Target diseases when considering foliar fungicides
These results clearly reveal that disease pressure plays a critical role in the magnitude and consistency of yield response to a foliar fungicide application in corn. The difficult part is being able to predict before the VT to R1 stages what type of disease pressure may develop later in the season. To make such a prediction, one needs to consider disease risk factors and to scout for disease.
Disease risk factors include these:
- Susceptibility level of corn hybrid. Seed companies typically can provide information on the susceptibility of their hybrids to gray leaf spot and northern leaf blight. In general, hybrids that are more susceptible to fungal foliar diseases will have a greater response to a foliar fungicide (if disease pressure is high enough).
- Previous crop. Because many foliar pathogens survive in corn residue, the risk of foliar diseases (such as gray leaf spot and northern leaf blight) increases when corn is planted back into a field that grew corn the previous year.
- Weather. Rainy and/or humid weather generally is most favorable to foliar diseases. In growing seasons when these conditions prevail, the risk for disease development increases.
- Field history. Some field locations have a history of high foliar disease severity. Fields in river bottoms or low areas or those surrounded by trees may be more prone to foliar corn diseases.
Scout for foliar diseases in corn just before tassel emergence. Current disease management guidelines suggest the following criteria by hybrid type for considering an application of foliar fungicide:
- Susceptible hybrids--if disease symptoms are present on the third leaf below the ear or higher on 50% of the plants examined.
- Intermediate hybrids--if disease symptoms are present on the third leaf below the ear or higher on 50% of the plants examined, if the field is in an area with a history of foliar disease problems, if the previous crop was corn, if there is surface residue of 35% or more, and if the weather is warm and humid.
- Resistant hybrids--fungicide applications generally are not recommended.
According to the data from our corn fungicide trials, if at least 15% of ear leaf area is affected by disease at the end of the season, a foliar fungicide applied between VT and R1 would likely have been beneficial. Using the disease risk factors and scouting observations collected just before tassel emergence will help you predict how severe disease may be several weeks from the VT to R1 stages and help you decide whether to apply a foliar fungicide.