Forage 2025 | TAR SPOT: NEW DISEASE HITS WESTERN CORN BELT
A new disease is plaguing cornfields across the Midwest.
Phyllachora maydis is the fungus that causes a corn disease called tar spot. The fungus creates black spots on corn leaves and husks and produces thousands of spores, which can be spread in the air by the wind and by splashing water. Tar spot reduces the ability of the plant to photosynthesize, thus resulting in lower yield through poor grain fill, reduced kernel weight, and in extreme situations, the death of the plant. Severe cases can also cause stalks to fall, or “lodge,” complicating harvest.
Originating in South America, tar spot was first reported in the U.S. in northeast Indiana in 2015, and since then, has moved across the Corn Belt. Last year, it was confirmed in eastern South Dakota (46 of 66 counties), in the eastern half of Nebraska, and in nine southeastern counties of North Dakota.
It’s caused angst among corn producers in the U.S., partly because not a lot is known about the disease, but also because, in extreme cases, it has caused yield loss of up to 60 bushels/acre.
To discern between tar spot and insect frass, tar spot cannot be washed away or scratched off with a fingernail, where frass can. Another fungus, the southern rust fungus, is orange during much of the growing season, but turns black towards the end of the season, which makes it hard to distinguish from tar spot.
SDSU Extension Plant Pathology State Specialist Madalyn Shires spent two and a half weeks last summer driving 2,500 miles, chasing the fungus as it invaded South Dakota.
“Our state is very new to the infection,” she said.
Shires said the fungus spreads in two ways: through windblown spores, and by over-wintering on residue.
Windblown spores are the primary method, and the one most common in the Great Plains. The tiny black circles on the plant can make millions of spores, Shires said, which blow in the wind.
The second method is when the disease is already present in the field and over-winters in residue. Both methods are treated the same way, but with the disease in the residue, it shows up sooner in the growing season on the lower plant, where the wind-borne spores will usually be found in the upper leaves, later in the growing season.
“Residue-borne (tar spot) has an opportunity to get going pretty quickly in the growth system,” Shires said.
It is thought that the fungus over-winters, even in cold environments; tar spot is found in Canada, where it survives through the winter.
How fast the fungus grows and spreads is dependent on the weather; it grows best in daytime highs of 65-85 degrees, with moisture; high humidity, heavy dews, or irrigation, and when leaves are wet for seven or more hours. Pathologists have found that pivot irrigated fields get the disease worse and faster than dryland fields.
“It likes hot, so long as it’s not too hot,” Shires said. A serious outbreak of tar spot occurred in the eastern Corn Belt in 2018, then it relapsed, then another major outbreak happened in 2021-2022. “Weather really, really affects how much disease you’ll see,” she said.
For that reason, Shires does not like to make predictions about how serious the tar spot infections might be this growing season. “That’s the reason I don’t want to proclaim, yes, we’ll have the disease every year.”
For Tamra Jackson-Ziems, Professor of Plant Pathology at the University of Nebraska-Lincoln, she has changed how she thinks about the weather affecting tar spot’s spread due to the volume of irrigated crop acreage in Nebraska.
“We always talk about wet weather, a lot of dew at night or cooler temperatures, which are more favorable for tar spot,” she said. “But if it’s really dry in Nebraska, we irrigate,” which provides the moisture and cooler temperatures which make for favorable conditions for tar spot.
Studies are preliminary, but research done by Talon Mues, a student of Jackson-Ziems, shows that tar spot shows up earlier in irrigated corn, with sometimes more heavy infestations in the inner one or two-spans closest to the pivot center, versus the outer span.
“I think it has more to do with how the pivot moves,” she said. “The outer part is moving faster than the inner part and the middle, so those leaves (on the inside and middle) are wetter longer.”
Research on tar spot in irrigated fields will continue this year, with a project supported by the Nebraska Corn Board and carried out by UNL.
Cultural control of the disease seems to help in some cases but not all; as the disease is new to the Great Plains, studies and research is still underway.
Crop rotation might help control tar spot, as soybeans are not a host for the fungus. And tilling the residue under after the growing season is over may offer a bit of control, so spores are less likely to be wind-blown.
But in the states to the east, these are not proving to be as effective, Jackson-Ziems said. “My understanding is in the states east of us, who have dealt with tar spot longer, crop rotation and tillage has not been as helpful. Some pathogens overwinter in the soil or have a wide range of hosts or may infect grasses in the fence line or in weeds.”
There are other solutions for controlling tar spot.
Shires recommends using genetic resistant seed corn. There is no seed that is fully resistant to tar spot, but there are some with partial resistance.
Fungicide application also helps control tar spot. The window for applying fungicide is small and depends on the disease’s threshold and the corn’s growth stage. Crop consultants and agronomists can help determine the best time to apply a fungicide.
If tar spot doesn’t become a problem until later in the growing season after the yield is set, there’s no reason to spray. In the northern Great Plains last year, “we didn’t see it early enough in the growing season for most places to need to spray,” Shires said. In eastern Minnesota, where they’ve been dealing with tar spot longer, “they’re spraying, because it shows up earlier and earlier,” she said.
The Crop Protection Network website has the online Disease Severity and Insect Defoliation Training estimation tool with pictures producers can use to train themselves to make accurate assessments of what percentage of a leaf is covered with the black spots, to know when treatment should take place.
Jackson-Ziems advises judging the spots on the ear leaf, for consistency, and says before the ear leaf is five percent covered, fungicide treatment should be considered, especially if the field has a history of tar spot. Five percent “is when it gets our attention a lot more.”
Take other factors into consideration, she said, like an increased risk if the field is irrigated or a decreased risk if the weather forecast doesn’t predict rain.
Jackson-Ziems also said coverage of the leaf with fungicide is important for good control. “We often don’t get good coverage down deeper in the canopy, in the lower half of the plant. Other states have reported that fungicide coverage was critical.”
Sometimes aerial applicators are busy and it takes more than a week to get a field sprayed. Jackson-Ziems suggests that chemigation of most fungicides can be done by the producer, “then you can be on your own schedule.” She stressed that more research needs to be done to know how the application methods compare.
Both women cautioned against applying fungicide without good reason.
“Just be good stewards of fungicides,” Shires said. “We want to know the disease is there, and not be spraying just to spray. Make sure the return on investment is worth the money you’re putting on for control.”
Tar spot can affect harvest times; it causes the plant to die down more quickly.
“It kills the leaves quickly,” Shires said. “In severe infections, it can take a field from green to brown in fourteen days.” Producers will want to harvest fields with tar spot sooner than fields without; the fungus causes lodging and brittle stalks.
The same goes for silage; fields with tar spot should be harvested sooner, because of dry-down. Tar spot is not dangerous for cattle; it is safe for them to eat, whether it’s silage or in cornstalks. But the lack of moisture in the silage can cause problems with fermentation.
There are a lot of unknowns about tar spot, both women say. Jackson-Ziems speculates the disease might lessen in severity as it moves west where conditions are more arid. It might slow down, “but I do think there’s enough moisture under pivots that we could still see some tar spot. We’re learning,” she said.
And, ironically, the disease has been spotted at big box stores in at least two states. Both women have heard of corn stalks being sold as decorations, with tar spot visible on the leaves. In one instance, the stalks were displayed outside of the store, where the spores were more likely to be spread by the wind.
Producers can stay abreast of movement of tar spot during the growing season in a couple of ways.
Scouting fields is the first step; Shires and Jackson-Ziems both encourage producers to scout, and if fields are pivot-irrigated, to especially scout towards the middle of the field. Scouting should start by mid-July for the northern Great Plains and by June for Nebraska. If the area has a history of tar spot, scouting should be done even earlier. Jackson-Ziems suggests scouting every two or three weeks in the summer, seeing how far up the plant the fungus is, and monitoring the percentage of the fungus on the ear leaf.
There’s an app for tar spot as well. The Tar Spotter app allows producers to input their field location or a GPS pin, and answer questions such as what stage the corn is at, when was fungicide last applied, or whether it’s irrigated, and it will give a real-time risk of tar spot.
“It helps inform your decision to spray or not,” Shires said.
Jackson-Ziems said the app, developed by the University of Wisconsin, may be a better fit for the upper Great Lakes than for Nebraska, because it doesn’t factor in many variables. “I think it’s a great tool, I just don’t know how helpful it is for us yet,” in Nebraska, “especially since we don’t have disease all the way across the Great Plains states.”
These are hard questions, Jackson-Ziems acknowledged, and said that producers should garner advice from others, such as crop consultants, seed company agronomists, and local University Extension personnel for help.
Jackson-Ziems’ student, Talon Mues, traveled across Nebraska last year, collecting data from 25 fields. Under ten percent of these fields reached a disease severity point of five percent.
“We’re still in the infancy stages of this disease and its development and movement. It’s been in some parts of Nebraska for three years now, and I don’t think we’ve seen the worst yet.”
It takes a few years before the fungus builds up, she said. “We don’t worry too much the first couple of years. We monitor its increase and its spread to new areas. It was really about the third or fourth year in states east of us before they saw it was impacting yield, and some fields needed a fungicide.”
There’s no reason for alarm, Shires said.
“The disease is so highly dependent on the weather. There will be monitoring, using the app, and seeing what’s going on.
“I really think that’s what producers are going to have to do. Keep an eye out. If and when you see tar spot, see if the weather is conducive, see if the neighboring counties have it, and take those proactive steps.
“Keep it in the back of your mind, but don’t panic.”
More information can be found online at Crop Protection Network (https://cropprotectionnetwork.org/), including info for 2025 on fungicide efficacy, at: https://cropprotectionnetwork.s3.amazonaws.com/corn-foliar-efficacy-2025.pdf
