Considerations for Fungicide Use in Corn

The economic return on a fungicide application is usually greater when fungicide use is based on the presence of a foliar disease. In corn, fungicides applied from tasseling to early silking tend to have the best possibility for economic return.

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The exact yield response to a fungicide treatment cannot be predicted with certainty considering all the different types of crop stress and crop genetics. Your results will be much more consistent when fungicide use is based on the presence of a foliar disease.

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Making a decision based on disease presence increases the odds of meeting or exceeding the break-even value for a fungicide application. According to research conducted by the University of Illinois from 2008 to 2014, under low disease pressure environments, the average yield response to a foliar fungicide applied between tasselling (VT) and silking (R1) was 2.8 bushels per acre. While under moderate to high disease pressure environments, the average yield response was 9.5 bushels per acre and the probability of achieving ≥ 3, 5, or 8 bushels per acre increased by 59%, 58%, and 46%, respectively.1

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In most cases, fungicide applications should be applied at or soon after tasseling. University of Illinois research showed that fungicide applications during the mid-vegetative growth stages (V5-V6) did not significantly reduce foliar disease severity or increase yields when compared with either untreated controls or applications made between VT and R1.1 Fungicides applied from VT to early R1 tend to have the best possibility for economic return.

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The use of foliar fungicides can also be considered as a tool to help relieve stress in corn, such as after hail damage. Fungicide product labels have changed to allow for mitigation of plant stress. For example, Headline AMP® fungicide is labeled for use in disease control and plant health in corn.2 The product label states that “the increase in plant health comes from the combined effect of disease control, improved growth efficiency, and improved stress tolerance.�? Therefore, the use of foliar fungicides in corn following a hail event has become increasingly common for disease prevention and to minimize stress on the plant. Hail damage to corn can cause reduced leaf area, plant bruising, stand loss, or injury to the ear.3 In multi-year testing conducted from 2012 to 2014 by Iowa State University, yield benefits were obtained by fungicide applications at the VT and R2 corn growth stages following simulated hail events.4 The testing suggested a beneficial crop response from using a fungicide to be more likely in years with significant disease pressure. Results also suggested that waiting at least a week may be more beneficial than an immediate application when applying a fungicide after hail damage to mid-season corn.

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Figure 1

Figure 1. Foliar disease symptoms of gray leaf spot (left), northcern corn leaf blight (center), and common rust (right).

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Begin scouting fields for foliar disease symptoms just before tasseling and continue through the grain filling stages of growth. Some common foliar diseases to watch for in corn include gray leaf spot (GLS), northern corn leaf blight (NCLB), and common rust (Figure 1). Since the majority of the plant's energy supply to fill the ear comes from the ear leaf and above, examine the ear leaf and leaves above and below the ear at several locations throughout a field. If disease is present on a majority (> 50%) of the leaves at R1, a fungicide application would protect yield potential. If disease is present at later growth stages up to R4 (dough), a fungicide application may be necessary based on if environmental conditions continue to be conducive to disease progression.

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The Corn Disease Working Group (CDWG) developed ratings for how well fungicides control major corn diseases in the United States. The CDWG deter31;mined the efficacy ratings for each fungi31;cide by field-testing the materials over multiple years and locations. Visit www.extension.iastate.edu for a copy of this table.