If he had to choose a plant pathogen to develop resistance to strobilurin fungicides, Carl Bradley says the frogeye leaf spot fungus would have been one of his top choices.

First off, the University of Illinois plant pathologist notes the disease doesn't travel very far. Unlike soybean rust, it tends to remain in a localized area.

"The other good news is there are soybean varieties available that are resistant to frogeye leaf spot," Bradley adds.

Strobilurin Resistant Frog-Eye Leaf Spot Now In Five States

Still, that doesn't change the fact that strobilurin-resistant strains of the frogeye leaf spot fungus, Cercospora sojina, have been confirmed in five states: Illinois, Kentucky, Missouri, Tennessee and Louisiana. In Illinois, resistant strains were found in Gallatin and Pope Counties in both 2010 and 2011. The first instance of strobilurin resistant frogeye leaf spot was found in Tennessee in 2010.

With its mode of action targeting a single site, Bradley says scientists have known for a while that there is a high risk of selecting for stobilurin-resistant strains of fungi.

Resistance prevention

Frogeye leaf spot typically is not a significant threat for most of Illinois. Still, Bradley says farmers need to be proactive in deploying practices that slow down fungicide resistance development in other pathogens of soybean and corn.

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"Fungicide resistance hasn't been a major problem in field crops like soybeans until now," he adds. "I have a feeling we may be finding more problems like this in the future if we are not careful how we use these products."

Bradley advocates fungicide applications only in instances where risk of disease or disease observations warrant them. In addition, Bradley recommends adding a fungicide with another mode of action to help slow down the selection of fungicide resistant strains.

Adding to the mix

When adding a mode of action, most immediately think of triazole fungicides. Possibly in time for use this season, BASF hopes to release another mode of action: carboxamide. According to Nick Fassler, BASF's technical marketing manager for row crop fungicides, the new active ingredient will be called Xemium. It will be paired with Headline. The new brand will be called Priaxor.

"Xemium really compliments Headline well," Fassler adds.

Fassler says crop rotation and tillage can also help manage soybeans diseases and decrease the chance that strobilurin fungicides will lose their effectiveness.

Of course, mixing a triazole with a strobilurin is another tactic for staving off resistance.

According to Syngenta's fungicide asset leader Eric Tedford, the key 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 puts Azoxystrobin with Difenoconazole.

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A wise grower doesn't put all his 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 the debris from the field each year can help limit over-wintering 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 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 notes.

Southern Farmer editor Brad Haire contributed to this report. Southern Farmer is a sister publication of Prairie Farmer.