SUDDEN-DEATH SYNDROME DEVELOPMENT IN SOYBEAN CULTIVARS DIFFERING IN RESISTANCE TO FUSARIUM-SOLANI

Sudden death syndrome (SDS), caused by Fusarium solani (Mart.) Sacc., can cause severe yield reduction in soybean [Glycine max (L.) Merr.]. We evaluated SDS leaf symptom development on six cultivars considered SDS-susceptible and six considered SDS-resistant. Microplot treatments used local noninfested soil inoculated (using an oat-based inoculum) with a pathogenic F. solani isolate from Arkansas; soil from two different F. solani-infested sites in Illinois; and, as a control, local noninfested soil. Plants were rated for severity and incidence of SDS leaf symptoms every 5 d from R1 through R6 developmental stages. Severe chlorosis and necrosis developed on the susceptible cultivars, but not on the resistant cultivars. Variation in symptom severity ranged from 0 to 100%. Cultivar rank based on the area under disease progress curve (AUDPC) was similar to the presumed rank. Correlation between AUDPC and the severity of SDS symptoms at R6 was very high (r = 0.98), indicating that the reaction of soybean to F. solani can be evaluated by rating symptoms at the R6 stage. Analysis of variance for R6 values indicated that cultivar X treatment effects were significant (P less-than-or-equal-to 0.05). Reactions of soybean cultivars to F. solani populations apparently differ and can cause the ranking of cultivars within a differential set to change. Severity at R6 correlated with reduced pods per plant, but not with plant height, maturity, seed weight, or seed quality. Symptoms that developed on F. solani-inoculated plants were similar to those that developed on plants grown in F. solani-infested soil. Reaction to F. solani can, therefore, be evaluated using an inoculum of F. solani-infested oats as an alternative to evaluation at naturally infested locations.