GENETICS OF RESISTANCE TO VERTICILLIUM WILT IN VERTUS ALFALFA

It is difficult to breed alfalfa (Medicago sativa L.) cultivars with > 50% resistance to Verticillium albo-atrum Reinke & Berthier. The objectives of this study were to determine if selection for resistance to V. albo-atrum within the alfalfa cultivar Vertus reduces general combining ability (GCA) compared to specific combining ability (SCA) and to compare disease evaluation in a controlled environment based on foliar symptoms and stem colonization. Two populations of Vertus were studied, one selected on the absence of foliar symptoms after repeated inoculation, the other selected at random. Correlations between stem colonization and foliar symptoms in both populations were low. Seedling height, stem colonization, and first and second-regrowth foliar symptoms were analyzed for GCA and SCA. Estimates of GCA variances for these characters were three to five times greater in the random population than in the resistant population. Estimates of SCA variances were similar for both populations, with one exception. Estimated GCA/SCA ratios were 4.7:1 and 5.0:1 for foliar symptom evaluation of two regrowth periods in the random population. Corresponding estimated GCA/SCA ratios in the resistant population were 2.9:1 and 1.9:1. This suggests that additive genetic variance will provide limited improvement of resistance levels and nonadditive genetic variance will not be a source of higher levels of resistance to verticillium wilt in this cultivar.