Frogeye Resistance Leaps

Midwesterners, it’s time to add fungicide-resistant frogeye leafspot to your list of soybean enemies.

Use pen, not pencil.

Scientists in Illinois, Indiana and Iowa have now confirmed strains of the fungus resistant to a class of fungicides called QoI (quinone outside inhibitors), known to most farmers as strobilurin fungicides.

QoI fungicide resistance in frogeye leafspot 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-leafspot-resistant soybean varieties and fungicide premixes with multiple modes of action can help Midwestern growers address this disease, plant pathologists say.

Choose Carefully. Frogeye leafspot 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 leafspot are your first and best lines of defense, says Mississippi State University plant pathologist Tom Allen.

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

It’s hard to find a fungicide premix that doesn’t have a QoI fungicide in it, Iowa State University plant pathologist Daren Mueller notes. That means if you have QoI-fungicide-resistant frogeye leafspot 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 fungicides), Allen says.

Resistance On The Rise. 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 will see resistance develop, Mueller says.

Take time to learn the different fungicide classes, using the Fungicide Mode of Action Lookup Tool from the Take Action website: bit.ly/2oQdJwL.

“This is sort of the canary in the coal mine,” Mueller says. Between grower practices and the pathogen that causes frogeye leafspot, Cercospora sojina, “you couldn’t have picked better traits for developing resistance,” he explains.

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 leafspot 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 says. This unique trait accounts for all fungicide-resistant frogeye leafspot strains known to researchers at this time.

You can find more information on frogeye leafspot from the Crop Protection Network at bit.ly/2D8p67k.

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