Soybean yield response to foliar fungicides under low frogeye leaf spot disease severity in Ohio

Frogeye leaf spot (FLS), caused by Cercospora sojina Hara, is a major soybean disease in Ohio, managed with resistant cultivars and foliar fungicides. This study assessed the effects of fungicide applications on (1) FLS incidence and severity, (2) soybean yield, and partial economic return when applied at the beginning of pod‐setting (R3) and seed fill (R5) stages under natural FLS infection, using two cultivars with contrasting levels of FLS resistance, and (3) examined the relationship between weather conditions and FLS development. Field trials were conducted across 16 Ohio environments (2022–2024) using a randomized complete block design with four replications. Treatments included two cultivars (moderately resistant and moderately susceptible), two fungicides (mefentrifluconazole + fluxapyroxad + pyraclostrobin and propiconazole), and two application timings (R3 and R5). Environments were grouped based on FLS severity (FLS present or absent). Fungicide applications failed to improve yield in environments without FLS. In environments with FLS, mefentrifluconazole + fluxapyroxad + pyraclostrobin fungicide applied at R3 on the moderately resistant cultivar produced a yield increase of 197.9 kg ha −1 compared to the nontreated control. The greatest reduction in FLS incidence (−23.0%) and severity (−0.5%) occurred when mefentrifluconazole + fluxapyroxad + pyraclostrobin was applied at R3 on the susceptible cultivar. Disease severity was low in the studied environments, and foliar fungicide applications did not generate an economic return. Higher precipitation was associated with more disease incidence. Farmers are advised to base their fungicide application decisions on scouting, soybean stage, field disease history, cultivar susceptibility, weather conditions, and costs.