Isolates of Cercospora sojina, causal agent of frogeye leaf spot of soybean (Glycine max), were collected across Alabama, Arkansas, Delaware, Illinois, Indiana, Iowa, Kentucky, Louisiana, Mississippi, Missouri, North Carolina, Ohio, Tennessee, and Virginia and were evaluated for quinone outside inhibitor (QoI) fungicide resistance. Collection of these isolates from these 14 states occurred between 2010 and 2017. QoI fungicide-resistant C. sojina isolates were detected in all 14 states surveyed and represent a total of 240 counties or parishes. In 2017, these 240 counties and parishes represented approximately 13% of the harvested soybean hectares in the United States. In light of this widespread occurrence of QoI fungicide-resistant C. sojina isolates, management of frogeye leaf spot should focus on integrated management practices such as planting resistant soybean cultivars, rotating with nonhost crops, and tilling to speed up decomposition of infested soybean residue. When foliar fungicide application is warranted, fungicide products that contain active ingredients from chemistry classes other than the QoI class should be applied for frogeye leaf spot management.

Resistance to quinone outside inhibitor (QoI) fungicides was detected in Cercospora sojina (causal agent of frogeye leaf spot) isolates collected from soybean (Glycine max) fields in four South Dakota counties during the 2018 growing season. A discriminatory dose assay was used to detect QoI-resistant isolates, and a follow-up polymerase chain reaction assay was used to determine the presence of the G143A mutation in QoI-resistant isolates. This is the first report of resistance to QoI fungicides in C. sojina isolates from South Dakota.

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Frogeye leaf spot, caused by Cercospora sojina, is an important disease of soybean (Glycine max) in the United States. An important tactic to manage frogeye leaf spot is to apply foliar fungicides. Isolates of C. sojina were collected from soybean fields in one county in Michigan, three counties in Minnesota, and 10 counties in Nebraska in 2019, and they were tested for resistance to quinone outside inhibitor (QoI) fungicides using a discriminatory dose assay, a PCR assay, and DNA sequencing. Results of the testing indicated that QoI fungicide-resistant isolates were detected in isolates from all counties. Testing results also indicated that the G143A mutation was responsible for the QoI fungicide resistance. This is the first report of QoI fungicide-resistant C. sojina isolates in Michigan, Minnesota, and Nebraska and expands the geographical distribution of QoI fungicide-resistant C. sojina isolates to 18 states in total.

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