No. 23 Article 11/October 8, 2010

Frogeye Leaf Spot Pathogen with Reduced Sensitivity to Fungicides Found in Tennessee Soybean Field

Since 2008, the Illinois Soybean Association has been funding a project in my lab to monitor for fungicide-resistant Cercospora sojina, the causal agent of frogeye leaf spot of soybean. The research has focused on fungicides in the quinone outside inhibitor (QoI) class of fungicides, generally referred to as "strobilurins." Strobilurin fungicide active ingredients registered for use on soybean include azoxystrobin, found in Quadris and Quilt (Syngenta Crop Protection); pyraclostrobin, found in Headline (BASF Corporation); trifloxystrobin, found in Stratego (Bayer CropScience); and fluoxastrobin, found in Evito (Arysta LifeScience).

At the start of the project, sensitivities of "baseline" isolates of Cercospora sojina to azoxystrobin, pyraclostrobin, and trifloxystrobin were determined using petri dish assays that measure inhibition of spore germination. These baseline isolates came from a historical collection of isolates that were all collected before strobilurin fungicides were registered on soybean, and had thus never been exposed to strobilurin fungicides.

The next steps included collecting and testing isolates of Cercospora sojina from commercial soybean fields and research plots where strobilurin fungicides had been applied. The project had focused primarily on Cercospora sojina isolates collected from Illinois. As better and more efficient lab methodologies were developed, we expanded the project and this year requested isolates from colleagues in other states.

Dr. Melvin Newman of the University of Tennessee sent us soybean leaves with frogeye leaf spot from his state. Some of the leaves came from a field where strobilurin fungicides had been applied twice, but the field continued to have severe frogeye leaf spot. Isolates from that field were obtained and tested using the petri dish spore germination assays. The assay results indicated that spores from these isolates germinated at high concentrations of azoxystrobin, pyraclostrobin, and trifloxystrobin. It took approximately 200 to 7,000 times higher fungicide concentrations to achieve spore germination inhibition with these isolates compared to the "baseline" isolates.

What Are the Implications of These Findings?

So far, the only Cercospora sojina isolates confirmed to have reduced sensitivity to strobilurin fungicides have come from this single field in Tennessee. However, this does not mean that similar isolates are not elsewhere. In light of our findings, consider these recommendations for managing frogeye leaf spot:

1. Plant soybean varieties resistant to frogeye leaf spot. This tactic is the best way to manage the disease. Resistant varieties are available for Illinois growers. Check with your seed dealer and the Illinois Varietal Information Program for Soybeans (VIPS).
2. If you plant a frogeye leaf spot-susceptible variety and are considering application of a fungicide, apply an effective triazole fungicide for control. Fungicides in the triazole chemistry class (also known as demethylation inhibitors) have a different site and mode of action on pathogenic fungi than strobilurin fungicides, and strobilurin-resistant isolates should not be cross-resistant to triazole fungicides.
3. In situations where other foliar diseases may be present along with frogeye leaf spot and a strobilurin fungicide may be needed to control the other foliar diseases, do not spray a solo strobilurin product. Either apply a strobilurin-triazole tank-mix, or apply a product that contains both a strobilurin and a triazole product.
4. Only apply a foliar fungicide to control plant diseases. Every time a fungicide application is made, a "selection pressure" is applied that selects out individuals in the pathogen population that may have reduced sensitivity to fungicides. Applying a fungicide only when it is needed--based on disease risk and scouting observations--will reduce the selection pressure placed on the pathogen population and slow the development and spread of fungicide-resistant isolates.

The Illinois Soybean Association has continued funding of this project through 2011, and Cercospora sojina isolates collected from the 2010 growing season will continue to be assayed through the winter months.--Carl A. Bradley

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