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Frogeye Leafspot may become resistant to fungicides

Posted Monday, November 29, 2010, at 3:42 PM

Frogeye leafspot in soybeans had only been a problem in soybeans in southern parts of Missouri until 1998. Since 1998, frogeye has expanded and it is now found throughout the state. Development of frogeye is favored by warm, humid weather. The fungus that causes frogeye leaf spot survives in infested soybean residues and infected seeds. Spores are spread short distances by wind or splashing rain. Dry weather severely limits disease development. With the weather patterns that we have experienced the last two years, expansion of the disease would be expected. In the last few years the use of fungicides to control various soybean diseases throughout the state has increased in the effort to improve yields.

Frogeye has been present in the south for a long time and the use of fungicides in southern growing regions is a very common practice. Strobilurin fungicides are the most widely used group of foliar fungicides applied to field crops to manage plant diseases. These fungicides can be sold as one-active ingredient products such as Headline or Quadris or in products that combine them with a fungicide in a different chemistry class sometimes referred to as triazoles.

Research conducted by the University of Illinois and the University of Tennessee confirms that the fungus that causes frogeye leafspot of soybean has shown resistance to strobilurin fungicides in a Tennessee soybean field. As of 2010 this resistance has only been identified in the Tennessee field. Strobilurin fungicides have been deemed high risk for fungal pathogens developing resistance to them. Plant pathogenic fungi have already developed resistance to strobilurin fungicides in potatoes and other crop and disease systems where multiple fungicide applications occur during the growing season. With all fungicides, 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.

Frogeye leafspot can be controlled with other management practices such as planting resistant soybean varieties or using effective triazole fungicides. Research from Illinois has shown that some triazole fungicides provide good control of Frogeye leaf spot and can be used alone or tank-mixed with strobilurin fungicides if the grower is concerned with more than just frogeye. The most effective manner to slow the spread of resistant isolates is to only use a fungicide when needed.

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As our local University of Missouri Agronomy Extension Specialist, Crook has been writing a column for the print edition Agriculture page for the past three years and we will now be sharing it on our web version. Crook has a bachelor and masters degree in agronomy from University of Missouri and received his doctorate in Agronomy from Kansas State University. He was in soybean variety development research for 22 years for various seed companies and has been Saline County's agronomy specialist for 10 years.
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