Target diseases when considering foliar fungicides
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.
click image to zoomFigure 1. A summary of University of Illinois corn foliar fungicide trials conducted from 2008 to 2011 across the state. Foliar fungicides were applied between VT and R1. A 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.