Pinney Purdue Virtual Field Day: Tracking tar spot in Indiana

WANATAH, Ind. — Reports of tar spot have starting trickling in across northern Indiana, said Darcy Talenko, field crop pathologist at Purdue University.

Tar spot has been tracked in Indiana since 2018, when a large epidemic occurred in the state.

“Last year, we confirmed 68 counties that had tar spot,” Talenko said. “Of those, the counties that I’m most worried about are those in northern Indiana, where we have the environmental conditions that may be conducive for this disease.

“Tar spot really likes high moisture conditions and generally lower temperatures, so around Lake Michigan is where we’re seeing the highest incidence of tar spot — and that’s where we’ve seen the greatest yield impacts.”

At the Pinney Purdue fields, researchers are beginning to find lesions of tar spot.

“If you think you have tar spot, the first thing we’re suggesting our growers to do is scratch at it,” Talenko said, “because many times these early leaf symptoms may be mimicked by insect frass.

“If it is insect frass, if you scratch at it, it’s going to come off. If it is a tar spot stroma, that stroma is growing and embedded in that leaf tissue, and it won’t scratch off.”

Many pathogens monitored in Indiana overwinter in crop debris, so it’s important to understand if the field has had a previous history of plant disease.

Once a pathogen is confirmed, pathologists look at whether the growing environment is favorable to disease growth.

“Each disease has its own set of temperature, moisture and leaf wetness requirements for that disease to occur,” Talenko said. “It’s really important to understand what those requirements are and if the environmental conditions are conducive for that disease to develop.

“For example, if it’s hot and dry, many of our foliar fungal diseases will not become a problem during the cropping season. But if the environment should change and we get a lot of leaf wetness and moisture, then there is greater risk for those diseases to develop.”

It’s important for farmers to scout fields and look for diseases such as tar spot in order to make well-timed fungicide application decisions.

“For tar spot, we’re still trying to determine that best window,” Talenko said. “We encourage many of our growers to hold on to their sprays because we want that fungicide there for the two to three weeks when it’s active, when the disease is going to be kicking up in the canopy.”

If you have questions about tar spot, you can send a picture or a sample to the Plant and Pest Diagnostic Laboratory, Talenko said.

Learn more about Purdue field crop pathology at https://extension.purdue.edu/fieldcroppathology/.