Performance of Frogeye Leaf Spot-Resistant and -Susceptible Near-Isolines of Soybean

Frogeye leaf spot (FLS) caused by Cercospora sojina Hara is a disease of soybean (Glycine max (L.) Merr.) that causes significant seed yield losses in warm, humid environments of southeastern United States. The Rcs₃ gene in soybean has been reported to condition resistance to all known races of C. sojina. The objectives of this study were to determine the effectiveness of Rcs₃ in limiting seed yield loss due to FLS and to compare the seed yield of the resistant and susceptible near-isolines (NILs) in the absence of significant FLS disease. Four pairs of NILs-Colquitt/Colquitt-Rcs₃, Gordon/Gordon-Rcs₃, Thomas/Thomas-Rcs₃, and Wright/Wright-Rcs ₃-were evaluated in 23 field experiments in Alabama, Florida, Georgia, Louisiana, Mississippi, and South Carolina during 1992 to 1994. The amount of damage to susceptible soybean caused by FLS was dependent on the specific environment. All four of the Rcs₃ NILs were resistant to the prevalent races of FLS in all environments. In the absence of significant FLS disease, each of the Rcs₃ NILs was at least equal to the respective susceptible line in its seed yield. In the presence of FLS infestation, the susceptible lines suffered significant seed yield loss (up to 31%) compared to their Rcs₃ NILs. The effect of FLS on seed yield was dependent on cumulative disease severity over the growing season. Thus, the area under disease progress curve was more useful than percent of leaf area infected at the end of the growing season (R7 stage of development) in explaining the seed yield loss due to FLS.