Frogeye leaf spot (FLS) symptoms are common today, especially in the South. The fungus that causes FLS is now showing resistance to fungicides that once controlled the problem.
Plant pathologists could be called the Rodney Dangerfields of agriculture. You might say the same of any scientist that warns of resistance. The fear of using too much of a good thing doesn’t generate much respect until the problem exists.
The latest resistance case involves strobilurin fungicides, a class of chemicals that has become popular for controlling fungal diseases in field crops and for reducing plant stress.
Research conducted by scientists at the University of Illinois and the University of Tennessee now confirms that the fungus that causes frogeye leaf spot (FLS) on soybean plants has shown resistance to strobilurin fungicides in a Tennessee field.
Strobilurin fungicides belong to the chemistry class known as the quinone outside inhibitors or QoI fungicides. "They are the most widely used group of foliar fungicides applied to field crops to manage plant diseases," says Carl Bradley, University of Illinois Extension plant pathologist.
These fungicides are sold commercially as one-active ingredient products such as Headline (BASF Corporation) or Quadris (Syngenta Crop Protection) and in products that combine them with fungicides in a different chemistry class known as demethylation inhibitors, which are sometimes referred to as triazoles. Products that include a strobilurin-triazole combination of active ingredients include Quilt (Syngenta Crop Protection) and Stratego (Bayer CropScience).
Strobilurin fungicides had already been deemed high risk for fungal pathogens developing resistance by the Fungicide Resistance Action Committee, an organization of scientists from manufacturers of at-risk fungicides.
Site-specific action. All strobilurin fungicides share a common biochemical mode of action—they interfere with energy production in the fungal cell. In fact, of the millions of biochemical reactions that occur in the fungal cell, these fungicides interfere with just one. This is important because just one mutation at that biochemical site can result in a fungicide-resistant strain. If that occurs, repeated applications of the fungicide can lead to a buildup of a fungicide-resistant pathogen.
"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."
Management strategies. The farm field in question received two applications of strobilurin fungicide during the growing season, but continued to experience high levels of FLS. Melvin Newman, University of Tennessee plant pathologist, says the farmer became suspicious and samples were collected and sent to Bradley’s lab.
"Currently, Tennessee is the only state in which we have documented resistance, but I doubt that it is an isolated case," Bradley says.
Newman’s research has shown that some of the triazole fungicides, when used either alone or in combination with strobilurin fungicides, provide good control of FLS. Growers are encouraged to rotate chemistries or use combination products if two applications are necessary.
"FLS-resistant and -tolerant soybean varieties are available and should be combined with crop rotation as a resistance management strategy," Newman says.
Tell-tale signs. The most diagnostic symptom of frogeye leaf spot is angular spots with light gray centers and distinct purple to red-brown margins. No chlorotic halos surround the lesions. Leaf spots can be single or merge to form larger lesions. This can result in premature leaf drop.
Lesions on stems and pod can occur later in the season, but they are less common and distinctive than lesions on the leaves. Seeds near pod lesions can be infected and develop light to dark gray or brown areas.
Written by Pam Smith and Jennifer Shike
- January 2011