Historically, foliar diseases have been a major threat to soybean producers in southern states, and that threat has been realized even more in the last 10 years, where losses due to foliar diseases had an estimated peak of nearly a $22 per acre loss in 2018 (Fig. 1). A similar trend also has been observed in northern states, where there was an estimated peak of nearly a $16 per acre loss in 2018 due to foliar diseases (data not shown, Crop Protection Network).

In response to the increasing threat of foliar diseases, an increase in foliar fungicide use in soybean has occurred the last several years in the U.S. Partly due to this increased use, some important pathogens have developed resistance to commonly used fungicides. Most notable is the frogeye leaf spot pathogen (Cercospora sojina), in which strobilurin fungicide-resistant strains have been detected in over 20 states, which span from the Canadian border to the Gulf coast to the Atlantic coast. In addition, pathogens that cause Cercospora leaf blight, Septoria brown spot, and target spot also have developed resistance to important foliar fungicides used on soybean.

While foliar diseases have become more important, marketing and advertising of foliar fungicides have also increased. Oftentimes, the marketing campaigns highlight the potential “yield increase” from the use of fungicides but may not focus much on actual disease management and how to effectively use the products as part of an integrated pest management program. In addition to the threat of plant pathogens becoming resistant to fungicides, resistance can spill over into human and animal health. An important human pathogen of health-compromised people, Aspergillus fumigatus, has developed resistance to triazole fungicides used in human health. Triazole fungicides are also an important group routinely used on soybean to manage foliar diseases. Recent research by Kang et al. (2021) indicated a potential association between triazole-resistant strains of A. fumigatus and use of triazole fungicides in agricultural settings. This topic was recently taken up by a committee of the National Academies of Sciences, Engineering, and Medicine and was discussed at a public workshop series on Antimicrobial Resistance in June 2022.

Potential implications of increased fungicide use over millions of crop acres are being discussed by the medical community and other groups outside of agriculture. Meanwhile, few tools have been developed to help soybean farmers make the most-informed foliar fungicide application decisions based on disease prediction. It is critically important that disease prediction tools that will help farmers make fungicide application decisions be developed and expanded. Such tools will benefit production and profitability of farms, while also reducing the potential risk of exposing both plant pathogens and human pathogens to fungicides that could lead to the selection of fungicide-resistant strains.

The Sporecaster prediction tool (Willbur et al. 2018), developed by the Damon Smith Laboratory at the University of Wisconsin through support from soybean checkoff funds, has been successful in helping farmers in northern states make fungicide application decisions for management of white mold (Sclerotinia stem rot). A recent project initiated in 2021 and funded by the North Central Soybean Research Program (NCSRP), led by Damon Smith (“Multidimensional approaches for improved productivity, sustainability, and management of major soybean diseases in the North Central U.S.”) is adapting the Sporecaster framework to develop and validate prediction tools for frogeye leaf spot. Our work on the current “Multi-Regional” project in years 1 and 2, has successfully expanded the NCSRP project to the Southern, Midsouthern, and Atlantic regions, and provides an opportunity for soybean pathologists across the nation to collaborate with each other and develop and validate tools for prediction of frogeye leaf spot as well as Cercospora leaf blight and target spot. A released beta version of the “Frogspotter” app for frogeye leaf spot prediction showed promising results in field validation trials across the U.S. in the 2023 and 2024 growing seasons. Ultimately, these tools will provide validated resources that farmers across the U.S. can utilize to make important disease management decisions.

Overall Goal: To develop improved management strategies, and communication of those strategies, for the sustainable management of soybean diseases that affect multiple production regions across the U.S.
Objective 1. Expand the development and validation of frogeye leaf spot prediction tool to new regions, using the Sporecaster framework
Objective 2. Develop and conduct experiments to adapt the Sporecaster framework for Cercospora leaf blight prediction
Objective 3. Develop and conduct experiments to adapt the Sporecaster framework for target spot prediction
Objective 4. Communicate results of the research to farmers and other stakeholders involved with soybean production

• Peer-reviewed publications that detail the development of prediction models and tools for Cercospora leaf blight, frogeye leaf spot, and target spot
• Peer-reviewed publication that reports on the most efficacious and economically viable fungicides for management of Cercospora leaf blight
• Peer-reviewed publication that reports on the most efficacious and economically viable fungicides for management of target spot
• Peer-reviewed publication that detail the geographical and temporal distribution of spores of important foliar fungal pathogens of soybean that will improve our epidemiological and biological understanding of these pathogens
• Updated foliar disease information on SRIN
• Updated Extension publication on Frogeye Leaf Spot and new Extension publications on Cercospora leaf blight and target spot

Updated July 31, 2025:
Uniform foliar fungicide trials have been planted at 15 locations across 11 states (AL, AR, DE, KY, LA, MS, NC, PA, TN, VA, WI). Growth stages at these locations range from V4 to R3. Due to frequent spring rains, many of the locations were planted later than normal. Frogeye leaf spot has been observed at many of the locations already, which should help provide for excellent fungicide efficacy comparisons among products. At 13 locations, spore traps have been deployed at two different heights within a “no fungicide” area of the field. The spores are being collected weekly throughout the season and are being analyzed via molecular quantification assays at the University of Wisconsin (Damon Smith Lab) and at Louisiana State University (Sara Thomas-Sharma Lab). Assays quantifying the frogeye leaf spot pathogen (Cercospora sojina) and the pathogens that cause Cercospora leaf blight (C. flagellaris C. sigesbeckiae, and C. kikuchii) from spore trap samples collected during the 2024 season are nearing completion. The data from these assays are being used to validate disease forecasting systems and to better understand the basic epidemiology of these important soybean diseases. The frogeye leaf spot forecasting system is now publicly available for farmers and others to use, as part of the Crop Protection Network’s Crop Disease Forecasting tool. The frogeye leaf spot forecasting system will help soybean farmers make critical soybean disease management decisions within the season and is the first of its kind to be publicly available. So far, this has been the largest success achieved by this project.

It is critically important that disease prediction tools that will help farmers make fungicide application decisions be developed and expanded. Such tools will benefit production and profitability of farms, while also reducing the potential risk of exposing both plant pathogens and human pathogens to fungicides that could lead to the selection of fungicide-resistant strains.