Black tar spot can be identified on corn plants by its distinctive raised black spots embedded on the leaves, stem and husk.
The yield-robbing disease was first found in 2015 in 10 counties in Northern Indiana. In 2019, that number increased more than five-fold, bringing the total numbers of counties with confirmed cases to 68.
According to Darcy Telenko, Purdue Extension field crops pathologist, those cases could be higher.
“I suspect we probably could confirm it in all 92—we just never got to those other southern counties,” she said. “It’s the northern part of the state where we’re most worried because it’s been there and we had yield effects in 2018.”
Like several diseases, moisture helps progress tar spot. It thrives in humid conditions with long periods of leaf wetness. Irrigation can exacerbate the problem, said Telenko.
“If we’re irrigating frequently with light irrigation events, we’re keeping that crop canopy wet longer, and that’s increasing the risk for the disease to develop,” she said.
For the farmers who run irrigation, Telenko says the key thing to do is scout early and scout often.
“Look and see if it’s active in the lower canopy, and then if you’re irrigating, stay on the water, and you may need to cut back if the disease is starting to explode,” she said. “We may need to water hard early and letting it dry off instead of multiple small rain events.”
There’s no variety on the market/that is tolerant to black tar spot, but Telenko suggests talking with your seed dealer/when you make your seed selections this winter.
“Being proactive and plan for what you need,” said Telenko. “There’s very few hybrids that have tar spot ratings, but work with your seed dealer to see if they know some that may be less of a risk. Be prepared to be out looking for it.”