Scout for tar spot, corn disease this summer

During an on-farm trial last year, AMP Fungicide was applied to corn at 10 fluid ounces per acre to treat tar spot.

INDIANAPOLIS — Last year tar spot was a major concern for some growers, contributing to yield losses ranging from 20 to 60 bushels per acre.

As of Aug. 10, fields in La Porte, Porter and St. Joseph counties in Indiana have tested positive for tar spot.

While incidence is still low, it’s expected that tar spot will continue to be discovered in the weeks to come.

Paula Halabicki, technical marketing manager at BASF, shared her knowledge about tar spot with AgriNews.

What is tar spot?

Tar spot is a pathogen that’s relatively new to the U.S. that’s impacting our corn acres. It was found in 2015 and has spread from there. Tar spot is a residue-born pathogen, so normally it will start building in the bottom of the canopy first.

How do you find tar spot?

It’s really good to get out and start scouting to look for the usual symptomology, which are black, raised dots or spots on the leaf. If you do see those symptoms, you can take samples or pictures and send them to your local pathologist to confirm the disease.

If you’re starting to find the disease on most of the plants within that field, then you definitely want to think about control measures.

Was tar spot a problem last year?

Last year, tar spot blew up based on environmental conditions, which were favorable for tar spot to develop right around that tassel or reproductive stage. Last year there were incidents where people were reporting 40 to 60 bushels per acre yield loss, which was quite severe.

What weather conditions might lead to tar spot?

It tends to be cooler, really humid conditions. You need leaf moisture for an extended period of time for tar spot to develop.

That doesn’t necessarily mean rain, it could be dews that last throughout the day. That’s why we find the issue mostly around the Great Lakes. Michigan, Ohio, Indiana, Illinois — those are the states being impacted.

And it’s really because of that level of humidity and cooler temperatures required for disease to develop.

What problems can tar spot cause?

It varies depending on the hybrid and relative maturity, but it could be anywhere from 0.3 to 1 bushel yield loss for every percent increase in severity. It can have quite a severe impact on yields.

How can you fight tar spot?

There are a number of fungicides on the market that do affect tar spot. From a BASF perspective, we have two products on the market – Headline AMP and Priaxor fungicides.

The active ingredients within those products have been used for a long time to control tar spot in South America, where they’ve been dealing with the issue for quite some time. They provide a really good level of efficacy on the disease. It’s always better to apply preventatively, to try and minimize the impact on the ear leaves of corn.

Any other words of wisdom?

It’s really important for our customers to be out scouting and to make sure they understand these diseases. Tar spot is very important because of the impact it had last season. But a lot of the same conditions in which tar spot develops cause Gray Leaf Spot and Northern Corn Leaf Blight, for example.

We encourage you not to forget about those residue-borne diseases. Make sure you’re out scouting and protecting corn plants to maximize yield potential.

Learn more at

Erica Quinlan can be reached at 800-426-9438, ext. 193, or Follow her on Twitter at: @AgNews_Quinlan.


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