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 sudden 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 resistance 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 121 days after planting (DAP). Six taproot sections per plant were tested for F. solani on a selective medium. Infection frequency (IF), the percentage of infected plants, and infection severity (IS), the percentage of infected root segments, were scored. Leaf symptoms were recorded and standardized to the R6 stage and the disease index (DX) was calculated. Infection was detected at the V1 stage (within 15 DAP) and reached a maximum by about R1 (55-68 DAP). All cultivars were infected by F. solani. However, seasonal mean IF and mean IS were significantly lower among the resistant cultivar class. After R1, infection was also significantly lower among the resistant cultivar class within individual sampling dates. Also, DX and IF were correlated in each environment (r = 0.38 and 0.61). Therefore, in the resistant cultivars Forrest, Ripley, Jack, PI520733, ExF44, ExF59, and ExF78, late season rate-reducing (partial) resistance decreased the DX, IF, and IS. Resistance probably extends the latent period of F, solani. Among susceptible cultivars, Essex and A5403, had reduced leaf symptoms but high IF and IS suggesting tolerance to F. solani that alleviated SDS.