Early tar spot confirmed in northern Missouri

Tar spot is showing up in Missouri earlier than usual and crop specialists are encouraging farmers to scout corn fields.

Mandy Bish, a plant pathologist with University of Missouri Extension, says the fungal disease isn’t usually confirmed until late August or September. She tells Brownfield minimal cases are being confirmed in northern Missouri (and parts of Iowa), likely due to mild June temperatures.

“We’re roughly 3 to 4 degrees cooler minimum air temperature (at night) in June 2023 compared to the previous few years.”

Tar spot is a fungal disease that creates small, raised black lesions on corn leaves, stalks and husks. Purdue University says the disease likes cooler, wetter conditions and is commonly found in fields with a corn-on-corn rotation. The disease can cause yield loss.

Agronomist Erin Marlow with Cropwise Consulting says tar spot has been confirmed in a few fields she monitors in northwest Missouri that aren’t quite tasseling and it’s been difficult to identify.

“The corn we found had very few lesions. I don’t think many people would have seen it, but I thought it looks too much like what I don’t want it to be, so let’s send it off and verify,” she says. “There were some other mimics on leaves we sent off. I’m not sure if it’s bug feces, but not everything we sent off was confirmed tar spot.”

Bish says bug feces is commonly mistaken for tar spot lesions, but the difference is tar spot sticks on the corn leaf.

“And if it is a bug issue, you’ll usually see other symptoms on the leaf, like chewing from the insects.”

Fungicides are an option to manage tar spot, applied between corn tasseling and blistering. Bish says the timing of fungicide applications will be crucial.

“If someone applies fungicides early, they’ll have to make a second pass if the disease spreads, which means management will cost more money, and early fungicide applications can sometimes delay ear growth.”

But if the fungicide application is well timed, Bish says one pass can be effective.

Bish also says the University of Missouri’s Plant Diagnostic Clinic will test for tar spot and there are other resources including the Crop Protection Network, newsletters and text alerts farmers can sign-up for to stay informed as the disease spreads.

Hear more from Mandy Bish about tar spot in this Brownfield interview.

Hear more from Erin Marlow about tar spot in this Brownfield interview.

Photo credit: Erin Marlow from June 2023

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