Frogeye leaf spot

Editor’s note: The following was written by Daren Mueller, Iowa State University associate professor and Extension plant pathologist, and Ethan Stoetzer, communications specialist, for the university’s Integrated Crop Management News website Jan. 27.

Iowa State University researchers, with funding from the 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 leaf spot in soybeans in Iowa) are resistant to quinone outside inhibitor (QoI) fungicides.

Frogeye leaf spot  map

Researchers at Iowa State University Extension found that nearly all of the isolates tested from all 51 counties had some level of resistance to azoxystrobin.

Throughout the months of September and October 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.


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.