Foliar fungicides are touted for a variety of reasons, including "plant health," "yield bumps," and "yield enhancements," but sometimes the most important benefit fungicides may provide is overshadowed or lost. When foliar fungicides do provide a benefit, it is generally because they protect against foliar diseases. In Illinois, the primary foliar fungal diseases that can reduce yields are gray leaf spot and northern leaf blight. In some years, southern rust also may reduce yields if it arrives to the state early enough in the season.

Beginning in 2008, the plant pathology program in the University of Illinois Department of Crop Sciences has conducted annual foliar fungicide trials on corn in six or seven Illinois locations. A number of products--including Headline, Headline AMP, Stratego, Stratego YLD, Quilt, Quilt Xcel, and Bumper--are applied between the VT and R1 stages (tassel emergence to silking). At each location, disease severity is measured by evaluating the ear leaves of each plot and estimating the percentage of leaf area affected by diseases. These measurements are collected about 4 weeks after foliar fungicides are applied.

A summary of these foliar fungicide trials is shown in Figure 3. On the horizontal axis, the final measurement of disease severity (the percentage of ear leaf with lesions) observed in the nontreated control is divided into three levels: less than 10%, between 10% and 14%, and 15% or greater. The number of trials for each category (N) is indicated, along with the mean yield response and the frequency of achieving a yield response of at least 5 bu/A.

Foliar fungicides for disease management in corn

The summary data indicate that disease pressure plays an extremely important role both in achieving a positive yield response with a foliar fungicide and in consistently achieving economically positive yield responses. Under low disease pressure (final disease severity of less than 10% of ear leaf area affected), the mean yield response was only 0.1 bu/A, and a yield response of at least 5 bu/A was achieved only 13% of the time (1 out of 8 times). Under medium disease pressure (final disease severity of 10%-14% of ear leaf area affected), the mean yield response was 4.8 bu/A, and a yield response of at least 5 bu/A was achieved 50% of the time (2 out of 4 times). Under high disease pressure (final disease severity of at least 15% of ear leaf area affected), the mean yield response was 14.1 bu/A, and a yield response of at least 5 bu/A was achieved 100% of the time (7 out of 7 times). Gray leaf spot generally was the most severe disease in these trials, with northern leaf blight being present in a few trials, and southern rust proving severe in one trial (at Dixon Springs in 2009).

These results clearly reveal that final disease severity plays a critical role in the magnitude and consistency of yield response to a foliar fungicide application. The tricky part is being able to predict before the VT to R1 stages what the disease pressure will be several weeks later. 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 was 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 may have a history of high foliar disease severity. Fields in river bottoms or low areas or surrounded by trees may be more prone to having foliar corn diseases.

Scout for foliar diseases in corn just before tassel emergence. Current disease management guidelines suggest the following criteria for considering an application of foliar fungicide:

  • For susceptible hydrids--if disease symptoms are present on the third leaf below the ear or higher on 50% of the plants examined.
  • For 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 35% 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 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.