Temporal dynamics of frogeye leaf spot caused by Cercospora sojina in soybeans

Cercospora sojina is the pathogen responsible for frogeye leaf spot (FLS) in soybeans. While this disease is common in West Tennessee, it does not always result in yield loss due to soybean varieties with low susceptibility or adverse weather conditions. Therefore, understanding the temporal dynamics of disease development is crucial for implementing effective management strategies. This study aimed to identify the epidemiological model that best explains the development of FLS in Tennessee . Non-treated soybean plots—comprising susceptible, moderately susceptible, and resistant varieties—were monitored for FLS severity at two locations during 2022 (three assessments), 2023 (four assessments), and 2024 (seven assessments). Disease progression curves and model fitting were conducted in R using the epifitter package. The models were assessed using Lin's Concordance Correlation Coefficient and Root Mean Squared Error. During the 2022 and 2023 seasons, the monomolecular model provided the best fit for the data, while the Gompertz model was consistently ranked second. In the 2024 season, the Gompertz best fits most conditions. The monomolecular model's superior fit in seasons with fewer observations is likely because sampling occurred late in the disease development epidemic, when disease progression had plateaued.  In contrast, earlier sampling during the 2024 season captured the initial development of the epidemic, forming a sigmoidal pattern typical of polycyclic diseases, making at this time Gompertz the model that better explains FLS temporal development.