Where there’s smoke, there’s fire – and smoke alarms are starting to go off to alert corn growers to the disease fire that’s already ignited in some parts of the Midwest: tar spot.
Farm Journal Field Agronomist Ken Ferrie is monitoring tar spot reports closely and working with growers to create and implement action plans for their specific fields to address the disease.
“We’re finding tar spot here on the bottom leaves, meaning it’s homegrown tar spot,” says Ferrie, who’s based south of Bloomington, Ill. “Finding tar spot here the last week in June means that the plants were infected already by the second week in June.
“There’s no way to sugarcoat this, guys. Tar spot showing up here in June means we’ll have hell to pay in August,” he adds.
The understanding of how the disease progresses quickly came at a cost to Illinois corn growers in June 2021, Ferrie recalls. Few agronomists or farmers understood the impact of what they were seeing unfold in cornfields.
“We weren’t sure what that meant at the time, but by the end of September, anyone in ‘tar spot alley’ realized just how devastating this disease could be. There were massive amounts of down corn with yield hits ranging from 20- to 60-bu. per acre,” Ferrie says.
“Northern Iowa farmers experienced something similar in 2022,” he adds.
You can listen to Ferrie’s complete Boots In The Field podcast, where he discusses how to effectively address tar spot here:
Here’s a look at where the Crop Protection Network has confirmed tar spot in 2025, so far. See county-level infections at Tar Spot of Corn Map:
How Tar Spot Differs From Other Corn Diseases
Ferrie says most leaf diseases in corn are a problem because they destroy the leaf surface and tear up the plant’s solar panels – the leaves that fuel corn’s ability to make food.
“When managing most leaf diseases, we are usually concerned about the leaves above the ear. Those are our money makers,” Ferrie says. “When we start scouting, we’re looking one leaf below the ear leaf, and we also look up, looking for any disease lesion and the halos around it.”
With tar spot, the attack is coming from the bottom of the corn plant. “So that is where you need to look when you’re scouting,” he says.
Another way tar spot differs from most diseases in corn is it’s parasitic.
“When it infects the bottom leaves, it will pull nutrients away from the corn plant. So not only will it tear up the solar panels and damage the leaves, but it’ll rob nutrients,” Ferrie explains. “It’ll siphon nutrients off from the plant.”
Like extreme parasite levels in livestock that go untreated, a tar spot infection can pull corn plants down and kill them prematurely.
The plant essentially cannibalizes itself, contributing to reduced yields and standability issues at harvest.
Have Your Plan Of Attack Ready
Every corn grower needs to have a plan in place ahead of time for how to handle the tar spot issue. Farmers needing help in selecting fungicides can check out efficacy ratings and other information the Crop Protection Network offers here.
Ferrie says his standard recommendation for a fungicide application is to wait for brown silk, if disease pressure is not at threshold.
“You want more backend horsepower for those D hybrids – those hybrids that make a big portion of their yield in kernel fill,” he says. “However, if you’re at threshold, spray the field, don’t wait for brown silk.”
Here is a look at tar spot fungicide application recommendations from the Crop Protection Network:
Ferrie shares two treatment scenarios for Illinois growers who are experiencing tar spot now – one for those growers in Scott County (where pollination is finishing up) and the other for Woodford County (where pollination is just starting).
“Our recommendation for the Scott County farmers is to spray as soon as the corn is done pollinating, which would be this week,” he advises.
“Throw a hard punch at these fields, because the tar spot and existing disease pressure is there. Then be ready to come back in 21 days with the second shot of fungicide, if warranted, of a cheaper pass. There’s where we throw the generics in, and that pass is to drive stability in your crop stand, so it’ll stand up.”
There are a couple of additional things guiding this application timing. For one thing, Illinois growers still have 60 days to go to protect grain fill in the crop. Another thing to keep in mind, he says, is that the curative factor of a fungicide only covers infections that have happened in the past 48 hours prior to application.
For those fields in Woodford County, Ferrie is telling farmers to wait for their fields to pollinate before pulling the fungicide trigger, because most fields are not at threshold with other diseases present above the ear leaf.
“Again, set up a plan to spray again in 20 to 30 days, and that’ll be weather and disease progression dependent. If we should have a drought – conditions that would slow the disease progress – we might not spray,” Ferrie says. “But If we get the weather pattern to give us a big crop, we most likely will need to spray.”
Ferrie says he understands, given the financial constraints this year, that many corn growers are struggling to spray a fungicide once much less doing so twice this season.
“For the cautious guys out there who are thinking, ‘it ain’t gonna happen, Ferrie; I am not spraying in a $4 corn market,’ this too is a plan. But you need to prepare to go early with your harvest, pre-book some dryer gas and look for pick-up reels,” he advises.
“The good news is we’re ahead of this, and we have experience to fall back on. We have time to react as this does or doesn’t unfold.”
Consider Getting Your Fungicide Applications Booked
Because fungicides need to penetrate the crop canopy and go lower on the plants to address tar spot, that can require adjusting how applicators spray the product.
“If you’re going by air, we need to narrow up the swath,” Ferrie says. “Bigger droplets being applied are the ones that penetrate that canopy, and this goes for planes, helicopters and drones.
“Be aware that when you ask an applicator to cut his swath width by a third, it will come at an increased cost, so be prepared for that. But this step will reduce the streaking issues we often see when spraying tar spot from the air.”
For ground applications to address tar spot, the thing to do is have conversations with your supplier now – plan ahead – as ground rigs cannot cover the same number of acres an aerial applicator can get across in a day. And note, your suppliers probably already had a full lineup of fields to spray before tar spot showed up.
“Remembering back to 2021, growers were waiting two weeks or more to get an application made. If 2025 is a repeat of 2021, we’ll need to go at this from multiple directions,” Ferrie says. “At the end of the day, we need to prepare and scout, scout, scout.”
Sign Up For Farm Journal Corn & Soybean College
Ferrie and his agronomic team at Crop-Tech Consulting are finalizing plans for the annual Farm Journal Corn & Soybean College, as well as the Crop-Tech events.
“These events been approved for 13 CEU credits across four different categories,” he says. “If you need some CEUs, this is a great place to get them.”
The theme for this year’s event is Making A Stand. Topics Ferrie and team are addressing during the two-day program, July 22-23, include:
- planter add-ons that pay off
- spray nozzle science
- in-field planter diagnostics
- ear count and rooting depth
- bean stress and variety response
- integrating soybeans for yield
You can sign up online here or call the Crop-Tech Consulting office to register at (646) 801-0591
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