Frogeye leaf spot (FLS), caused by Cercospora sojina, can cause yield reductions of soybean grown in the southern and north-central United States (Figure 1). A primary way of managing FLS is the use of foliar fungicides in the strobilurin class (Headline, Quadris, Evito, Gem). Because there is a high risk of fungal pathogens developing resistance to strobilurin fungicides, research was initiated to establish baseline sensitivities of C. sojina to strobilurin fungicides and to establish a program to monitor for strobilurin fungicide-resistant strains of C. sojina.
A historical collection of C. sojina isolates (courtesy of Dr. Dan Phillips, University of Georgia) was used to develop baseline sensitivity levels. The collection consists of isolates collected prior to strobilurin fungicide use in the U.S. To determine the baseline sensitivities, spores of the isolates were placed onto media amended with different levels of strobilurin fungicides, and spore germination was determined. The effective concentration of fungicide at which 50% of spore germination was inhibited relative to the no-fungicide control (EC50) was determined for each isolate-fungicide combination.
In 2010, C. sojina isolates were collected from soybean fields treated with strobilurin fungicides from Illinois and several other states. Isolates collected in 2010 from two locations in Illinois, one in Kentucky (courtesy of Dr. Don Hershman, University of Kentucky), and two in Tennessee (courtesy of Dr. Melvin Newman, University of Tennessee) were found to have EC50 levels over 200-fold higher than the mean EC50 levels of the baseline C. sojina isolates (Figure 2). These results indicate that these isolates are resistant to strobilurin fungicides.
A greenhouse trial was conducted with the resistant isolates collected in 2010 to confirm that they were resistant to strobilurin fungicides applied at normal use rates and to determine if fungicides with other modes of action could control these isolates. In the greenhouse trial, FLS caused by a strobilurin fungicide-resistant isolate could not be controlled with strobilurin fungicides, while FLS caused by a baseline isolate could be. Triazole fungicides and thiophanate methyl (Topsin) were able to control FLS caused by both the isolates resistant to and sensitive to strobilurin fungicide (Figure 3). In light of our findings, the following practices are recommended for managing frogeye leaf spot:
Carl A. Bradley
Associate Professor of Plant Biology
Guirong R. Zhang
Graduate Research Assistant