Fungicide Considerations for Corn and Soybean

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. In soybean, fungicide applications made prior to the R1 growth stage, or after the R6 growth stage are often not economical. (Hershman)

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Corn - 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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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.

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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 determined the efficacy ratings for each fungicide by field-testing the materials over multiple years and locations. Visit www.extension.iastate.edu for a copy this table.

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Soybean - Keeping a close watch on fields for disease symptoms and a review of soybean growth stages can help properly time fungicide applications. Anthracnose, brown spot, Cercospora leaf blight, frogeye leaf spot, white mold, and soybean rust are diseases that may be managed with a timely fungicide treatment. A relatively early fungicide application at R2.5 (full flower) growth stage may have yield benefit over fungicide applications at R4 (full pod) growth stage.2

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Fungicide application in soybean differs from corn as there is more leaf development after application. Therefore, the more leaves present at the time of spraying, the more potential protection from foliar disease. A soybean plant treated with a fungicide at R3 (beginning pod) may have 14 to 16 nodes. The plant with16 nodes at application time has two more nodes protected from disease compared to the 14-node plant at the same beginning pod growth stage.3 Application timing is essential to effective disease control, and fungicide applications made prior to the R1 (beginning flower) growth stage, or after the R6 (full seed) growth stage are often not economical.4 Foliar diseases are often not an issue until the R3 growth stage. If left unchecked, potential yield loss may occur due to premature leaf drop reducing the photosynthesizing ability of the crop to produce grain.5

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Weather conditions can also help determine proper fungicide timing in soybean. Response from fungicide treatment may be best when conditions favor disease development and the product lacks resistance. If this type of environment is anticipated at R3 growth stage, treatment with a fungicide may be warranted. More than one fungicide application may be needed in environments with high disease pressure. Additionally, a compatible herbicide or insecticide may be included if labeled for soybean and there is a weed or insect pest present that warrants treatment. For more information on fungicides for soybean diseases, please see BP-161-W from Purdue University.​6