Some companies are promoting foliar fungicide use on corn at early growth stages, such as V5 or V6 (when five or six leaf collars have developed). While there are some advantages of this timing as far as the ability to apply fungicide with a ground applicator and to tankmix it with a postemergence herbicide, this early application may not provide any advantage in disease control.

University of Illinois Extension Plant Pathologist Carl Bradley says the most important reason to apply a fungicide is to control disease. However, at the V5 or V6 growth stage, most foliar diseases frequently observed in Illinois would not be present yet.

“Unfortunately, few replicated field research trials have been conducted to evaluate these early growth-stage timings of foliar fungicides on corn,” he says.

This spring, Bradley requested results from the University of Wisconsin, Purdue University, Iowa State University, the University of Nebraska and the University of Illinois regarding the yield responses of corn with early fungicide applications.

Based on the responses he received regarding products currently registered for corn, only Headline fungicide (BASF) had been evaluated at early timings in more than two trials. In these trials, yield responses to Headline applied at either V6 or tassel emergence/silking varied. Yield responses of at least an additional 10 bu./acre were observed in one of the eight trials with V6 applications and in three of the eight trials with tassel emergence/silking applications.

The overall average yield response across all trials was 1.5 bu./acre with V6 applications and 8 bu. with tassel emergence/silking applications. The level of disease pressure varied by trial and location; for instance, a high level of southern rust was present in some of the Nebraska trials.

“Overall, the tassel emergence/silking timing appears to be more advantageous for disease control and yield response compared to the V6 application,” Bradley says. “The biggest yield responses with foliar fungicides on corn will be observed in fields that have the highest risk of foliar diseases. Disease risk increases with more susceptible hybrids and in corn-on-corn situations.”

If growers decide to apply foliar fungicides on corn at an early growth stage, Bradley suggests that multiple untreated strips are left to serve as a check.

“It’s important for growers to be able to determine if these early fungicide applications are beneficial in their own fields, and the ability to compare yields and disease levels between treated and untreated strips is critical,” he says.

For more information, check out The Bulletin, an online publication written by U of I Extension specialists in crop science.