New insights into the epidemiology of frogeye leaf spot of soybean (Glycine max L.).

Frogeye leaf spot (FLS), caused by Cercospora sojina, is an important foliar disease of soybean in the United States, yet quantitative information describing FLS epidemic dynamics during soybean reproductive stages remains limited. This study characterized the temporal FLS progress under natural field conditions and evaluated disease risk using survival analysis. Field observations were conducted in university-managed soybean fields in northwestern Missouri during the 2017, 2019 and 2024 growing seasons, without artificial inoculation or fungicide application. Disease severity was quantified as lesion counts on tagged upper-canopy leaflets repeatedly photographed and analyzed over time. Disease progress was modeled using exponential, monomolecular, logistic, and Gompertz nonlinear growth models. Logistic and Gompertz models consistently provided the best fit. Apparent infection rates estimated from logistic models ranged from 0.10 to 0.19 across epidemics, and lesion accumulation showed evidence of density-dependent dynamics with progressive deceleration. Survival analysis estimated the time required for leaflets to reach a threshold of 40 lesions, corresponding to approximately 4 to 6% leaf area affected. Kaplan-Meier curves indicated that in 2017, the median time to reach this threshold was approximately 26 to 28 days after initial assessment, whereas in 2019, fewer than 50% of leaflets reached the threshold during the observation period. In 2024, disease development remained minimal, and lesions did not reach quantifiable levels. These results provide quantitative insights into FLS epidemics during soybean reproductive stages and highlight the value of integrating repeated field monitoring with nonlinear epidemic modeling and survival analysis to improve disease risk assessment and management decisions.