Fungicide-resistant strains of the pathogen that causes frogeye leaf spot was first found in Iowa in 2017 and Septoria brown spot in 2018. These findings have changed the efficacy of fungicide products used in the Midwest. We conducted a state-wide survey of strains of the fungus in 2019 and found that EVERY isolate of the pathogen was resistant to the QoI (sometimes called strobilurins) fungicides. Similar findings have been reported in several other Midwestern states. This confirms that the resistant strain of the pathogen is widespread and this should affect our decision on which fungicides to use moving forward. One place to gather information on fungicide efficacy is the Crop Protection Network webpage.
Soil Science Extension
University of Wisconsin Madison