Tar spot can be devastating for corn growers, reducing yields up to 50 bushels per acre or more under worst-case scenarios.¹ To combat this disease, it’s important to understand the environmental conditions that help it spread, the signs of tar spot in corn and the best practices for using fungicides for tar spot in corn.
Top environmental factors that help the spread of tar spot in corn
Two main elements set the stage for tar spot in corn, though there are also a few other risk factors.
Temperature
Mild temperatures over an extended period are the top driver of tar spot spread. It thrives when daily highs hover between 64 F and 73 F for 30 days or more.
Humidity
Moisture in the air ranks second as an environmental spur for tar spot development. When relative humidity stays in the 75% to 90% range for two to three weeks, spread is most common because moisture helps spores germinate.²
Related risk factors
Beyond temperature and humidity, several other factors can create favorable conditions for tar spot in corn. These include:
- Leaf wetness that lasts more than seven hours
- Ten or more foggy days per month
- 6” or more of monthly precipitation³
Good record-keeping and regular scouting can help you monitor for early signs of tar spot in corn. Bring a phone or camera to capture pictures of tar spot in corn or of plants you’re unsure about. A trusted adviser can help you properly diagnose your crop and treat it with the help of these visual cues.
Tools, tips and tricks for confirming tar spot in corn
To avoid the spread of tar spot in corn, it’s best to combine technology and in-person monitoring.
Tools to understand ideal conditions for tar spot in corn
The free online Crop Disease Forecasting web tool has been validated as 90.1% accurate for predicting times of year when your corn crop is at greatest risk of tar spot.⁴
You can also take a step back and evaluate current and past county-level risk in your local area using the Tar Spot Distribution map.
Tips and tricks to identify tar spot
When you head out to do early scouting for tar spot, start with any fields in a corn-on-corn rotation. Those are most likely to face early tar spot. Tar spot inoculum in the field from the previous year, i.e. tar spot in the field last year in a corn on corn rotation, can mean early infection for this year’s crop. Tar spot can also spread from neighboring fields during the season, but infection levels and severity tend to be less than in corn on corn rotations.
Look at the bottom section of your corn plants. Tar spot can live on corn residue over the winter. It often travels from ground level up into the canopy, though it can also be transported via wind and farm equipment.
If you’re unsure whether a black blotch is tar spot, another corn disease or insect residue, try a simple test: Use a wet cloth to clean off the leaf. If the spot stays in place, it’s likely tar spot. Everything else on the leaf will rub off.⁵
Tar spot has a matte texture and often sports a brown color around the margins. The lesions are raised and typically 1/16” to 3/4" in diameter.⁶
How to time your use of fungicides for tar spot in corn
If fungicides are needed to treat for tar spot in corn, follow these guidelines:
- Normally applying one fungicide pass will give the best economic ROI. In cases of early season infection, two passes may be required.
- Use fungicides between R2 and R3 corn growth stages, in cases of early infection, a first application at or prior to VT may be justified.
- Consider fungicide products such as BASF’s Veltyma® for tar spot
- Review these additional BASF resources on getting ahead of emerging corn disease and on early fungicide application in corn with Veltyma®
Experts are available to help you make your corn fungicide decisions. Reach out to your seed retailer, a nearby extension office agent or a professional like your regional BASF representative.
________________________________________________
Endnotes
- Telenko, Darcy, et al. Tar Spot of Corn. Crop Protection Network, 16 Mar. 2021, cropprotectionnetwork.org/web-books/tar-spot-of-corn?section=tar-spot-yield-losses. Accessed 5 Feb. 2026.
- Webster, Richard W., et al. Tar Spot Prediction in Corn: The Weather Matters. Crop Protection Network, 18 Dec. 2023, cropprotectionnetwork.org/publications/tar-spot-prediction-in-corn-the-weather-matters. Accessed 5 Feb. 2026.
- Mues, Talon, et al. “Early Tar Spot Detected in Nebraska Corn; Irrigation Shown to Elevate Disease Risk.” CropWatch, University of Nebraska–Lincoln, 20 June 2025, cropwatch.unl.edu/early-tar-spot-detected-nebraska-corn-irrigation-shown-elevate-disease-risk/. Accessed 5 Feb. 2026.
- Webster, Richard W., et al. Tar Spot Prediction in Corn: The Weather Matters.
- Drake, Madeline. “Tar Spot Confirmed in Kansas Cornfields for 2025, K-State Pathologist Urges Early Scouting.” K-State Research and Extension News Service, Kansas State University, 30 June 2025, extension.k-state.edu/news-and-publications/news/stories/2025/06/agriculture-tar-spot.html. Accessed 5 Feb. 2026.
- Chilvers, Martin. “Managing Tar Spot in Corn.” MSU Extension, Michigan State University, 7 Aug. 2025, www.canr.msu.edu/news/managing-tar-spot-in-corn. Accessed 5 Feb. 2026.
