Just because it’s too early to spray for tar spot doesn’t mean you shouldn’t be looking. Especially if you have found tar spot before, disease specialists urge you to be on the lookout.

“If you haven’t seen [tar spot] before but think you found it, let an Extension educator or state specialist confirm it,” says Mandy Bish, plant pathologist at the University of Missouri. “Rub the black specks. If they rub off, it was likely just insect frass. If black specks remain and are visible on both sides of the leaf, it could be tar spot — but get it confirmed.”

Why confirm it? Because state specialists track various diseases, including tar spot. “We rely on reports from the field to track where the disease is, and where it might go,” explains Darcy Telenko, Purdue plant pathologist. “But we won’t light up a county as having tar spot if we can’t confirm it is indeed tar spot.”

In most states, including Missouri and Indiana, you can contact a plant and pest disease lab associated with the state university to report findings. “Many people send pictures, but I recommend pulling samples at the same time,” Telenko says. “In many cases, we can’t make a positive identification, and we will want the sample.”

Tracking tar spot

Disease specialists like Bish and Telenko put out updates regularly, but you don’t need to wait for them. Bish explains that the National Institute for Food and Agriculture and its partners developed ipmPIPE to manage soybean rust across the Southern U.S. many years ago. The system is used now for various purposes, including tracking tar spot and southern rust.

“Any grower can click on the website, move to the tar spot screen, and see where it is confirmed in real time,” Telenko says.

On June 17, Jefferson, Doniphan and Atchison counties in the northeastern tip of Kansas were made yellow, indicating tar spot was confirmed, along with Marshall, Tama, Jasper and Poweshiek, four adjacent counties in east-central Iowa, plus Dubois, Warren, Carroll, Porter and La Porte counties in Indiana.

Counties shaded gray mean tar spot was confirmed in past years. For example, in Benton County, Mo., the disease was initially found in September 2023. Across the county line to the south, in Hickory County, shaded white, tar spot has never been detected.

Forecasting tar spot

Two apps developed at the University of Wisconsin help predict when environmental conditions have been favorable for tar spot.

“They continue improving the model as we learn more about the disease, and it is becoming more accurate all the time,” says Telenko, who collects information to help developers improve it. “Tarspotter is in its fifth iteration.

“If Tarspotter says tar spot risk is high in your area, still scout and find it before making management decisions.”

Tarspotter, maintained by the University of Wisconsin, functions by using data related to temperature and moisture conditions which favor the disease to predict when it should appear.

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