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Watch for tar spot, but don’t panic!

Weather conditions in July could determine if this becomes a strong tar spot year.

Tom J. Bechman, Midwest Crops Editor

June 20, 2024

3 Min Read
Black tar spots on corn leaves
RECOGNIZE TAR SPOT: Here are the classic black specks that show up on corn leaves when tar spot first appears. Make sure they are truly lesions and don’t rub off. Darcy Telenko

Wet spring weather, especially in late May, brought cries from those who offer fungicides. Their message was simple: This might be an intense tar spot year, so be prepared to spray. However, several university disease specialists offer a somewhat different message: Be prepared, but wait and see what happens in July and August — the most economical time to spray fungicide on corn is the VT to R1 time frame.

Indeed, tar spot does like wet weather, and positive identification of tar spot was reported first in northeast Kansas, then in east-central Iowa and northern Indiana during the second week of June. However, specialists insist that what happens in July and August will determine if tar spot threatens corn yields, not what happens in June.

“We confirmed tar spot in central Missouri on June 14,” says Mandy Bish, a plant pathologist with the University of Missouri. “This is slightly earlier than in 2023, when we found it on June 23. However, in 2023, it turned dry, and disease progression slowed until mid-August. If it turns hotter soon, as some suggest, the disease could slow down.

“Our best advice is to be patient. Scout, especially where you know there is inoculum, like the northern half of Missouri. But remember that the most consistent results have come for a well-timed fungicide application between VT and R3.”

Related:How to manage tar spot in corn

What others say

One place where tar spot has posed a threat since it first appeared in the U.S. almost 10 years ago is Indiana, and especially northern Indiana. Darcy Telenko, a Purdue University plant pathologist, works on tar spot year-round and echoes Bish’s advice.

“Yes, it was a wet spring late, and yes, tar spot has been positively identified now in a few locations,” Telenko says. “But it is only June. What matters is what happens as we move into V8-, V9- and V10-stage corn, and then on into July and August.

“Tar spot prefers humid but relatively cool weather. If it heats up or dries out, disease developing in early June may not progress. If conditions stay favorable for tar spot, we will track its development.

“We have studied tar spot for multiple years, and the only time so far it paid to treat tar spot at V10 and again at R1 was in 2021. It started early, and conditions remained favorable for the disease. What our data shows is that the best bet for a positive return on investment is to hold off spraying until VT to R3. Once you get to that point, when you pull the trigger depends partly on disease pressure at the time, both from tar spot and other diseases.”

Free resource: Tar spot management tips

In Minnesota, Dean Malvick, a plant pathologist at the University of Minnesota, notes that in past years, tar spot was documented in 36 counties, mostly in southeastern Minnesota. No detections were reported as of June 11. “We advise growers to scout if it has been in their county before,” he says. “Think about which hybrids in your lineup are most susceptible.

“As we move into August, that’s the time to think about potential fungicide applications. Be thinking about where you would spray and how you would spray if it became necessary.”

About the Author(s)

Tom J. Bechman

Midwest Crops Editor, Farm Progress

Tom J. Bechman became the Midwest Crops editor at Farm Progress in 2024 after serving as editor of Indiana Prairie Farmer for 23 years. He joined Farm Progress in 1981 as a field editor, first writing stories to help farmers adjust to a difficult harvest after a tough weather year. His goal today is the same — writing stories that help farmers adjust to a changing environment in a profitable manner.

Bechman knows about Indiana agriculture because he grew up on a small dairy farm and worked with young farmers as a vocational agriculture teacher and FFA advisor before joining Farm Progress. He works closely with Purdue University specialists, Indiana Farm Bureau and commodity groups to cover cutting-edge issues affecting farmers. He specializes in writing crop stories with a focus on obtaining the highest and most economical yields possible.

Tom and his wife, Carla, have four children: Allison, Ashley, Daniel and Kayla, plus eight grandchildren. They raise produce for the food pantry and house 4-H animals for the grandkids on their small acreage near Franklin, Ind.

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