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Farmers encouraged to monitor corn for Tar Spot disease

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TIPPECANOE COUNTY (WLFI) — Farmers are encouraged to be on the lookout for a new disease affecting corn.

Darcy Telenko, a Purdue Field Crop Extension Plant Pathologist, has been studying Tar Spot disease since it was first identified in the United States in 2015.

“Tar Spot is a new disease in corn,” said Telenko. “It was first identified back in 2015, with the first reports coming from Indiana and Illinois."

Due to the disease being found late in the season, no yield was lost 2015. 

"It's a little black dot, exactly what the disease sounds like; Tar Spot,” said Telenko. “Like someone took a brush of tar and just splattered the corn."

It’s unknown how the disease got to the United States, but over the years it has continued to spread across the country.

“The disease has been around for over 100 years in Mexico and Central America, but we never found it in the United States until 2015,” said Telenko. “Since finding it in 2015 we really didn't think much of it, but 2018 we had he perfect storm of conditions and that was the first year we experienced yield loss here in Indiana."

Telenko is referring to a yield-reducing epidemic of Tar Spot that affected 38 counties in Indiana in 2018.

The disease can first be identified as a small black dot, but it can quickly spread within a few weeks.

"Other things can mimic it, like insect frass that can put little black spots on the leafs, so that's why I'm telling growers to scratch at it because it's really embedded in the leaf," said Telenko. "If it's insect frass it will rub off the leaf."

Telenko says Tar spot is corn specific and grows in the leaf tissue.

“How it got here we really don’t specifically know,” said Telenko. “There’s a lot of biology we don’t know about this pathogen, but what the disease does is it causes rapid blighting of the corn and it doesn't get to black layer.”

The Field Crop Extension Specialist says that’s exactly why Tar Spot can ultimately lead to yield reduction. 

Telenko says she can identify a positive county with just one lesions on the crop.

She says typically Northern Indiana is where the conditions are more conducive to the disease, which is where yield losses have occurred. 

"Generally we're not usually seeing this until late august or September,” said Telenko. “But this year it's kicking up early and I think it may be similar to what we saw in twenty 2018 where we had the first epidemic occur."

So far for 2021, 11 counties in Indiana have reported positive cases. Telenko says that number will keep rising as the growing season continues.

“If you don’t know if you have it or not you can send samples in to our lab, the Purdue Plant and Pest Diagnostic Laboratory,” said Telenko. “When you send it in we will be able to confirm what pathogens are growing on your crop.”

She also says Tar Spot disease can survive in the debris within the soil year-to-year.

Since the disease was first identified in 2015, Teleko says they have been evaluating fungicides for their efficacy.

“We’re still trying to learn but we do know some basic biology from the literature from back in the 60’s,” said Telenko. “The spores require a long period of leaf wetness and then relatively wet temperatures, but we’re thinking it’s more of the leaf wetness that makes the disease ramp-up.”

While crops are continuing to grow for the 2021 season, she's encouraging growers to scout their fields on a regular basis. 

"What I want to tell our growers is if you know you've had it in your field get out and start looking for it," said Telenko. "If you haven't seen it the field be aware of it and keep your eye out because we are trying to track it."

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