United Soybean Board Director Johnny Dodson, a soybean farmer from Halls, Tennessee, has felt the effects of soybean disease fungicide resistance in his soybean fields.

“I have experienced loss of yield and quality to fungicide resistance,” Dodson says. “It has forced me to select varieties that might not be the highest yielding in ideal growing conditions but will perform well when disease pressure is high.”

At the same time that many U.S. soybean farmers are battling herbicide-resistant weeds, now they have another issue for which their previous management methods may no longer work.

Carl Bradley, Ph.D., plant pathologist at the University of Illinois, says soybean farmers and agriculture in general could be facing a big obstacle.

“If fungicide resistance becomes more widespread, we will lose an important tool in disease management,” he says.

As soybean prices have increased, so has the use of fungicides to increase yields and manage fungal diseases, such as anthracnose, Septoria brown spot, Cercospora leaf blight, frogeye leaf spot, pod and stem blight and soybean rust.

Before using a foliar fungicide, Bradley advises farmers to scout and determine the type of disease present to help determine which products to use.

Farmers can proactively implement best-management practices now to curtail fungicide resistance, including:

Become aware of common diseases. If farmers see frogeye leaf spot on their farm year after year, then management of that disease should become more of a priority. Integrating different practices for disease management will result in the highest level of disease control.

Choose a resistant variety. Bradley says this is the most important decision a farmer can make in managing a disease. If the farmer decides to apply a foliar fungicide, then it is important to use a product that contains multiple active ingredients (or tank mix products) from different chemistry classes.

Crop rotation is a disease-management practice that most farmers utilize; however, some continuous-soybean fields do exist. In fact, the first case of fungicide-resistant frogeye leaf spot documented was from a field that had been in continuous soybean for several years, according to Bradley.

The Fungicide Resistance Action Committee website contains basic information about fungicide resistance.

The same principles that apply to herbicide-resistance in weeds and insecticide resistance in insects apply to fungicide resistance. Naturally occurring mutations in pathogen populations create individual fungicide-resistant isolates. These are a small portion of the pathogen population at first, but multiple applications of a fungicide year after year will begin to select out these individual mutants until they become the majority.