Frogeye Leaf Spot Fungicide Resistance Confirmed in Iowa Soybean

Researchers in the Iowa State University and University of Kentucky Departments of Plant Pathology have confirmed that isolates of Cercospora sojina, the pathogen that causes frogeye leaf spot of soybean, found in Iowa have shown resistance to quinone outside inhibitor (QoI, strobilurin) fungicides.

Frogeye leaf spot occurs across the United States and significant yield loss can occur when this disease is widespread within a soybean field. Plant pathologists estimate this disease was responsible for more than 17.5 million bushels of lost yield in 2015 across the U.S., with a value of $158.1 million. One method of frogeye leaf spot management and subsequent yield protection has been application of foliar fungicide during pod development. However, overuse or misuse of fungicides can result in decreased management efficacy if targeted pathogens acquire resistance to a fungicide.

“This type of fungicide resistance has been found in neighboring states, so it isn’t surprising that it was found in Iowa,” said Daren Mueller, associate professor and extension plant pathologist at Iowa State. “This pathogen population has a lot of variation, which increases the chance we can select for resistance.”

The pathogen that causes frogeye leaf spot is genetically diverse, which is an important reason why fungicide resistance has occurred. C. sojina resistance to QoI fungicide was first documented in Tennessee in 2010 and has been detected in several other states since then. Isolates of C. sojina were sampled from multiple locations across the state and were tested for resistance using established laboratory protocols at the University of Kentucky. All samples evaluated were found to be resistant. The C. sojina isolates from Iowa in 2017 are the first reported to have QoI fungicide resistance in the state.

“An integrated method of management that doesn’t rely solely on fungicides should be employed,” said Carl Bradley, extension plant pathologist at the University of Kentucky. “Farmers should consider other disease management practices such as crop rotation, planting of frogeye leaf spot-resistant cultivars and applying fungicides with multiple modes of action.”

Fungicides are very important for disease management, and it is critical to preserve the usefulness of these crop protection tools. Fields should be scouted approximately two weeks after fungicide application to determine if the fungicide is working. If you believe fungicide resistance may be an issue in your field, contact an Iowa State University Extension and Outreach field agronomist. For resources on fungicide resistance, visit the Take Action website (

Funding for this research has been provided by the Soybean Checkoff through the Iowa Soybean Association and United Soybean Board.

Source: Iowa State University

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