Fungicide Efficacy Guides for Foliar Diseases in Corn and Soybean: Development and Validation

Fungicide efficacy guides, updated annually through the Crop Protection Network, inform fungicide selection for foliar and seedling diseases of corn (Zea mays) and soybean (Glycine max). These guides rank fungicides based on multi-state field trials across the U.S. and Ontario, Canada. Trials were analyzed to validate these rankings by assessing efficacy of fungicides under varying disease severities. Under high disease severity (≥5%), fungicides with the best efficacy ratings significantly reduced gray leaf spot (GLS; caused by Cercospora zeae-maydis) and southern rust (SR; caused by Puccinia polysora) in corn when applied at tasseling (VT) to silking (R1) growth stages, and frogeye leaf spot (FLS; caused by Cercospora sojina) in soybean when applied at beginning pod (R3) growth stage. GLS severity was reduced by 8.6 to 8.8%, SR by 14.6 to 20.6%, and FLS by 15.3%. Corn yields were 420.4 kg/ha (6.7 bu/A) greater than the non-treated control, and yield response in 57.9 to 63.6% of the trials exceeded the economic breakeven point of 288.4 kg/ha (4.6 bu/A) for fungicide application. Soybean yields were 417.0 kg/ha (6.2 bu/A) greater than the non-treated control, with 83.3% of trials reaching the economic breakeven point of 134.5 kg/ha (2 bu/A). Under low disease severity (<5%), disease control and yield benefits diminished across all fungicide efficacy categories. These results validate the fungicide efficacy ratings as predictive tools for disease control and yield response, especially under high disease pressure, highlighting their importance for fungicide decisions in corn and soybean across the U.S. and Canada.