or necrosis was used to measure resistance. All genotypes were rated fo r these characters on two separate dates in 1994 and 1995, Hypersensitive n ecking was also evaluated in the 1995 seedlings to determine its relationsh ip with downy mildew resistance. Crosses with at least one resistant parent had a larger number of resistant offspring than crosses between two suscep tible parents. General combining ability (GCA) effects were highly signific ant for 1994 and 1995. Specific combining ability effects were significant, but were relatively small compared to GCA, suggesting additive gene action was a primary influence on downy mildew resistance. Heritability estimates for sporulation, chlorosis, and necrosis were the highest at the second ra ting in 1994 (0.88, 0.74, and 0.57, respectively) and 1995 (0.50, 0.60, and 0.60, respectively). Reciprocal crosses indicated that maternal inheritanc e did not influence downy mildew resistance. A small percentage of progeny with hypersensitive flecking were identified from the germplasm. Seedlings with the flecking characteristic tended to have lower sporulation, chlorosi s, and necrosis ratings earlier in the growing season.