Profitability of foliar fungicides in double crop soybean under low-disease environments

Abstract

Double-crop soybean production involves planting soybean (Glycine max) directly following winter wheat (Triticum aestivum) harvest. Frogeye leaf spot (FLS), caused by Cercospora sojina, is an important late-season foliar disease affecting soybean fields in the United States. In some instances, foliar fungicides have been used in double-crop soybean production with little to no FLS present, raising questions on the profitability of these applications. This study analyzed yield data from 25 fungicide trials across five states (Illinois, Indiana, Kentucky, Missouri, and Tennessee) conducted under low FLS pressure, from 2008 to 2021, on double-crop soybean. Fungicide classes evaluated in the trials included quinone outside inhibitors (QoI), demethylation inhibitors (DMI), and methyl benzimidazole carbamates (MBC) applied alone, and mixtures of chemistry classes that included DMI+succinate dehydrogenase inhibitor (SDHI), MBC+DMI, QoI+DMI, and QoI+DMI+SDHI. A network meta-analytic model estimated yield differences between fungicide-treated and nontreated plots, which ranged from -28 to 72 kg/ha among the fungicide treatments. Negative yield response values were estimated for the single fungicide classes MBC (-15.7 kg/ha) and QoI (-28.4 kg/ha). Yield difference as high as 72.8 kg/ha was estimated only for DMI+SDHI. Economic analyses indicated that, due to the lower yield responses, probabilities of breaking even were lower than 50% for all fungicide treatments, regardless of the fungicide cost or soybean sale price. Therefore, the low yield responses associated with foliar fungicides in low-disease environments linked to the higher risk of not offsetting the costs, suggest that growers should consider disease risk prior to making the fungicide application.