frogeye leaf spot  on a leaf
SYMPTOMS: Looking like its name, frogeye leaf spot is a common foliar disease in Iowa, appearing as gray lesions surrounded by a reddish-brown border on soybean leaves.

Fungicide resistance confirmed in Iowa soybeans

Frogeye leaf spot, a common soybean leaf disease, is showing resistance to fungicides.

Plant pathologists at Iowa State University and University of Kentucky have confirmed the pathogen that causes frogeye leaf spot, a common soybean disease, has shown resistance to fungicides in Iowa. The types of fungicides frogeye are resistant to are quinone outside inhibitor (QoI, strobilurin).

Frogeye leaf spot occurs across the U.S., and significant yield loss can occur when this disease is widespread within a soybean field, says Daren Mueller, ISU Extension plant pathologist. Plant pathologists estimate this disease was responsible for more than 17.5 million bushels of lost yield, valued at $158.1 million, across the U.S. in 2015. 

The pathogen that causes frogeye leaf spot, Cercospora sojina, is genetically diverse, which is an important reason why fungicide resistance can occur. This genetic variation increases the chance of selecting for resistance, Mueller says. He says C. sojina resistance to QoI fungicide was first documented in Tennessee in 2010 and has been detected in several other states since then, including those that border Iowa.

To track down this disease in Iowa, isolates of C. sojina were sampled from multiple locations across the state in 2017 and were tested for resistance using established laboratory protocols at the University of Kentucky. All samples evaluated were found to be resistant to the fungicides. The C. sojina isolates from Iowa in 2017 are the first reported to have QoI fungicide resistance in the state.

Tips on managing frogeye leaf spot
One method of frogeye leaf spot management and subsequent yield protection has been the application of foliar fungicides during soybean pod development. However, overuse or misuse of fungicides can result in decreased effectiveness if targeted pathogens acquire resistance to a fungicide.

An integrated method of frogeye leaf spot management that does not rely solely on fungicides should be employed, advises Mueller. Farmers should consider other disease management practices, such as crop rotation, planting of frogeye leaf spot-resistant soybean varieties and applying fungicides with multiple modes of action.

Fungicides are essential to manage disease, and it is critical to preserve the usefulness of these crop protection tools, Mueller says. Fields should be scouted about 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 specialist. For resources on fungicide resistance, visit the Take Action website.

Funding for this research was provided by the soybean checkoff through the Iowa Soybean Association and United Soybean Board.

Source: Iowa State University

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