PERFORMANCE OF FROGEYE LEAF SPOT-RESISTANT AND SPOT-SUSCEPTIBLE NEAR-ISOLINES OF SOYBEAN

Frogeye leaf spot (FLS) caused by Cercospora sojina Hara is a disease of soybean (Glycine mau (L.) Merr.) that causes significant seed yield losses in warm, humid environments of southeast ern United States. Th e RCS3 gene in soybean has been reported to condition resistance to al l known races of C. sojina. The objectives of this study were to deter mine the effectiveness of RCS3 in limiting seed yield loss due to FLS and to compare the seed yield of the resistant and susceptible near-is olines (NILs) in the absence of significant FLS disease. Four pairs of NILs-Colquitt/Colquitt-Rcs(3), Gordon/Gordon-Rcs(3), ThomaslThomas-Rc s(3), and Wright/Wright-Rcs(3)-were evaluated in 23 field experiments in Alabama, Florida, Georgia, Louisiana. Mississippi, and South Caroli na during 1992 to. 1994. The amount of damage to susceptible soybean c aused by FLS was dependent on the specific environment. All four of th e RCS3 NILs were resistant to the prevalent races of FLS in all enviro nments. In the absence of significant FLS disease, each of the RCS3 NI Ls was at least equal to the respective susceptible line in its seed y ield. In the presence of FLS infestation, the susceptible lines suffer ed significant seed yield loss (up to 31%) compared to their Rcs(3) NI Ls. The effect of FLS on seed yield was dependent on cumulative diseas e severity over the growing season. Thus, the area under disease progr ess curve was more useful than percent of leaf area infected at the en d of the growing season (R7 stage of development) in explaining the se ed yield loss due to FLS.