University of Illinois plant pathologist Carl Bradley said that, although foliar fungicides are touted as increasing corn yields, these effects have not been observed consistently in U of I trials when foliar disease pressure is low.
Since 2008, the plant pathology program in the U of I Department of Crop Sciences has conducted annual foliar fungicide trials on corn across the state. In these trials, a number of products, including Headline, Headline AMP, Stratego, Stratego YLD, Quilt, Quilt Xcel, and Bumper, were applied between the VT and R1 growth stages (tassel emergence to beginning of silking). At each location, disease severity was measured four weeks after applying the fungicides by evaluating the ear leaves of each plot and estimating the percentage of leaf area affected by diseases.
The results indicated 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 predicting before the VT stage the level of disease pressure likely to develop later in the season.
Predictions have to take into account disease risk factors, which include:
- Susceptibility level of the corn hybrid. Seed companies can provide information on their hybrids' susceptibility to gray leaf spot and northern leaf blight. Hybrids that are more susceptible to fungal foliar diseases tend to respond best to a foliar fungicide (if disease pressure is high enough).
- Previous crop. Many foliar pathogens survive in corn residue, so the risk of foliar diseases (such as gray leaf spot and northern leaf blight) increases when corn is planted back into a field that was corn the previous year.
- Weather. The risk for disease development increases in rainy and/or humid weather.
- Field history. Fields in river bottoms, low areas, or surrounded by trees may be more prone to foliar corn diseases.
Scouting observations should also be taken into account. Scout for foliar diseases in corn just before tassel emergence. Current disease management guidelines suggest applying foliar fungicide under the following conditions:
- For susceptible hybrids, if disease symptoms are present on the third leaf below the ear or higher on 50 percent of the plants examined.
- For intermediate hybrids, if disease symptoms are present on the third leaf below the ear or higher on 50 percent 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 35 percent or more surface residue, and if the weather is warm and humid.
- For resistant hybrids fungicide applications generally are not recommended.
"According to the data from the corn fungicide trials, if at least 15 percent of the ear leaf area is affected by disease at the end of the season, a foliar fungicide applied between VT and R1 probably would have been beneficial," Bradley said.