Farmers look to fight fungicide resistance

While there has only been one documented soybean field in Missouri found to show frogeye leaf spot resistance to strobilurin fungicide, Allen Wrather says farmers should be on the lookout for more.

Farmers look to fight fungicide resistance

While there has only been one documented soybean field in Missouri found to show frogeye leaf spot resistance to strobilurin fungicide, Allen Wrather says farmers should be on the lookout for more.

“It is reasonable to think that it is occurring in multiple fields in southeast Missouri,” the University of Missouri plant scientist says. “And we do not know how much further north it has spread.”

Wrather says it will take a formal study to determine just how much resistance is in farmers’ fields. Frogeye leaf spot resistant to widely used strobilurin fungicide has been reported in southern Illinois, Kentucky, Tennessee, Louisiana and Missouri.

According to Wrather, a crop consultant working in southeast Missouri examined a soybean field last September that had many plants infected with frogeye leaf spot. The field had previously been treated with a strobilurin fungicide. While the fungicide should have stopped development of frogeye, it had not.

The affected soybean leaves were collected and sent for testing to Carl Bradley, a professor at the University of Illinois.

Key Points

• State’s first case of strobilurin-resistant frogeye was found in soybeans in 2011.

• “The sky isn’t falling,” says Melvin Newman. Other treatments are available.

• Strobilurin still provides a broad spectrum of protection for soybeans.


“He [Bradley] isolated the pathogen from 10 different leaves and tested each for sensitivity to strobilurin fungicide,” Wrather explains. “Six of the leaves were resistant to strobilurin fungicide and four were sensitive to strobilurin fungicides.”

Wrather says farmers and consultants should observe soybean fields that have been treated with a strobilurin fungicide to determine if frogeye leaf spot is present and still spreading. If so, plants in the field could be treated with a fungicide other than strobilurin to stop the spread of this disease.

“Producers and consultants should be vigilant,” Wrather says. “If they find a field treated with strobilurin and two to three weeks later it [frogeye leaf spot] is developing rapidly, they should test it. Then plan to use other products in the future.”

Managing resistance

The first documented case of strobilurin-resistant frogeye was found in soybeans in Tennessee in summer 2010, says Melvin Newman, a plant pathologist with the University of Tennessee Extension. The frogeye disease in general is Tennessee’s No. 1 soybean disease, causing 5% to 10% yield losses each year.

Strobilurin-based fungicides were “a near miracle on soybean diseases for a long time, 10 years or more,” Newman says. “And I’m surprised we were able to keep resistance away for as long as we did. … But the sky isn’t falling. We have other tools we can use to treat this.”

Growers who suspect they have strobilurin-resistant frogeye should:

n Plant a frogeye-resistant soybean variety. “I’ve found through my evaluations that varieties that are resistant to normal frogeye hold up that resistance to the strobilurin-resistant frogeye, which is a good thing,” Newman says.

n Consider using another mode of action, such as a triazole-based product, “which works very well against frogeye,” he says.

n Rotate away from soybeans “for at least two years, and that will get you a long way down the road against this,” he says.

Use the right mix

But even with strobilurin resistance, you don’t have to throw the baby out with the bath water, so to speak.

“Resistance is always an important issue for all of our active ingredients, but even when you have some resistance, it doesn’t spell the end of the day for the product [in this case, strobilurin],” says Eric Tedford, Syngenta fungicide asset leader.

Strobilurin still provides a broad spectrum of protection for soybeans from other diseases, he says. In the last few years, the company has concentrated and championed the advantage of mixed products, or using more than one mode of action to protect soybeans; in this case, mixing strobilurin-based products with a triazole.

The key, Tedford says, is to find the right triazole and get the right mix. In response to the strobilurin-resistant frogeye, the company now markets Quilt Xcel, which matches azoxystrobin with propiconazole, and Quadris Top, which pairs azoxystrobin with difenoconazole.

Wise growers don’t put all their hopes into one practice or tool to do the job, Tedford says. Resistant varieties and cultural practices also need to be used to fight diseases. For example, frogeye can over-winter in field residue or stubble, and removing debris from the field each year can help limit overwintering of the disease and limit its emergence in early summer.

“It is important to us to keep a product around as long as possible. It costs a lot of money to develop products, and it’s expensive to find new active ingredients that work for growers. That’s why we need to stay on top of resistance issues and do what we can to reduce it and the spread,” Tedford says.

1842.tifRESISTANT: Farmers should inspect fields where frogeye leaf spot continues to thrive after fungicide treatments for potential resistance.

This article published in the June, 2012 edition of MISSOURI RURALIST.

All rights reserved. Copyright Farm Progress Cos. 2012.

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