8 ISOLATES OF DIDYMELLA-BRYONIAE FROM GEOGRAPHICALLY DIVERSE AREAS EXHIBIT VARIATION IN VIRULENCE BUT NO ISOLATE BY CULTIVAR INTERACTION ON CUCUMIS-SATIVUS

Eight isolates of Didymella bryoniae from geographically diverse areas were tested for differences in virulence on nine genotypes of cucumber (Cucumis sativus) in two greenhouse experiments. Cucumber genotypes tested represent the range of resistance currently available. Isolates were collected in Arizona, California, The Netherlands, North Carolina, South Carolina, Sweden, and Wisconsin. The original host for one isolate was unknown, six were from cucumber, and one from muskmelon (C. melo). No significant isolate by cultivar interaction was detected in either experiment. Ranks of isolates were usually consistent across cultivars and experiments, and ranks of cultivars were usually consistent across isolates and experiments. Thus, resistance in cucumber to D. bryoniae appears to be nonspecific in nature. Single degree of freedom contrasts showed that the two foreign isolates (from The Netherlands and Sweden) were significantly more virulent than the U.S. isolates. Regression analysis indicated that the variance among cultivar ratings was not significantly correlated with mean isolate rating and that the variance among isolate ratings was not correlated with mean cultivar rating, indicating that an additive model of host-pathogen interaction may control resistance. The finding that resistance to D. bryoniae in cucumber is nonspecific suggests that breeders can use a single virulent isolate of D. bryoniae to screen for resistance.