Progression of Gummy Stem Blight Epidemics on Watermelon with and without Fungicide Inputs

Gummy stem blight (GSB), caused primarily by the fungus Stagonosporopsis citrulli in the southeastern United States, affects cucurbits and is particularly destructive on watermelon. Previous epidemiological models of GSB constructed for greenhouse cucumber showed leaf wetness and temperature were the primary and secondary environmental factors, respectively, that explained epidemic progress. The objective of this study was to construct a model that predicted GSB severity on field-grown watermelon based on environmental factors. Disease and weather data from six fungicide experiments in Charleston, South Carolina, in spring and fall 1997 and fall 2017, 2018, 2019, and 2022 were used as inputs. Fungicide treatments were grouped into nonsprayed, protectant (chlorothalonil and mancozeb) and GSB-specific (cyprodinil, difenoconazole and fludioxonil) applications. Cumulative hours of leaf wetness was the primary explanatory variable that modeled the increase in proportion GSB severity ≥2% across all epidemics. Incorporation of temperature or other environmental variables did not improve the model. Fit of the overall model was evaluated with k-fold cross validation, where individual experiments were each excluded from the model fitting process. Slopes of predicted disease progress curves were lowered significantly compared to the nonsprayed treatments by applications of protectant fungicides. Applying GSB-specific fungicides alternated with chlorothalonil further reduced slope values. The model successfully predicted progress of GSB epidemics under different weather patterns and fungicide applications.