RATE-REDUCING RESISTANCE TO FUSARIUM-SOLANI F-SP PHASEOLI UNDERLIES FIELD-RESISTANCE TO SOYBEAN SUDDEN-DEATH SYNDROME

Yield losses occur in soybean [Glycine max (L.) Merr.] because of sudd en death syndrome (SDS) caused by Fusarium solani (Mart.) Sacc. f. sp. phaseoli (Burk.) Snyd. & Ham, type A. Our objective was to determine whether tolerance or partial resistance to infection underlies field r esistance to SDS. Seven field resistant cultivars and five susceptible cultivars were planted at two locations. Twenty taproots per cultivar were sampled every 7 to 21 d from the V0 to R8 growth stages, 8 to 12 1 days after planting (DAP). Six taproot sections per plant were teste d for F. solani on a selective medium. Infection frequency (IF), the p ercentage of infected plants, and infection severity (IS), the percent age of infected root segments, were scored. Leaf symptoms were recorde d and standardized to the R6 stage and the disease index (DX) was calc ulated. Infection was detected at the V1 stage (within 15 DAP) and rea ched a maximum by about R1 (55-68 DAP). All cultivars were infected by F. solani. However, seasonal mean IF and mean IS were significantly l ower among the resistant cultivar class. After R1, infection was also significantly lower among the resistant cultivar class within individu al sampling dates. Also, DX and IF were correlated in each environment (r = 0.38 and 0.61). Therefore, in the resistant cultivars Forrest, R ipley, Jack, PI520733, ExF44, ExF59, and ExF78, late season rate-reduc ing (partial) resistance decreased the DX, IF, and IS. Resistance prob ably extends the latent period of F, solani. Among susceptible cultiva rs, Essex and A5403, had reduced leaf symptoms but high IF and IS sugg esting tolerance to F. solani that alleviated SDS.