Economic returns on fungicide use across soybean cultivars with varying tolerance to target spot (Corynespora cassiicola)

This study investigated the economic viability of fungicide use in managing target spot disease, caused by Corynespora cassiicola, across soybean cultivars with varying tolerance levels. An extensive dataset was collected over 11 growing seasons (2012/2013 to 2022/2023) from 173 trials in six Brazilian states. A total of 56 soybean cultivars were classified into high, medium, and low tolerance categories based on calculated relative damage coefficients (DC; or the percent reduction in yield per percent point [p.p.] increase in severity) derived from linear regression models between yield and late-season disease severity. Stochastic simulations, incorporating factors such as attainable yield, fungicide cost, and efficacy, were performed to estimate the probability of break-even on costs with fungicide, the economic returns, and the economic damage thresholds (EDT; % severity) under scenarios of attainable yield and disease pressure scenarios. The results showed that low-tolerance cultivars (mean DC = 1.0 %/p.p) consistently achieved higher yield benefits (up to 2,510 kg/ha) and profitability with fungicide applications, especially under scenarios of high disease pressure (>30%) and high attainable yield (>4,000 kg/ha). Conversely, high-tolerance cultivars (mean DC = 0.2 %/p.p) exhibited lower economic returns, greater resilience to declining fungicide efficacy over multiple seasons, and the highest EDT (10.1%), underscoring their potential for sustainable disease management. Medium-tolerance cultivars (mean DC = 0.6 %/p.p) showed intermediate outcomes in terms of profitability and yield difference. While low-tolerance cultivars benefit most from chemical interventions, high-tolerance cultivars offer a sustainable alternative by reducing reliance on fungicides, minimizing environmental impact, and mitigating economic risks associated with fungicide resistance and reduced efficacy. These findings provide valuable insights for decision-making in soybean disease management, emphasizing the integration of cultivar selection and economic thresholds into targeted fungicide programs.