Skip to Main Content

Corn Yield Robber: Tar Spot

Counties with tar spot as of 9-23-21; Image from Purdue Extension Field Crops Pathology websiteReports of early corn harvest in Whitley County, and in surrounding counties, are troubling. As farmers entered fields expecting corn yields to be one of their best in recent years, they were surprised to learn that something has robbed them of those top yields, and in many cases, its name is tar spot.

Darcy Telenko, Purdue Extension field crops pathologist, has been talking about this relatively new disease for the past few years. She was a guest speaker at our regional Northeast Purdue Agricultural Center field day in August, where tar spot was a part of her talk.

Telenko expects this year’s outbreak to result in significant yield loss.

Rainfall and long periods of leaf wetness early in July and throughout the growing season produced favorable conditions for disease development. Infected plants display small, raised black and circular spots or fungal structures called stromata on healthy or dead tissue of leaves, stalks and husks.

Telenko, whose research focuses on promoting sustainable and economically sound disease management practices in Indiana field crops, monitors for tar spot with her team each year beginning early in the season. As of now, only 11 Indiana counties remain on the team’s map without reports of tar spot. She suggests growers use their map and the Tarspotter App to help guide future disease management decisions. Find her tar spot map at

Telenko said that suggested management includes selecting moderately resistant hybrids, if available, and timely application of fungicide. “We are working hard to try to understand this new disease to minimize losses,” Telenko said in a recent Purdue Pest & Crop newsletter article. “The good news is that we found a number of fungicides are highly efficacious against tar spot here in Indiana when applied from tassel (VT) to R2 [blister].” Ongoing field trials have included fungicide application timings before and after these growth stages to determine the most efficacious application timings.

“Tar spot can survive in residue from previous seasons, so you need to understand field history to make informed management decisions,” she said. “If you have fields with significant tar spot, I would talk to your seed dealers to see if they have any hybrids with moderate resistance.”

Tar spot can be identified visually, but a laboratory diagnosis is required to distinguish it from other pathogens. Contact the Purdue Plant and Pest Diagnostic Laboratory to submit a sample. Find information on the laboratory at

Tar spot of corn is caused by the fungus Phyllachora maydis. It was first confirmed in the U.S. in 2015 on dent corn in seven counties in northwest Indiana and 10 counties in north-central Illinois. Telenko’s field research data and maps can be found at

Finally, find publications from Purdue Extension and Crop Protection Network on tar spot at


Banner image by Darcy Telenko

To Top