With much of Illinois suffering from abnormally dry conditions to severe drought, the risk of corn foliar diseases in most of the state is currently low. Although foliar fungicides are touted for a variety of reasons--including "plant health," "yield bumps," and "yield enhancements"--these effects have not been observed consistently in University of Illinois trials when foliar disease pressure is low.

Since 2008, the plant pathology program in the University of Illinois Department of Crop Sciences has conducted annual foliar fungicide trials on corn across the state. A number of products, including Headline, Headline AMP, Stratego, Stratego YLD, Quilt, Quilt Xcel, and Bumper, are applied between the VT and R1 growth stages (tassel emergence to beginning 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 disease. These measurements are collected about 4 weeks after foliar fungicides are applied.

Target diseases when considering foliar fungicidesA summary of the trials is shown in Figure 1. On the horizontal axis, the measurement of disease severity (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. This yield response level can be used as a measure of profitability. With current corn prices, fungicide prices, and application costs, a 5 bu/A yield response would be close to the "break-even" or "profitable" mark.

This summary indicates that disease pressure plays an extremely important role in achieving a positive yield response with a foliar fungicide and in consistently achieving economically positive yield responses. Under low disease pressure (less than 10% severity), the mean yield response was only 0.7 bu/A, and a response of at least 5 bu/A was achieved only 23% of the time. Under moderate disease pressure (10% to 14% severity), the mean yield response was 4.8 bu/A, and a response of at least 5 bu/A was achieved 50% of the time. Under high disease pressure (at least 15% severity), the mean yield response was 12.7 bu/A, and a response of at least 5 bu/A was achieved 100% of the time. In these trials, gray leaf spot generally was the most severe disease, but northern leaf blight and southern corn rust also were present in a few trials.

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.