Tar spot of corn was found in several Nebraska counties this past week. You can see the full map at: https://cropprotectionnetwork.org/maps/tar-spot-of-corn. Samples can be sent in to the UNL Plant and Pest Diagnostic Lab if you’re unsure and want to confirm. So far we’re only seeing it at low incidence of a few lesions on plants.

In this week’s UNL CropWatch, our Extension Plant Pathologists shared, “We do not recommend fungicide treatment at these early stages. Results from multi-state fungicide trials summarized on the Crop Protection Network show that treatment during the vegetative stages — V6 through V11 — was not economical even though disease had already begun. 

Fungicide applications made between VT–R1 through R3 (milk) or occasionally early R4 (dough) stages were the most effective at controlling tar spot and had the best chance of generating a favorable net return. In those experiments, fungicide applications made to corn before tar spot severity exceeded 5% on the ear leaf by early R4 were still economical. Applications made after disease on the ear leaf exceeded 6% at application were not economical. Additionally, products with a mixture of active ingredients from two or more fungicide classes were most effective.”

You can see a photo of what 5-7% disease severity looks like at https://jenreesources.com/wp-content/uploads/2025/06/when-to-spray.png. There’s also a yield/price/fungicide risk assessment tool that allows you to compare the ROI of a fungicide application: https://connect.doit.wisc.edu/cpn-roi-tools/.

Considerations for now:
1-Scout fields and wait till a 5% threshold on leaves before applying fungicide
2-Observe fields as to which hybrids have more tolerance to tar spot
3-When irrigating, consider less frequent and deeper irrigations, https://go.unl.edu/vipj
4-Consider plant nutrition instead. I will focus on a series of plant nutrition articles in the coming weeks. Please contact me in the meantime if you’re interested in knowing more about this option.

The fungus that causes tar spot prefers temperatures in the 60-70F range, leaf wetness of at least 7 hours, and relative humidity above 75%. That’s why we can see it early in the growing season and then again later in the growing season. We will continue to see this favorable environment for the coming week. Once the environment (especially higher temperatures) is no longer favorable, the fungus isn’t able to grow and reproduce and that’s why we didn’t see lesion development throughout the summer in 2025. Susceptible host, pathogen able to infect, and the correct environment all have to come together at the same time for disease development.

SCAL Weed Science Field Day: Just a reminder of the Weed Science Field Day at SCAL near Clay Center on June 24. Attendees will see weed control options for corn, soybean, and sorghum with chemical and cover crop tools. The Ecorobotix machine will be showcased as another precision spraying tool. No charge, walk-ins are welcome, but RSVP is preferred for meal count to:  https://agronomy.unl.edu/fieldday.