Quantification of Fungicide Resistance in Cercospora sojina Populations and Development of a Fungicide Application Decision Aid for Soybean in the Mid-Atlantic U.S.
Soybean is an important source of protein in animal feed, and growing demand for meat consumption worldwide has led to increased soybean production. Over 120 million metric tons of soybean were harvested in the United States in 2018, approximately one-third of the world production. In the Mid-Atlantic region, soybean is one of the most valuable field crops. Major foliar diseases that reduce soybean yield in the Mid-Atlantic region are frogeye leaf spot (FLS) and Cercospora leaf blight. In addition to crop rotation and host resistance, foliar fungicides, often with quinone outside inhibitor (QoI) active ingredients, are used to manage these soybean foliar diseases. Yield benefits of foliar fungicides have been inconsistent and this may be the result of low disease pressure, unfavorable environmental conditions for disease development, or the presence of fungal pathogen populations that have developed resistance to fungicides. The objectives of this research were 1) to develop a pyrosequencing-based assay to rapidly quantify QoI resistance frequencies in Cercospora sojina, the causal agent of FLS, 2) to examine the effects of fungicide application timings, disease pressure, and environmental factors on soybean yield, and 3) to develop a weather-based soybean foliar fungicide application decision aid for the Mid-Atlantic U.S. using a threshold decision rule. A pyrosequencing assay targeting the G143A mutation was designed, and a Virginia survey of C. sojina populations indicated that the G143A mutation conferring QoI resistance is widespread. In small plot fungicide application timing experiments, five weekly fungicide applications starting at beginning pod (R3) resulted in the greatest yield, but for single fungicide applications, R3 or 1 week after R3 resulted in the greatest yields. There was positive relationship between the cumulative number of disease favorable days (mean daily temperature 20-30°C and ≥ 10 hours of relative humidity >90%) from planting to R3 and disease severity at the full pod stage (r = 0.97, P = <0.01). Higher disease severity was associated with greater yield loss (r2 =0.53, P = 0.10) suggesting foliar fungicide applications are more likely to have yield benefits as the number of disease favorable days prior to R3 increase. A disease favorable-days threshold (FDT) using the environmental parameters indicated above was evaluated in on-farm experiments throughout Virginia, Maryland, and Delaware. Based on decision rules, FDT = 8 three weeks prior to R3 was the best predictor of a yield benefit with an R3 fungicide application. The decision aid was also able to correctly predict when a fungicide application would not be profitable ≥90% of the time. This weather-based decision aid along with monitoring of fungicide resistance development within the region will provide soybean growers in the Mid-Atlantic U.S. with tools to maximize yields and profitability.