Concern is particularly high this season about our most serious fungal leaf blight of corn, gray leaf spot (GLS). Has the wet spring and early growing season put the corn at risk for fungal leaf blight?

University of Illinois Extension Plant Pathologist Carl Bradley has been researching these issues for the past several seasons. His research shows some interesting results for both disease management and profit. He notes that foliar fungicide applications provide profitable yield benefits on a consistent basis only when disease pressure is severe enough to cause economic losses. The idea that fungicides can protect against yield losses when disease pressure is severe is certainly not new. He notes that it has become evident that the issue of severity level is not stressed clearly enough in fungicide product advertisements. Claims of yield enhancement, improved growth efficiency, and stress tolerance are often the messages being touted, with little to no mention of diseases. Bradley's work indicates that you will get the most return for your money when fungicides are applied to manage fungal disease. Hybrid selection and production practices play a big role for management.

Several criteria are available to help you make a decision about fungicide application for gray leaf spot (GLS), they developed between University of Illinois and Iowa State University. First, consider the GLS resistance in the hybrid you are growing. Hybrids with moderate to excellent GLS resistance typically do not need a fungicide application. If growing hybrids with intermediate GLS resistance consider fungicide if field has history of disease, the previous crop was corn with at least 35% residue, disease is present on third leaf below ear or higher on 50% of plants prior to tasseling, and we have warm and humid weather thru July and August. Finally, if you are growing GLS susceptible or moderately susceptible hybrids consider fungicide if disease is present on the third leaf below ear or higher on 50% of the plants prior to tasseling.

Management of fungal leaf blights may have additional benefits for the crop with regard to stalk rots. Bradley's research also looked at the relationship between fungal leaf blight and ensuing damage from fungal stalk rots. When foliar disease pressure is severe, the diseased leaves cannot produce enough photosynthates (sugars) to adequately fill the ear. When this happens, the plant may rob the stalk for additional sugars, which can damage the integrity of the stalk and allow additional colonization by stalk rotting pathogens. His research shows that when leaf disease is present and managed with fungicide, less stalk rot is evident later in the season. Bradley concludes that foliar fungicides can impact stalk rot, but they likely do not directly control stalk rot pathogens; rather, they control foliar pathogens, which allows the plant to get more photosynthates from leaf photosynthesis because of reduced foliar disease severity. He concludes that it is likely that we will only see improved stalk quality with fungicides when foliar disease pressure is high.