CULTIVAR RESPONSES TO SUDDEN-DEATH SYNDROME OF SOYBEAN

Forty-two soybean (Glycine max) cultivars representing maturity groups IV - VIII were evaluated for their response to sudden death syndrome (SDS), caused by Fusarium solani, over 3 yr in a field with a history of the disease. Disease severity was assessed weekly from the middle of July through September using a rating scale of 0-5 where 0 = 0%, 1 = 10%, 2 = 11-30%, 3 = 31-70%, 4 = 71-90%, and 5 = > 90% of the leaf area exhibiting foliar symptoms of SDS. Area under the disease progress curve was calculated for each cultivar. The susceptible cultivar Lee 74 was planted adjacent to each cultivar tested and served as a standard for comparison of disease levels. Disease progress on plants of Lee 74 was used to estimate disease for each of the plots with plants of the test cultivars using the geostatistical technique, "kriging." Estimates of relative amounts of disease were used as covariates in the cultivar analysis. Foliar symptoms of SDS were noted at or shortly after flowering with cultivars of maturity group IV but before flowering with many of the cultivars in maturity groups V-VIII, depending on the year. Significant differences in disease development between cultivars occurred at the R3 growth stage and were more distinct at the R6 growth stage. Cultivar rankings at R3 were generally highly correlated to their rankings at R6. Most cultivars had similar levels of disease between years and with previous observations. However, some cultivars had significant differences in disease development between years. Disease development in a few cultivars differed markedly with previous observations. These differences appeared to be associated with the response of the cultivar to race 6 of the soybean cyst nematode (SCN), Heterodera glycines, the predominate race in the test field. Cultivars susceptible to race 6 were susceptible to SDS and those that were moderately resistant or resistant to race 6 had low levels of SDS. With other cultivars, susceptibility to SCN was not related to susceptibility to SDS.