Development and validation of a fungicide scoring system for management of late season soybean diseases in Argentina

Abstract

The use of foliar fungicides is a common disease control practice among soybean producers around the world, yet there is still no clear understanding about the timing and opportunity of fungicide applications to manage late season diseases (LSD) in soybean crops. The unnecessary use of fungicides in extended areas increases production costs, risk of resistance and risk of negative environmental impact. The objective of this study was to develop and validate a scoring system to guide the application of fungicides in soybean crops to manage LSDs in Argentina, with special reference to Septoria brown spot and Cercospora leaf blight. To develop the scoring system, a risk matrix with weighted epidemiological risk factors was developed based on previous research data. The scoring system recommends application of foliar fungicides based on the total points accumulated from the risk factors. Scoring greater than 33 indicates a higher probability of obtaining a positive yield response, whereas scoring below 23 indicates no expected response and thus no need for fungicide applications. Intermediate values indicate that the application of fungicides would produce uncertain results thus detailed analysis of risk factors would be required. To validate the scoring system, 19 field trials were carried out over five growing seasons in three Argentinian provinces. The fungicide used in all trials was a mixture of a quinone outside inhibitor and a demethylation inhibitor fungicide. In most cases, the scoring system resulted in appropriate decisions to apply the fungicide within the so-called “window of opportunity”, which lies between the R3 and R5 soybean developmental stages. The greatest yield responses were achieved when the scoring system recommended the fungicide application at R3 or R4 or R5, depending on the obtained sum of points. In all cases, except when the scoring system recommended no application of the fungicide, disease severity values were significantly greater in untreated than in treated plants. Regarding net income, phenology-based applications showed negative margins in cases where the scoring system recommended no applications, demonstrating that in such situations the use of fungicide caused losses rather than yield advantages. In contrast, when the scoring system recommended the application of fungicide regardless of timing (R3, R4 or R5), net margins were always positive and generally provided a higher income. The scoring system presented in this study can be a valuable tool to reduce the number of fungicide applications in soybean crops, especially in seasons when conditions for LSD development are not favorable.