Tar Spot Found in Corn

Corn Tar Spot Infections Discovered Early in 2024

Pamela Smith
By  Pamela Smith , Crops Technology Editor
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Examples of corn leaves infected by tar spot. The spots (stromata, in black squares) will be embedded in the leaf, raised (bumpy to the touch), and will not rub or wash off. In addition, they may be surrounded by a slight halo. (Photos courtesy of Darcy Telenko, Purdue University)

DECATUR, Ill. (DTN) -- Darcy Telenko isn't accustomed to finding tar spot in corn at the same time she sees Fusarium head blight (FHB) in wheat. But that's exactly what the Purdue University plant pathologist found on June 11. It was the earliest and lowest in the canopy she's ever seen tar spot in the field.

Active tar spot infections have also been confirmed in Iowa and Kansas this year. Farmers are advised to start scouting for the disease early to understand what fields may be infected and to properly time fungicides.

"Even though we have found it early, it is still best to wait until the important leaves emerge to get coverage (ear leaf and above)," Telenko told DTN. "Spraying before V8 will not provide the upper canopy protection needed down the road. It's not panic time. It's time to be aware and watching. Our data has shown that the optimum timing of fungicides for tar spot is between tassel (VT) to milk (R3) for best ROI."

Telenko said farmers will really need to look to spot tar spot this time of year because lesions (stromata) are small. Typically, most scouting is concentrated in fully expanded leaves that are knee to hip height in the canopy.

"What we've found so far is on the lower leaves (V1-V3) and on corn planted mid-April that was at the V5 to V7 growth stage," she said. "I've not found it in anything planted later, yet."


Telenko advises that growers pay particular attention to fields with a history of tar spot. Knowing a hybrid susceptibility to the disease is also helpful.

"Some fields have been clean as a whistle, and others are showing symptoms of it and some other minor diseases such as Anthracnose," she said. "We're not urging farmers to spray right now. We want them monitoring for evidence of the disease, so they know whether they need to use a fungicide and when it makes the best economic sense to do that.

"But we also don't want to wait too long. If significant disease develops in the upper canopy, then a fungicide application may be too late," she added.

For tar spot, you are looking for small, black, raised spots that may be circular or oval. They can appear on one or both sides of the leaves and leaf sheaths. The spots (stromata, in black squares) will be embedded in the leaf, raised (bumpy to the touch), and will not rub or wash off. In addition, they may be surrounded by a slight halo.

A quick and dirty "scratch test" to see if you can rub the spot off the leaf, especially if you have leaves with just a few small spots is advised.


State Extension specialists are tracking this disease closely. Another good spot to watch for confirmations is through the Corn IPM Pipe map: https://corn.ipmpipe.org/….

The national Corn Disease Working Group has developed a very useful fungicide efficacy table for corn diseases: https://cropprotectionnetwork.org/….

Find more thoughts from Telenko on the recent discovery here: https://indianafieldcroppathology.com/….

The University of Missouri's recent alert on tar spot can be found here: https://ipm.missouri.edu/….

Find more from DTN on tar spot here: https://www.dtnpf.com/….

Pamela Smith can be reached at pamela.smith@dtn.com

Follow her on social media platform X @PamSmithDTN

Pamela Smith

Pamela Smith
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