Performance and profitability of rain-based thresholds for timing fungicide applications in soybean rust control

Abstract Soybean rust (SBR), caused by the fungus Phakopsora pachyrhizi, is the most damaging disease of soybean in Brazil. Effective management is achieved by means of calendar-timed sprays of fungicide mixtures, which do not explicitly consider weather-associated disease risk. Two rainfall-based action thresholds of Disease Severity Values (DSV50 and DSV80) were proposed and compared with two leaf wetness duration-temperature thresholds of Daily Values of Infection Probability (DVIP6 and DVIP9) and with a calendar (CAL) program, with regards to performance and profitability. An unsprayed check treatment plot was included for calculating relative control. Disease severity and yield data were obtained from 29 experiments conducted at six sites across four states in Brazil during 2012-13, 2014-15 and 2015-16 growing seasons, which represented different growing regions and climatic conditions. The less conservative rainfall action threshold (DSV80) resulted in fewer fungicide sprays compared with the other treatments and the more conservative one (DSV50) resulted in fewer sprays than the DVIP thresholds. Yield was generally higher with the increase of spray number, but the economic analysis showed no significant differences on the risk of not offsetting the costs of fungicide sprays regardless of the system. Therefore, based on the simplicity and the profitability of the rain-based model, the system is a good candidate for incorporating into management of SBR in soybean production fields in Brazil