Research conducted by the University of Illinois and the University of Tennessee confirms that the fungus that causes frogeye leaf spot (FLS) of soybeans, Cercospora sojina, has shown resistance to strobilurin fungicides in a Tennessee soybean field.
“Strobilurin fungicides belong to the chemistry class known as the quinone outside inhibitors (QoIs), which are the most widely used group of foliar fungicides applied to field crops to manage plant diseases,” said Carl Bradley, U of I Extension plant pathologist.
These fungicides can be sold as one-active ingredient products such as Headline (BASF Corporation) or Quadris (Syngenta Crop Protection) or in products that combine them with a fungicide in a different chemistry class known as the demethylation inhibitors, sometimes referred to as triazoles, he said. Products that include a strobilurin-triazole combination of active ingredients include Quilt (Syngenta Crop Protection) and Stratego (Bayer CropScience).
Strobilurin fungicides have been deemed high risk for fungal pathogens developing resistance to them. This high-risk status has been determined by the Fungicide Resistance Action Committee (FRAC), an international committee that evaluates fungicides’ likelihood of developing resistance.
“Plant pathogenic fungi developing resistance to strobilurin fungicides is not new,” Bradley said. “This has already occurred in potatoes and other crop and disease systems where multiple fungicide applications occur during the growing season.”
In the major soybean production areas in the United States, soybean fields are generally treated once during the season with a fungicide (if treated at all), Bradley said.
“However, we were somewhat surprised to find resistance so soon,” he added. “Every time you apply a fungicide, you increase the selection pressure and the opportunity to select out individuals in the pathogen population that have resistance or reduced sensitivity to the fungicide.”
In 2008, Bradley’s laboratory began a project funded by the Illinois Soybean Association to develop a fungicide resistance monitoring program. Since then, his lab has been obtaining samples, conducting tests and monitoring isolates collected from Illinois.
“This year, we decided to cast our net a little farther, particularly in the South,” he said. “In Tennessee, FLS is a major soybean foliar disease. Melvin Newman of the University of Tennessee sent me samples from a field that had been sprayed twice with strobilurin fungicides, but still continued to have high levels of FLS, which was an indication of potential fungicide resistance.”
Bradley’s team confirmed that the sensitivity of the Tennessee isolates was reduced as compared to the sensitivity of baseline isolates.
In petri dish tests conducted at the U of I, spores from isolates of Cercospora sojinagerminated in the presence of high concentrations of azoxystrobin, pyraclostrobin, and trifloxystrobin, which are the strobilurin active ingredients found in Quadris, Headline, and Stratego.
“This proved we were dealing with isolates that have reduced sensitivity to strobilurin fungicides,” he said. “Currently, Tennessee is the only state in which we have documented isolates like these, but we are continuing to perform tests on isolates collected from fields in Illinois and other states.”
U of I’s research will continue into the 2011 season with funding from the Illinois Soybean Association.
In the meantime, Bradley reminds growers that FLS can be controlled with other management tactics such as planting soybean varieties that have high levels of resistance to FLS or using effective triazole fungicides.
“Newman’s work has shown that some triazole fungicides provide good control of FLS and can be used alone or tank-mixed with strobilurin fungicides if the grower is concerned with more than just FLS,” he said.
The most effective manner to slow the spread of resistant isolates is to only use a fungicide when needed.
“If we overuse fungicide products, we won’t be able to use them for very long because we will select out resistant populations,” he said. “There’s a lot of marketing to use fungicides for yield increases, but little talk about where those increases come from. They come from protection of yield from diseases. In some cases they pay off because conditions have been favorable for diseases. But in years where conditions aren’t favorable for disease, we generally don’t see a big yield increase.”
Bradley’s crew is expanding their work in monitoring fungicide resistance in pathogens of corn. They are currently developing strobilurin sensitivity baselines for the gray leaf spot and northern corn leaf blight pathogens.