During the 2011 season several foliar diseases were present in Iowa, including brown spot, Cercospora leaf blight and frogeye leaf spot. The disease that was noticeably higher in 2011 compared to previous years was frogeye leaf spot, at least in susceptible varieties. The pathogen that causes frogeye leaf spot, Cercospora sojina, has been in the news several times the past couple of years because of reports of resistance to strobilurin fungicides.
Because of these reports, Iowa State researchers collected leaves with frogeye leaf spot from our fungicide trials and sent the leaves to University of Illinois to be tested for sensitivity. All of the isolates collected from the Iowa leaves tested sensitive, which means that they are not resistant to strobilurin fungicides.
These leaves represent only a small percentage of frogeye leaf spot in Iowa. While the 2011 Iowa isolates were not resistant to strobilurin fungicides, it is still good to be thinking about long-term use of this important class of fungicides. Isolates of C. sojina resistant to strobilurins have been recovered from neighboring states, namely Illinois and Missouri. Since spores of the fungus are wind dispersed, the likelihood of resistant isolates being found in Iowa is real.
The possible development of resistant strains of the frogeye leaf spot pathogen to strobilurin fungicides should be a consideration as growers are deciding about foliar fungicides for 2012. Further advice from Dr. Carl Bradley, the scientist from Illinois leading the fungicide-resistance testing, is summarized in his article in early October.