Frogeye leaf spot got finicky in soybeans in 2010 when the first strobilurin-resistant strains showed up. The case has plant pathologists like Carl Bradley pleading that growers take the proper management steps to prevent the spread and development of further resistance.
"Plant pathogenic fungi developing resistance to strobilurin fungicides has already occurred in potatoes and other crop and disease systems where multiple fungicide applications occur during the growing season," Bradley says.
"However, we were somewhat surprised to find resistance in this particular pathogen
so soon," he adds. "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."
The farm field in question received two applications of strobilurin fungicide during the growing season, but continued to experience high levels of frogeye leaf spot. Melvin Newman, University of Tennessee plant pathologist, says the farmer became suspicious and samples were collected and forwarded to Bradley’s lab at the University of Illinois.
Bradley’s recommends the following management practices to prevent the spread and development of strobilurin fungicide resistant C. sojina (the causal agent of frogeye leaf spot):
- Plant soybean cultivars with resistance to frogeye leaf spot.
- If a susceptible cultivar has been planted, and a fungicide application is being considered, then apply an effective triazole fungicide for control.
- 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 a apply a product that contains both a strobilurin and a triazole fungicide.
- Only apply a foliar fungicide when warranted for disease control.
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