Identifying Tar Spot Before It Impacts Yields


Tiny specks can spell big problems for your corn yields. Regionalized Tar Spot outbreaks were a major challenge for some Midwestern farmers last year, making it an issue farmers have kept an eye on through the 2022 growing season. Joe Bollman, corn product manager for NK Seeds, talks about how quickly Tar Spot can spread if left unchecked.

“Tar Spot is a fungus that, if left unchecked, can spread rapidly. We’ve seen fields just starting to turn brown and then, a week later, they can be completely dead from Tar Spot, especially on hybrids that are sensitive to Tar Spot. So, it’s something that once you see that infestation, it can rapidly get a lot worse and significantly impact the yield potential.

Tar Spot can be challenging to identify because its distinguishable signs mimic many other corn diseases. He offers some tips to help farmers correctly identify the disease.

“It’s got these raised black, circular, fungal structures, so it actually looks like tar specks on the leaf itself. The lesions are bumpy, but they don’t easily rub off, and that’s something important for Tar Spot versus some of the other diseases out there. These spots also have a halo, or what we call a fisheye appearance around them that’s fairly distinguishable from a Tar Spot standpoint. The disease does begin in the lower corn canopy and moves up the plant canopy, so that’s something fairly distinguishable as well. It can also be found on healthy and dead tissue, so depending on the time of the year that you’re in the field, you’ll be able to identify these fungal structures both on healthy and dead structures. So, if in doubt, there are always questions if Tar Spot is new to somebody, feel free to contact their local county extension agent or local university to send in samples so they can get some diagnostics to confirm if it’s Tar Spot or not.”

Once an outbreak of Tar Spot occurs, it can significantly impact yields. He talks about how farmers can limit the damage.

“In-season when you start to identify challenges. right now, you’re looking at fungicide applications. Depending on where you’re at in the United States, we might be past that stage, but fungicide applications such as a Miravis Neo will work to help prevent some of the impacts from a Tar Spot standpoint. First and foremost, when it comes to managing Tar Spot, the hybrid selection is the first thing I would talk about. You can look in the NK Seed Guide to look at hybrids that have strong Tar Spot ratings. Crop rotation and tillage also do help. Tar Spot itself is not highly mobile, so rotating away from corn and tillage to bury that residue does have some success as far as minimizing the impact of Tar Spot. But hybrid selection’s always the number one thing I talk about with growers when it comes to looking at Tar Spot if they’re going to look go corn on corn.”

Bollman talks about what happens if a farmer misses that early application window.

“If they’re missing that herbicide application at that V4 to V8, that early timing, they can always definitely come back around that tassel timing. With the potential yield impact of Tar Spot, the return on investment, especially on sensitive hybrids or fields where you have cooler wet conditions, if you have a cooler environment that keeps that plant moist for a while, a fungicide application of depth will be warranted and likely be a good return on their investment. So, even if they missed the early timing due to whatever the reason may be, coming back at that later timing with a fungicide application is likely going to be beneficial in most years, especially if you’ve had a history of Tar Spot. There’s just enough inoculant built up in the soil for you to really protect yourself against a severe outbreak.”