Tar spot infections in corn are beginning to show up across the U.S., and strip-tillers should be vigilant about the disease and explore preventative measures.

In Pennsylvania, the first detection occurred in Berks and Snyder counties in the last week of June, and in Lancaster and York counties on July 1.

Tar spot was also confirmed on June 11 in northeast Kansas, and it’s been identified recently in multiple counties in Iowa and Indiana.

Since the disease began to emerge in 2018, experts are starting to learn more about its distribution and how to manage it. 

Kansas State University Row Crop Pathologist Rodrigo Onofre says tar spot is a disease that survives in corn residue, “so folks that have seen this spot before are going to be dealing with this disease for a long time.”

Here are some management tips for the disease shared by researchers: 

  • Onofre emphasized the importance of knowing hybrid susceptibility to tar spot, as there are some that are more susceptible — “which means you’re going to need to pay more attention to the crop."

  • Understanding how tolerant your corn is to both tar spot and grey leaf spot can help you determine the benefits you may or may not receive from a fungicide treatment, says Alyssa Collins, associate research professor at Penn State University Extension (PSUE). This may allow you to properly identify less-typical symptoms in resistant hybrids.
  • It can be tough to decipher tar spot from other corn diseases. “Get a wet cloth and try to clean the leaf. “If it’s tar spot, it’s not going to rub off, but everything else will clean off,” Onofre says. 
  • Don't jump the gun on spraying, PSUE says. While some symptoms on very young corn may be seen, the best results come from a fungicide application between V10 and R3. “The priority for protection is from the ear leaf to the top of the canopy. Scout to stay aware of disease progress in your fields, and if severity approaches 5% on the ear leaf or higher, yield impact is likely,” says , and Paul Esker, research pathologist at. PSUE. Growers can use this tool to train your eye for scouting.
     
  • The Crop Protection Network says the timing of fungicide application to hedge your bets against tar spot generally is at tasseling (VT) to the silking (R1) growth stage. Recent regional research has demonstrated that while there might be little yield benefit with an application at the V6 growth stage, a single application of fungicide at VT-R1 on average can result in as much as 7 bushels or more yield compared to not treating. This is compared to just 2-3 bushels at the V6 timing, which suggests that farmers are more likely to recover their fungicide costs if applying just one application at VT-R1. 
     
  • One spray for two diseases. Ongoing research at PSUE suggests a single, well-timed fungicide application can effectively manage both gray leaf spot and tar spot. Visit the corn fungicide efficacy guide to view the best products for the management of both diseases. 
  • Esker and Collins urges growers to check out the new Crop Disease Forecasting Tool — a web-based tool that utilizes local weather conditions to provide risk assessments for several field crop diseases, including tar spot. They suggest growers check risk levels when corn approaches tasseling to assist with fungicide application decisions.

For those who are already affected by tar spot, Onofre recommended steps to take now to prepare for 2026. As tar spot survives through the winter in crop residue, Onofre says those who have been affected by tar spot in the past will see it start from the bottom into the canopy. 

“The best thing that we could do for next year is start to think about more resistant hybrids, early scouting and a good herbicide management program,” Onofre says.