With reports of tar spot already showing up in some corn fields this growing season—combined with the wetter conditions over the past couple of weeks—a common question is: when is the right time to pull the trigger on spraying a foliar fungicide?
Before pulling the trigger, let’s pump the brakes and step back a moment and consider the following:
- Favorable conditions for disease development – While the past 10 days have brought wet and humid weather, that’s only part of what fosters disease development. Temperature also plays a critical role. For example, tar spot prefers more moderate daily averages (60–70°F), while gray leaf spot thrives in warmer conditions (70–90°F). Even though tar spot has been detected, the very warm recent conditions have been less conducive to its development and more favorable for gray leaf spot to start appearing in fields.
One tool that can help assess disease risk is the new Crop Risk Tool on the Crop Protection Network, designed for both gray leaf spot and tar spot. Note: this tool should only be used when corn is in the V10 to R3 growth stages.
Apart from environmental conditions, check disease ratings for hybrids being grown. Since both the tar spot and gray leaf spot pathogens survive in corn residue, inoculum is likely present but remember some pathogens, such as those that cause common and southern rust blow into Iowa from our southern neighbors. Maps on the Crop Protection Network provide data on where southern rust has been reported this growing season.
Take time to scout your fields and understand what you’re seeing.
- Properly ID the disease – While a lot of the foliar diseases in corn are caused by fungal pathogens, not all are! In Iowa two bacterial diseases that can be confused for fungal diseases include bacterial leaf streak and Goss’s Wilt. A fungicide will not control a bacterial disease. Feel free to reach out to your local extension field agronomist if you aren’t quite confident on the disease identification.
- Fungicide timing – what does the data say? VT to R2 is the general window we see most foliar fungicides applied between; however, for gray leaf spot (GLS) it may be more advantageous to spray earlier based on research ISU has done looking at application timings and fungicide products. While a V12 fungicide application usually reduces GLS better than an R1 application, applications at VT-R2 are still recommended for GLS. In contrast with tar spot, we tend to see a greater reduction in disease severity with a R1 application. Regardless of the fungal disease present, a fungicide application after R3 is not likely to provide a return on investment.
- Fungicide Efficacy: If you are going to spray a fungicide, you can check how well different fungicide products work on different diseases using the Corn Fungicide Efficacy table. Some products work better on tar spot or gray leaf spot whereas others are more effective on rusts.
- Fungicide ROI – Given the grain market prices for corn, it’s also important to consider the ROI on a fungicide application. You can use the new Corn Fungicide ROI Calculator on the Crop Protection Network to look at different scenarios (grain prices, expected yield, disease severity) to see the potential ROI on a fungicide application.
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- Professor of Plant Pathology and Microbiology
- Extension Field Crops Pathologist
- Field Agronomist in EC/SE Iowa