Instances of Frogeye Leaf Spot Resistance to QoIs Abundant in Iowa

January 27, 2020
ICM News

Iowa State University researchers, with funding from soybean checkoff through the United Soybean Board and Iowa Soybean Association, have confirmed that over 70 isolates of the pathogen Cercospora sojina (cause of frogeye leafspot in soybeans in Iowa) are resistant to quinone outside inhibitor (QoI) fungicides.

Experimental design

Throughout the months of September and October of 2019, Iowa State extension plant pathologists collected soybean leaves displaying symptoms (small round lesions with dark reddish-brown borders) of frogeye leaf spot across 73 soybean fields, spanning 51 counties. Fungal spores were collected from each leaf lesion and isolated for a strain of the C. sojina pathogen. One isolate from each of the 73 fields was tested for sensitivity to azoxystrobin, a QoI fungicide. Researchers compared these test results with two control groups of C. sojina isolates with known sensitivity to azoxystrobin.

frogeye resistance map of counties in Iowa


Resistance to azoxystrobin was classified as fungi germinating in the presence of 1 parts per million (ppm) of azoxystrobin. Researchers found that nearly all of the isolates tested from all 51 counties had some level of resistance to azoxystrobin, having higher than a 50% germination rate (a single field in Adair County did not have such a high rate). In fact, most isolates were able to germinate in 10 ppm of azoxystrobin.

What does this mean?

As azoxystrobin is part of the QoI class of fungicides (FRAC Code 11), it’s important to know that the C. sojina pathogen’s isolates are most likely resistant to other fungicides within that same class, as resistance to QoIs is often the result of a single gene/single site mutation, most commonly the G143A mutation that occurs at the fungal cytochrome b gene.

Frogeye leaf spot resistance to QoIs was confirmed in Iowa back in the 2018 growing season by the Mueller Lab. The continued development of resistance among the pathogen’s different isolates illustrates that using QoIs as the primary control of frogeye leaf spot is no longer a solution to control the disease.

QoI-resistant strains can still be managed effectively with other fungicide groups, but introducing alternative disease management practices will be even more important to preserve future use of these fungicides. Selecting a frogeye leaf spot-resistance cultivar and incorporating crop rotation with non-host crops in to an operation can provide better control of the disease. 

For more information about the frogeye pathogen, disease development, and the mechanisms of fungicide, please review our previous article on the subject.


Links to this article are strongly encouraged, and this article may be republished without further permission if published as written and if credit is given to the author, Integrated Crop Management News, and Iowa State University Extension and Outreach. If this article is to be used in any other manner, permission from the author is required. This article was originally published on January 27, 2020. The information contained within may not be the most current and accurate depending on when it is accessed.


Daren Mueller Professor

Daren Mueller is an associate professor and extension plant pathologist at Iowa State University. He is also the coordinator of the Iowa State Integrated Pest Management (IPM) program. Daren received his bachelor's degree from the University of Wisconsin-Madison in 1996, and his master's degree a...