Tar spot in Branch County, Mich., was confirmed on June 24. With the potential to rob 50 to 100 bushels per acre in severe cases, tar spot should be on the radar for growers across all major corn-growing states.

“It can also cause severe lodging through weakened stalks,” says Martin Chilvers, Michigan State University professor of plant pathology. “While the infection has been identified in Branch county, I fully expect it to be in other counties as well.”

Infection is driven by moderate temperatures and moisture events. “Disease has hit hardest in areas that have received frequent moisture events throughout the season,” Chilvers says. “It’s just a matter of having favorable conditions for disease. Irrigated producers are at increased risk as irrigation can help drive disease.”

Farm Progress has developed a free special report on tar spot, providing information to growers about the disease and management options.

The National Institute for Food and Agriculture and its partners developed IPM Pipe to manage soybean rust across the southern United States many years ago. The system is used now for various purposes, including tracking tar spot and southern rust. A grower can visit the IPM Pipe map to see where it is confirmed in real-time.

In Michigan, tar spot was first found in 2016 and then spread across the west side of the state. An epidemic occurred in 2018 in western and central Michigan, and another epidemic across most of the state in 2021. These epidemics, Chilvers says, were driven by the wet growing seasons.

Scout regularly

Tar spot presents as small, black lesions on leaves, which quickly begin to destroy plant tissue. Plentiful rain, morning dew and fog kept leaves wet, allowing the pathogen to thrive in 2018 and 2021. Drier years such as 2022 saw far less incidence of disease.

“Growers should scout regularly, especially as we approach tasseling,” Chilvers says. “The disease develops quickly and can be somewhat challenging to scout for because the disease can go through multiple disease cycles in a season, building with each cycle. Be sure to confirm that it is tar spot. We’ve already seen confusion with bug poop this season. Use some water or spit to see if the lesion will dissolve. Tar spot lesions will also penetrate through both sides of the leaf.”

If growers are planning to make a single fungicide application, university data has demonstrated, in most years, a well-timed fungicide application between VT/R1 and R3 provides the best return on investment when tar spot is present. Early vegetative fungicide applications do little to slow the development of tar spot, Chilvers says.

Apps such as Tarspotter can be used to help gauge the risk of disease to time fungicide applications. Tarspotter, maintained by the University of Wisconsin, functions by using data related to temperature and moisture conditions that favor the disease to predict when it should appear.

A second app, Field Prophet, also predicts when to expect tar spot. This app is useful for predicting white mold in soybeans and turf diseases, as well.

The Crop Protection Network publishes a Corn Fungicide Efficacy Guide, which is updated annually and includes efficacy information on tar spot.

Select good hybrids

The best preventive treatment is to select hybrids with some resistance to the disease, Chilvers advises. No hybrids are immune, and there is a range of susceptibility.

With corn prices where they are, Chilvers says growers need to be mindful of the cost of fungicide applications. “The best ROI from a fungicide comes when we are treating to manage a disease,” he says. “In the absence of disease, the ROI value of fungicides is greatly diminished.”

What’s the threat in Michigan?

“My crystal ball doesn’t work that well,” Chilvers says. “However, if moisture events continue through the season, it could develop into a severe tar spot year.”

Concerning a cutoff date for treatment, Chilvers says it’s often hard to justify a fungicide application beyond the R3 growth stage, especially if the disease has taken hold. “A fungicide on a heavily infected crop will do little to protect yield,” he says.

If a fungicide is applied, leave a check strip to gauge return on the fungicide investment. “Without the check strip, you will have no idea how things would have been without it,” he says.