Joe Stephan, an AgriGold area business manager in northern Indiana and southwest Michigan, first saw the tell-tale black specks of tar spot while touring corn research plots in the fall of 2016. At that time, tar spot was a new corn fungal disease, found only in limited locations in the Midwest.

“We had heard about tar spot and recognized the signs. But the disease tended to show up late in the growing season, so we didn’t expect much impact on yield. There was little data available,” recalls Stephan.

Since then, Stephan observed that tar spot expanded its geographic footprint and started to affect plants much earlier in the season. 

“In tar spot-infected fields, yield dropped by as much as 20, 30 or 40 bushels per acre,” Stephan says. The disease robs yield by reducing leaf surface area for photosynthesis so there’s less energy available for plant and ear development.

Tar spot has since spread throughout the Midwest and eastern Corn Belt – affecting Iowa, Illinois, Indiana, Wisconsin, Michigan, Kentucky, and many other states. Last season it was identified as far west as Nebraska. 

Creating a management plan 

To reduce tar spot disease pressure and prevent potential yield losses, Stephan advises farmers to work with their local agronomist or crop advisers to develop individualized farm and field management plans. 

Stephan’s recommendations include understanding the risk factors, planting corn hybrids with proven tar spot tolerance in fields that are unlikely to get fungicide applied and planning for timely fungicide applications. 

Track the disease 

The Tarspotter app developed by the University of Wisconsin-Madison is a useful tool to help forecast disease pressure and assist farmers in making management decisions. 

Tarspotter uses GPS coordinates to determine if weather has been favorable for the development of tar spot fungus during corn flowering in a specific field. 

Fields at risk for tar spot

As with other corn fungal diseases, hot and humid weather creates optimal conditions for tar spot. 

“Irrigated fields are more susceptible because leaves stay wet longer. And irrigated corn tends to be taller with wider leaves, holding more moisture in the canopy,” Stephan says. “River bottoms, low-lying areas or fields surrounded by woods may also have an elevated risk, as well as fields planted in corn after corn because inoculum remains in crop residue.”

Decoding tar spot ratings

Stephan says planting tar spot-tolerant hybrids is the first step in managing the disease, and the industry has done a good job of identifying hybrids with tar spot tolerance. 

By studying tar spot early and identifying less susceptible hybrids, AgriGold was one of the first seed brands to publish tar spot ratings for its hybrids. Ratings are available online. 

Stephan emphasizes that other seed brands have different methods of reporting tar spot tolerance. AgriGold, for example, ranks tolerance on a scale of one to five, with five being the highest level of tolerance. 

“Make sure you understand tar spot ratings and consult with your seed supplier to find the appropriate tolerance level for your field,” Stephan says. “Focus highly tolerant hybrids on fields that may be difficult to spray with fungicides, such as small fields or fields that are close to residential areas.”

Tar spot tolerance results

Matt Sebasty, who farms 3,500 acres of corn and soybeans in Indiana, sought out tar spot tolerance after previously experiencing yield losses. 

“We first noticed tar spot in the 2020 season,” he recalls. “We were taken by surprise by tar spot and lost over 30 bushels per acre.”

Sebasty was better prepared to tackle tar spot in 2021 – despite a wetter-than-normal growing season that set up ideal conditions for the disease. He tried a hybrid highly rated against tar spot.

“The hybrid performed exceptionally well against tar spot pressure and outpaced our 2021 farm average by 21 bushels per acre. Many of the other hybrids we grew took large yield losses due to tar spot,” he says. 

Fungicide strategies

It’s important for corn farmers to understand that no hybrid is resistant to tar spot. Highly tolerant hybrids should stay healthier longer but may still be affected by the disease. Therefore, farmers concerned about tar spot should plan their fungicide strategy now.

“Farmers should scout for the disease as the canopy starts to close and humidity rises,” Stephan says. “Keep in mind if it’s too wet to scout, conditions are ideal for tar spot.”

Fungicide timing can vary but in most cases two applications are needed. 

“Some farmers apply fungicides at planting and then plan for aerial application at the R3 stage. Others choose to spray at V8 to V10 stages and then follow up with another spray later,” Stephan says. “For those looking to make one application, the ideal time is just after tasseling or before tasseling if humidity is high.”

Don’t wait to act 

Stephan encourages farmers to avoid a “wait and see” attitude when it comes to tar spot due to potential limitations in fungicide supply and infrastructure to get it applied.

“Farmers should be prepared and have their fungicide purchased and ready to go,” he says. “Spraying for tar spot is usually very cost effective when you factor in a potential loss of 20 or more bushels per acre.”