Research conducted by the University of Illinois and the University of Tennessee confirms that the fungus that causes frogeye leaf spot (FLS) of soybeans, Cercospora sojina, has shown resistance to strobilurin fungicides in a Tennessee soybean field.

“Strobilurin fungicides belong to the chemistry class known as the quinone outside inhibitors (QoIs), which are the most widely used group of foliar fungicides applied to field crops to manage plant diseases,” said Carl Bradley, U of I Extension plant pathologist. 

These fungicides can be sold as one-active ingredient products such as Headline (BASF Corporation) or Quadris (Syngenta Crop Protection) or in products that combine them with a fungicide in a different chemistry class known as the demethylation inhibitors, sometimes referred to as triazoles, he said. Products that include a strobilurin-triazole combination of active ingredients include Quilt (Syngenta Crop Protection) and Stratego (Bayer CropScience).

Strobilurin fungicides have been deemed high risk for fungal pathogens developing resistance to them. This high-risk status has been determined by the Fungicide Resistance Action Committee (FRAC), an international committee that evaluates fungicides’ likelihood of developing resistance.

“Plant pathogenic fungi developing resistance to strobilurin fungicides is not new,” Bradley said. “This has already occurred in potatoes and other crop and disease systems where multiple fungicide applications occur during the growing season.”

In the major soybean production areas in the United States, soybean fields are generally treated once during the season with a fungicide (if treated at all), Bradley said.