Cercospora sojina Isolates from Kansas Soybean Fields with the G143A Mutation Conferring Resistance to Quinone Outside Inhibitor Fungicides

Frogeye leaf spot, caused by Cercospora sojina, is one of the most important foliar diseases of soybean in the United States. Isolates of C. sojina with resistance to quinone outside inhibitor (QoI) fungicides were first identified in 2010 from Illinois, Kentucky, and Tennessee soybean fields and since have been found across multiple soybean-producing states. Resistance to QoI fungicides has been due to a mutation in the cytochrome b gene of C. sojina that leads to a substitution of glycine with alanine at position 143 (G143A mutation). Soybean leaf samples with symptoms of frogeye leaf spot were collected from soybean fields across multiple counties in Kansas during the 2022 growing season. Isolates of C. sojina from these soybean leaf samples were tested for the presence of the G143A mutation. The G143A mutation was found in isolates collected from soybean fields in seven Kansas counties, with a total frequency of 84.7% of the isolates having the G143A mutation for QoI fungicide resistance.