Frogeye Leaps Into Iowa

Fungicide-Resistant Frogeye Leaf Spot Moves Into the Midwest

Emily Unglesbee
By  Emily Unglesbee , DTN Staff Reporter
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Frogeye leaf spot resistance to QoI fungicides (strobilurins) is becoming widespread in the Midwest, as well as the South. (Photo courtesy MAFES/Sead Sabanadzovic)

ROCKVILLE, Md. (DTN) -- Midwesterners, it's time to add fungicide-resistant frogeye leaf spot to your list of soybean enemies.

Use pen, not pencil.

The disease recently added Iowa to its list of I-state conquests when scientists there discovered strains of the fungus resistant to QoI fungicides (quinone outside inhibitors), the most common of which are strobilurins.

QoI-fungicide resistance in frogeye leaf spot was first found in Tennessee eight years ago. By 2017, it had surfaced in more than a dozen states stretching from the Gulf of Mexico up to the southern shores of Lake Michigan and east to the Atlantic.

Crop rotation, frogeye leaf spot-resistant soybean varieties and fungicide pre-mixes with multiple modes of action can help Midwestern growers address this disease, plant pathologists told DTN.


Frogeye leaf spot attacks the leaves of a soybean plant especially during the plant's reproductive stages. The pathogen produces gray lesions with a reddish border on the upper leaves of the plant. It can lead to premature leaf drop and spread to pods and stems in severe situations.

The fungus can survive in soybean residue, where it can serve as a source of inoculum for subsequent soybean crops.

Crop rotation and soybean varieties with genetic resistance to frogeye leaf spot are your first and best lines of defense, said Mississippi State University plant pathologist Tom Allen.

Fungicide applications can also be effective, but overuse has led to this widespread failure of QoI fungicides against the disease.

It's hard to find a fungicide pre-mix that doesn't have a QoI fungicide in it, Mueller noted. That means if you have QoI-fungicide-resistant frogeye leaf spot in your field, a premix fungicide with two fungicides may only have a single effective mode of action.

Triazole fungicides (in the DMI class of fungicides) remain effective against the disease, as well as thiophanate methyl (in the MBC class of fungicide), Allen said.


Fungicide use in field crops like corn, soybean and wheat has increased significantly in the past decade, so this is not the last time growers are likely to see resistance develop, Mueller added.

Take time to learn the different fungicide classes, using the Fungicide Mode of Action Lookup tool from the Take Action website:…

"This is sort of the canary in the coal mine," Mueller said. Between grower practices and the pathogen that causes frogeye leaf spot, Cercospora sojina, "you couldn't have picked better traits for developing resistance," he explained.

QoI fungicides are extremely common in field crops, and using a single mode of action in applications used to be a common practice. Moreover, frogeye leaf spot is well equipped to develop resistance, thanks to its abundant genetic diversity.

A single-point mutation in the DNA of this pathogen has allowed it to tolerate QoI fungicides and thrive, Allen added. This unique trait accounts for all fungicide-resistant frogeye leaf spot strains known to researchers at this time, he said.

You can find more information on frogeye leaf spot from the Crop Protection Network here:…

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Emily Unglesbee