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Release Date: 07/01/2020

By: Dr. Marty Chilvers and Kristin Poley

What is Tar Spot?

Tar spot is caused by the fungus Phyllachora maydis, and can be identified by the raised, black spots that appear on corn leaves and husks. Surrounding some of the black spots, there may also be a tan halo, which is called a fish-eye lesion. Each one of those black tar spots is the reproductive structure of the tar spot fungus, which can produce thousands of spores. Research has demonstrated that the tar spot fungus can overwinter in those black spots releasing spores the next year. Don’t confuse insect frass with tar spot. Frass will rub off the leaf with some water, while tar spot structures will be embedded in the leaf and often protrude through the underside of the leaf.

Recent Tar spot pressure in MI – 2018 and 2019

Tar spot pressure in 2018 was significant with losses of up to 50 bu/A. Frequent rainfall in 2018 and increasing disease inoculum load in the region led to those dramatic losses. The 2019 season saw a much drier July and August which put the brakes on tar spot, and resulted in much lower losses. Although tar spot pressure was relatively light across Michigan in 2019, the disease did continue to spread and is now present across most of our corn acres.

Outlook for Tar spot in 2020

  • What weather patterns could lead to greater tar spot pressure?

Cool, humid conditions with prolonged leaf wetness can lead to higher tar spot pressure in fields. Windy and rainy conditions may also spread spores to new fields, or new areas within a field. Irrigators should be careful that they do not promote tar spot with light frequent irrigation which may increase leaf wetness events.

  • At what point in the season should scouting start?

Tar spot has been found in early July in some years. As we approach silking it’s a good time to scout for tar spot and other diseases such as northern leaf blight and gray leaf spot.

  • What control measures are available?

There are no hybrids currently available that are completely resistant to tar spot, but some varieties may have more resistance than others.

Testing of several fungicides is underway across the state, and the Midwest, for management of tar spot. One of the biggest factors for successful tar spot suppression is application timing. To date, early applications at the V8 growth stage are too early and provide little benefit. Research this season is going to focus on optimizing fungicide timing. It appears that scouting is critical to get fungicide applications at the beginning of an epidemic.

How can farmers get confirmation of tar sport in their fields?

Feel free to reach out to get assistance in identifying tar spot. Pictures can be emailed to or via twitter @MartinChilvers1. Physical samples can be submitted to the MSU Plant and Pest Diagnostic Clinic

How can farmers get more information?

Additional articles and resources can be found on the MSUE News for Ag website, and the Crop Protection Network

CPN article on tar spot:

Fungicide efficacy table:

YouTube demonstrating tar spot symptoms

Photo credits: The Chilvers Lab