Efficacy and cost-benefit analysis of fungicide combinations against soybean rust and Cercospora leaf blight in Paraguay

Abstract

This study evaluated the efficacy, seed yield and economic returns of nine fungicide mixtures for the integrated management of Asian soybean rust (Phakopsora pachyrhizi) and Cercospora leaf blight (Cercospora spp.) under field conditions in Canindeyú, Paraguay, during the 2023/2024 and 2024/2025 seasons. Treatments included site-specific premixes combining QoI, DMI and SDHI modes of action, as well as protectant-only programs based on chlorothalonil or mancozeb applied sequentially at four timings. Disease severity was recorded over time to generate disease progress curves and the area under the disease progress curve (AUDPC); control efficacy was derived relative to the untreated control. Grain yield, thousand‐grain weight, gross income, total treatment cost, net benefit and benefit–cost ratio (B/C) were also determined. Across seasons and diseases, multi-MOA premixes (QoI + DMI ± SDHI) consistently resulted in the lowest AUDPC values, providing the strongest suppression of the foliar disease complex compared with the untreated control. Soybean rust pressure differed between seasons, with later and lower severity in 2024/2025; therefore, treatment separation for rust was smaller in absolute terms in that season, while AUDPC rankings remained consistent. Yield and seed weight responses generally followed disease suppression, with the best premixes showing the highest numerical yields and thousand-seed weight. Although protectant-only programs provided intermediate disease reduction, their lower input costs often resulted in high B/C ratios, highlighting their value as low-cost benchmarks under certain scenarios. Overall, the results indicate that locally validated multi-MOA fungicide programs can provide effective and economically viable management of late-season soybean foliar diseases in Paraguay, under variable epidemic conditions.