With only one or two applications of a strobilurin fungicide per growing season in corn and soybeans, some fungal diseases have begun to develop levels of resistance to strobilurin mode-of-action fungicides. The first resistance was recognized in the Mid-South and South, but there is concern of fungicide resistance spreading across the Corn Belt unless steps are taken to restrict the problem.

Frogeye leaf spot is the first culprit resistant to strobilurin mode-of-action fungicides in soybean production confirmed in a few states after first identified in Tennessee in 2010. Controlling resistant frogeye leaf spot and other diseases must entail using more than one mode-of-action fungicide.

Countering disease resistance needs dual-mode fungicide“I’m concerned and many agronomists are concerned that continuous use practices can lead to resistance unless we use a more integrated approach. Disease resistance is not going to be the end of the world, but we are going to need to do more than using a strobilurin fungicide alone,” said Matt Harbur, Ph.D., Trupointe, resource agronomist for the Ohio-based cooperative.

“Just as we’re using another herbicide mixed with Roundup for resistant weeds, we need to use the same concept with fungicides. What we are seeing is major fungicide producers of strobilurin fungicides mixing that fungicide with another class of fungicides to introduce a second mode of action,” he added.

Harbur continues to recommend dual-mode of action fungicide applications for plant health reasons. But additionally an integrated approach for preventing resistance by Trupointe agronomists includes helping the soybean and corn plants produce their own in-plant defense chemicals by applying liquid fertilizer—monopotassium phosphate or urea phosphite—in furrow or foliar. University research that proves in-plant defense activity was first published years ago, said Harbur.

A vigorously growing plant’s natural defenses are more immune to pest resistance occurring because the plant produces several defense compounds on its own to fight off the disease fungi, Harbur said. Trupointe’s Progressive Crop Technology fertilizers are thought to stimulate the plant’s production of more natural defense compounds.

“There always is an educational aspect that has to occur when you have an occurrence such as frogeye leaf spot resistance,” said Alan Hopkins, Bayer CropScience seed technology manager located in Tennessee. “It is important that growers be educated on the use of two fungicide modes of action to help them mitigate resistance to fungicides and help them control those diseases that have or might show resistance in their fields.”

The set up for resistance is because strobilurin fungicides have been used on so many acres continuously (corn and soybeans) and because this class of fungicides is so effective on a wide spectrum of diseases.

One of the advantages Hopkins notes to Stratego YLD fungicide is that it includes two fungicide modes of action that ag professionals have now recognized as necessary to minimize potential resistance. It contains a strobilurin and a triazole fungicide.

“The strobilurin component is very effective against a large number of diseases, and you have the triazole component that takes care of the frogeye leaf spot that may be resistant to strobilurins. You are also at the same time diversifying the fungicide exposure. Using the combination of modes of action in Stratego YLD can help delay the onset of resistance to either fungicide component when used together.”

He concluded by saying, “There is a whole complex of diseases that may not get much attention, but the fact is if you can keep the soybean plant clean, healthy and productive, it is going to have a whole lot more capacity to use its photosynthesis to produce higher yields than if the crop is suffering from even a low level of disease.”

Hopkins can be reached at alan.hopkins@bayer.com for more information.