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The economic return on a fungicide application is usually greater when fungicide use is based on the presence of a foliar disease. Fungicides applied from tasseling to early silking tend to have the best possibility for economic return.
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.
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
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.
Begin scouting fields for foliar disease symptoms just before tasseling and continue through the grain filling stages of growth. 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.
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.
1 Bradley, C.A. 2015. Getting to know the foliar diseases of corn. The 2015 University of Illinois Corn & Soybean Classic. University of Illinois. 160604155040