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Review AccountProactive Monitoring Matters in Fight Against Tar Spot
Aug 13, 2025
Tar spot is to a corn crop what high blood pressure is to a person. If you don’t check it, you won’t know you have a problem. But both can cause extreme harm. Tar spot can kill corn.
Tar spot is relatively new to Federated’s service areas, and as it spreads (the spores travel by air), growers need to be aware of its potential economic impact. It’s not something to ignore.
Tar spot disease looks like specks of tar on the leaves of a corn plant, but the spots can’t be rubbed off. The spots will penetrate a leaf from top to bottom, spreading and overtaking plants until they die – as if hit by a killing frost.
At this time of year (or earlier in future seasons) growers “need to be checking their fields often, at least once a week, or every two weeks at most,” said Rod Gustafson, manager of the Albertville location. The pathogen resides in crop residual from year to year and, depending upon environmental conditions, it can seriously affect yields. “We saw it in crop residue last year when we did some soil sampling,” he said. “It’s here.” And it’s more than likely here to stay (studies show it can linger in the residue a minimum of one year or more).
Thus far this year, Federated Agronomists have found tar spot on corn in Hugo and Albertville fields, according to Kevin Carlson, Federated’s agronomy sales manager. Gustafson said the disease level is low [in his area], but its very presence indicates the need for closer monitoring.
“We need to take the disease seriously, and be actively looking for it,” said Carlson. In areas where tar spot has taken hold, yield losses from the disease have ranged from 20 to 100 bu./ac., depending on the point at which the disease infested the plant. If tar spot affects the plants late in the season – mid- September, for example – it won’t likely affect yields significantly. But early infection – such as mid-August or sooner – can be devastating.
If tar spot is found on corn plants at this point in the season (mid-August), it’s too late to do anything about it – but it’s the best time to make plans for future disease management. “If the conditions are right [for the disease] in the future, you can take preventative action,” said Carlson. It’s not a “see and spray” disease, he said. The only way to manage tar spot is to employ management practices to deal with it ahead of time. Advanced planning is essential.
When it comes to managing tar spot, Gustafson noted that hybrid selection is “the first step.” The seed companies Federated works with all offer hybrids with tar spot resistance ratings. Though no hybrid is totally resistant, one with a higher resistance rating will provide an added layer of protection from the disease.
Applying a fungicide, such as Miravis® Neo, at the VT stage can provide control that’s unavailable later in the season. However, the window for application in the VT stage is typically very small, so being ready with a plan – and your Federated Agronomist’s phone number – is especially important.
This video from Corteva® effectively describes the life cycle of tar spot. Call your Federated Agronomist with additional questions about tar spot and how you can plan to control it in coming seasons if this year’s monitoring reveals its presence.
Tar spot is relatively new to Federated’s service areas, and as it spreads (the spores travel by air), growers need to be aware of its potential economic impact. It’s not something to ignore.
Tar spot disease looks like specks of tar on the leaves of a corn plant, but the spots can’t be rubbed off. The spots will penetrate a leaf from top to bottom, spreading and overtaking plants until they die – as if hit by a killing frost.
At this time of year (or earlier in future seasons) growers “need to be checking their fields often, at least once a week, or every two weeks at most,” said Rod Gustafson, manager of the Albertville location. The pathogen resides in crop residual from year to year and, depending upon environmental conditions, it can seriously affect yields. “We saw it in crop residue last year when we did some soil sampling,” he said. “It’s here.” And it’s more than likely here to stay (studies show it can linger in the residue a minimum of one year or more).
Thus far this year, Federated Agronomists have found tar spot on corn in Hugo and Albertville fields, according to Kevin Carlson, Federated’s agronomy sales manager. Gustafson said the disease level is low [in his area], but its very presence indicates the need for closer monitoring.
“We need to take the disease seriously, and be actively looking for it,” said Carlson. In areas where tar spot has taken hold, yield losses from the disease have ranged from 20 to 100 bu./ac., depending on the point at which the disease infested the plant. If tar spot affects the plants late in the season – mid- September, for example – it won’t likely affect yields significantly. But early infection – such as mid-August or sooner – can be devastating.
If tar spot is found on corn plants at this point in the season (mid-August), it’s too late to do anything about it – but it’s the best time to make plans for future disease management. “If the conditions are right [for the disease] in the future, you can take preventative action,” said Carlson. It’s not a “see and spray” disease, he said. The only way to manage tar spot is to employ management practices to deal with it ahead of time. Advanced planning is essential.
When it comes to managing tar spot, Gustafson noted that hybrid selection is “the first step.” The seed companies Federated works with all offer hybrids with tar spot resistance ratings. Though no hybrid is totally resistant, one with a higher resistance rating will provide an added layer of protection from the disease.
Applying a fungicide, such as Miravis® Neo, at the VT stage can provide control that’s unavailable later in the season. However, the window for application in the VT stage is typically very small, so being ready with a plan – and your Federated Agronomist’s phone number – is especially important.
This video from Corteva® effectively describes the life cycle of tar spot. Call your Federated Agronomist with additional questions about tar spot and how you can plan to control it in coming seasons if this year’s monitoring reveals its presence.