Source: University of Illinois
By Carl A. Bradley
In 2010, Tennessee soybeans were documented with frogeye leafspot that was resistant to strobilurin fungicides. Following that report, similar resistance was also found in southern Illinois and western Kentucky.
Researchers testing the same fields this season have found the counties in Illinois and Tennessee with fungicide-resistant strains of Cerocospora sojina (the causal agent of frogeye leaf spot) in 2010, also have fungicide-resistant strains present in 2011. In addition, new counties in Tennessee, Kentucky, and Missouri have been confirmed to have strobilurin fungicide-resistance.
Strobilurin fungicide active ingredients currently registered on soybeans include azoxystrobin (found in Quadris, Quilt, and Quilt Xcel), fluoxastrobin (Evito), pyraclostrobin (Headline), and trifloxystrobin (Stratego, Stratego YLD).
The Fungicide Resistance Action Committee classifies the strobilurin fungicides as being at high risk for having resistance develop in targeted fungi. Any time a fungicide is applied to a crop, a selection pressure takes place, and individuals in the fungal pathogen population with reduced sensitivity to that fungicide are selected out. With additional applications of fungicides in the same fungicide class, a greater selection pressure is applied, and the risk increases for selecting out fungicide-resistant strains of plant pathogenic fungi.
To reduce this risk, the following tactics are recommended for managing frogeye leaf spot:
- First and foremost, plant a variety with high resistance to frogeye leaf spot, particularly in areas with a history of damaging levels of the disease. Resistant varieties are the best way to manage frogeye leaf spot.
- If you have planted a susceptible variety in a risk-prone area and plan to apply fungicide, choose fungicides from different classes. Preliminary results from greenhouse and field studies at the University of Illinois with strobilurin fungicide-resistant strains of C. sojina indicate that many triazole fungicides (Proline, TopGuard, Domark, etc.) and thiophanate methyl fungicide (Topsin and other trade names) were effective in reducing frogeye leaf spot severity compared with strobilurin fungicides and the nontreated control.
- Apply a foliar fungicide only to control plant diseases. Every time a fungicide application is made, pressure is applied that selects out individuals in the fungal pathogen population that may have reduced sensitivity to fungicides. Applying a fungicide only when it is needed--based on disease risk and scouting observations--reduces selection pressure and slows the development and spread of fungicide-resistant isolates.
My thanks to the Illinois Soybean Association for funding much of this research. In addition, I thank Melvin Newman (University of Tennessee), Don Hershman (University of Kentucky), and others for sending samples to my laboratory for testing.