Tar spot has already been recognized in some areas across Midwest cornfields, and recent stretches of wet weather have begun to raise grower concerns. Because of this, many producers are throwing around the idea of whether it is time to apply a foliar fungicide. In a recent Iowa State University (ISU) Integrated Crop Management Blog, extension pathologist Alison Robertson and field agronomist Rebecca Vittetoe highlighted the importance of making a surefire decision and stepping back to take a closer look at the field conditions. After all, it may save you time and money in the long run.

Disease pressure continues to build, but it is not entirely due to rainfall. Tar spot thrives in moderate temperatures between 60°F and 70°F, while gray leaf spot (GLS) enjoys warmer, humid conditions between 70°F and 90°F. While temperatures continue to soar, GLS may be the bigger threat for many, even as tar spot cases continue to climb, they noted.

To assess disease risk more precisely, growers can turn to the Crop Protection Network’s Crop Risk tool, which models risk for both tar spot and GLS. This tool is best used when corn is between stages V10 and R3. Hybrid selection also plays a key role. Resistance ratings vary, and while both GLS and tar spot overwinter in corn residue, other diseases such as southern rust can make their way up from Southern states. Disease maps are available on the Crop Protection Network to help producers track their paths.

Scout fields and be sure to correctly identify what you are seeing. Misidentification can waste time and money, and it is important to note that fungicides won’t work on bacterial diseases. When in doubt, lean on your local extension agronomist for support.

When it comes to timing fungicide applications, there is a sweet spot for when to apply. “VT and R2 is the general window we see most foliar fungicides applied between; however, for GLS it may be more advantageous to spray earlier based on research ISU has done looking at application timings and fungicide products,” explained Robertson and Vittetoe. However, for early GLS pressure, a V12 application may be more effective. Tar spot, on the other hand, responds better to applications at R1. Spraying after R3 is less likely to take care of the issue.

However, it is important to note that not all fungicides are of the same quality and strength. The corn fungicide efficacy table, published annually, rates products by their performance on specific diseases like tar spot, GLS, and rusts.

In closing, don’t forget about your return on investment. As market prices continue to tighten, every pass counts. The corn fungicide return on investment (ROI) calculator can help growers crunch numbers by inputting expected yield, corn price, and disease severity to determine if the application pencils out.

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