By Andi Anderson
In 2025, a statewide evaluation examined how commercial foliar fungicides affected corn diseases yield and profitability across Iowa.
The study was conducted at several Iowa State University Research and Demonstration Farms located in northwest, northern, northeast, central and southeast Iowa. One research site was lost due to wind damage before tasseling and was excluded from final results.
Southern rust was the most common disease observed across all locations. Other diseases such as tar spot, northern corn leaf blight and bacterial leaf streak were also present at some sites.
Disease pressure varied widely with the lowest southern rust levels recorded in central Iowa and the highest in southeast Iowa. This variation allowed researchers to compare fungicide performance under different conditions.
Results showed that all fungicides reduced southern rust compared to untreated corn. No single name brand product consistently performed better than others across locations.
A lower cost generic product applied twice provided strong disease control and often matched the effectiveness of higher priced products. A single application of the generic product reduced disease but was generally less effective than two applications.
Yield results showed that fungicide applications increased corn yields at most locations except one northern site. When fungicides were applied at silking, no statistical yield differences were found among products. Applying fungicide twice did not always increase yield compared to a single application. Overall fungicide use improved yield when disease pressure was present.
Researchers also compared returns on investment using a grain price of five dollars per bushel. Generic fungicide and some brand products delivered positive returns at most locations. Returns varied by product timing and disease severity but supported the economic value of fungicide use under disease risk.
Based on the findings researchers recommended applying foliar fungicides at silking for the best disease control. In many cases, one application was sufficient to protect the crop through grain fill.
Southern rust does not survive Iowa winters, so disease risk resets each year. However, other diseases can overwinter making weather conditions and hybrid selection important factors in future disease management decisions.
Photo Credit: gettyimages-dszc
Categories: Iowa, Crops, Corn