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5 Questions Growers Are Asking About Tar Spot


The top questions [and answers] about this harmful fungus

Tar spot first hit the corn belt in 2015. Originally a South American disease, it first showed its ugly face in Indiana as it spread to the U.S., and has now made it to the front door of South Dakota and even into Nebraska. For seven years, research has been done on tar spot and is still being conducted to learn more about this disease that comes in the form of a fungus, Phyllachora maydis. Here’s what we know.

1. What does tar spot look like?

Leaves with tar spots have small, raised, round or irregular-shaped black spots, which are fungal structures called stromata. Occasionally, tan to brown lesions with dark borders can develop. These are known as fisheye lesions because of their appearance.

This corn disease got its name because it looks as though tar was splattered on the corn leaves. Tar spot can be identified visually, however, a laboratory diagnosis is required to distinguish it correctly from other pathogens as it can get confused or mistaken for rust.

2. What causes tar spot?

There are still a lot of unanswered questions about tar spot and what causes it to show up and spread. So far, researchers say that the ideal climate for this disease is cooler temps, around 60-70 degrees at night, with high humidity. This situation creates moisture on the corn leaves. If moisture stays on the leaves for over seven hours, that makes the ideal scenario for tar spot to grow.

BW Fusion Agronomist Sean Nettleton, who resides in southeast Illinois, says they have not seen much tar spot so far this year in his region, as the weather has not been as conducive for the fungus.

“As we cool down and carry moisture way deeper into the mornings, we are going to see it start to show up,” Sean says.

3. How damaging is tar spot?

Like most fungi, the biggest impact of tar spot is yield loss. Tar spot will turn the leaf brown and make it die early. It affects corn in the beginning R stages with a reduced ear size and, if affected later, can be linked to poor kernel fill and ear weight. Reports show an average 30 bushel per acre yield loss when tar spot is in fields and left untreated. There have been some growers with severe infestation that resulted in around 60 bushel per acre losses. Observations also suggest that stalk rot and lodging are increased when tar spot severity is high. Severe tar spot can also reduce forage quality.

4. What should I do if I find tar spot in my fields?

If you believe you have a corn field with tar spot, send a tissue sample to a National Plant Diagnostic Network university diagnostic lab for a confirmed diagnosis. You can find the lab you need to send your sample to on their website at

For treatment, foliar fungicides have been successful in clearing tar spot, but they must be applied near disease onset to be most effective. If you have questions about the best fungicides to use, give us a call and we would be happy to help guide you in your decision making.

5. Can I prevent tar spot from hitting my corn fields?

Again, there is still a lot of research being done on this fungus. Right now, there is no prevention, so your best bet is to be proactive. Keep an eye on your fields and pick a fungicide that works on multiple diseases, including tar spot. Sean also suggests a product application following fall harvest.

“One thing I would consider if you have had tar spot in your fields is a Meltdown application to potentially break down the host,” Sean says. “Research is showing that tar spot may winter in the corn stalk stubble.”

He added that it has been seen in other diseases that breaking down plant residue can make it harder for the disease to come back the following year. This added disease protection is just one benefit on top of all the other results Meltdown’s team of microbes brings to fields.

Moving Forward

Pathologists across the Midwest have been collaborating on various research projects learning more about the disease and the impact it has on corn growers. Stay up to date with tar spot reporting at

As always, if you have more questions or concerns, give us a call. We are here to help protect your crops and your profits.

Tar Spot
Tar spot, also known as stromata, on a corn leaf. Photo courtesy of The Producer.

Tar Spot Field
A corn field that has been hit by tar spot. The left side of the field had a more severe hit causing the leaves to die and turn brown. Photo courtesy of Michigan Farm News.

Tar Spot Map
Current map of areas that tar spot has hit during the 2022 growing season in yellow. Reach of tar spot in previous years in gray.