Today (20 October), a report out of the University of Illinois confirmed that a field in Tennessee where frogeye leaf spot was found and sprayed twice with a strobilurin fungicide but still had high levels of the disease was because the pathogen that causes the disease (Cercospora sojina) was resistant to the fungicide. This finding was based on laboratory assays that examined the sensitivity of the isolate obtained from the field in Tennessee with baseline isolates and compared against active ingredients like azoyxstrobin, pyraclostrobin, and trifloxystrobin. These are the active ingredients that are found in fungicides such as Quadris, Headline, and Stratego.
What does this mean for Wisconsin? Most importantly, this serves as a very important reminder that the use of fungicides should be done based on several factors, including knowledge of the variety planted and if there is resistance to the targeted diseases of interest, followed by active scouting during the growing season to assess if conditions would warrant a fungicide application. Misuse or overuse of a foliar fungicide can increase the risk for resistance. Specifically for frogeye leaf spot in 2010, we did see symptoms in many fields, but severity was low on average. However, this is the sort of information that should be used to build a working knowledge of the specific diseases that may affect production fields in order to most effectively build a long-term management program.
Based on our data from Wisconsin over the past several years, in the majority of situations a foliar fungicide was not found to be needed and would have been an additional cost to production.