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Alyssa Collins, director of Penn State's Southeast Agricultural Research and Extension Center, holds up corn infected with tar spot at Crops Day at the Lancaster Farm and Home Center on Jan. 17, 2023.

LANCASTER, Pa. — If you’re growing corn in Pennsylvania and haven’t noticed tar spot yet, chances are that will change.

“If you don’t have tar spot already, don’t worry, you’ll get it,” said Alyssa Collins, director of Penn State’s Southeast Agricultural Research and Extension Center.

Collins spoke during a Jan. 17 Extension crops day at the Lancaster Farm and Home Center.

Tar spot, a fungus that causes raised black spots surrounded by dead tissue on corn leaves, was first detected in Pennsylvania on Sept. 29, 2020, in western Lancaster County.

Since then, the disease has spread to most of the south-central counties, and has appeared in western and northwestern Pennsylvania.

Maryland had its first tar spot case in August 2022, and the disease hit western New York last fall.

Collins stressed that the presence of tar spot is not a reflection on the farmer’s management techniques.

The spores spread easily on the wind, and Pennsylvania’s climate has good conditions for the fungus to overwinter. Once you see it in your fields, you’ll have to continue watching for it in subsequent seasons.

Tar spot thrives in cool temperatures with high relative humidity. Leaf wetness is also key for spore germination.

The dry weather in 2022 kept the disease at bay for the most part, but if conditions are right this year, Collins expects to see it throughout the state.

One positive thing about the fungus is it’s easy to spot.

“Once you see it once, you don’t really have too much trouble recognizing it and identifying it after that,” Collins said.

Southern rust and sooty mold can look similar to tar spot, but there’s a simple way to tell if the black stuff you see on corn leaves is truly tar spot.

Collins suggested rubbing an alcohol-based sanitizer on the leaves to see if it comes off.

“Do a little rubbing,” she said. “If it comes off, it’s not tar spot.”

Though Pennsylvania didn’t see tar spot until 2020, it first appeared in the U.S. in 2015 in the Midwest.

Fungicide trials from the Midwest showed that all fungicide options led to reduced levels of the disease, but applications with two or three modes of action performed best.

Those products with multi-action ingredients also helped the most in preserving yield.

In 2022, Penn State ran trials to gauge the best timing of fungicide applications. Applying at tassel and again three weeks later showed the best results.

“Take it with a grain of salt,” Collins said. “Because 2022 was a dry year, we did not get a lot of pressure.”

So while fungicides have proven to help, the biggest question that remains is when they should be applied.

Collins said research on fungicide applications is continuing, both in Pennsylvania and throughout the U.S.

There are also hybrids that have shown to limit susceptibility, which Collins suggested planting if possible.

“If you didn’t ask about tar spot this year, ask your seed dealer about tar spot next year and what products they have that can give us some resistance to this,” she said.

But regardless of management practices, Collins said the fungus is here to stay.

To prove her point, she held up a decorative dried corn shock that she purchased at a Michael’s craft store. The corn was “riddled with tar spot,” she said, adding that a colleague in Minnesota found tar spot-infected decor as well.

“Tar spot is well and truly here,” Collins said. “It is not going to go away any time soon.”


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