Could Your Corn Fields Have Tar Spot?

Steve Sick

Sep 27, 2018

If you farm in the Midwest, you may not have noticed it, but there are more and more corn fields marked by plants with leaves dotted with curious black spots. These are tar leaf spots. And while this disease may not have stolen much yield from you this year, it’s something you should keep a lookout for, because tar spot has been popping up in more and more Midwestern corn fields during the past few years.

Where does tar spot come from?

Tar spot was first confirmed in the United States in 2015 on corn in seven counties in Indiana and 10 in Illinois. Soon after, it popped up sporadically in Iowa, Michigan, Wisconsin and Florida. This year, there have been sightings all across the upper Midwest.

The disease first appears in the lower to mid canopy of a corn plant, with leaves that look like they have been flecked with tar, sometimes surrounded by brown lesions. The spots can spread throughout the plant, eventually showing up on the husks. While it may seem small, severe infestations can be seen from a drone or plane as dark patches in the field.

It is not a seed-borne disease, but it can travel in storms and windblown rain. The original spores likely came up from a weather event that originated in Mexico or central America.

What is causes tar spot, and how much damage can it do?

The most intense damage from the disease happens when two pathogens appear in combination: Phyllachora maydis, which causes the black spots, and Monographella maydis, which causes the surrounding brown fisheye lesions. All reports to date seem to indicate only P. maydis is here in the U.S., as the fisheye damage from M. maydis has been seen, but not isolated.

The disease likes moderate temperatures, particularly cool, humid conditions, with extended periods of leaf wetness that encourage disease growth. These temperature specifics are likely part of why the disease is popping up primarily in the Midwest, but not in the South.

What should I do about tar spot if I notice it in a field?

Fields that were not treated with a fungicide at tassel are the most likely to be infected; however, even those that were treated may begin to show symptoms. Because the disease shows up late in the season, it is not expected to have a significant impact on grain fill. The biggest concern is standability, as the stalk may weaken as the plant is in the final stages of feeding the ear. Consider harvesting any infected fields first, to avoid the pitfalls of downed corn.

If you see tar spot in your fields this year, there’s a good chance you’ll see it again, as that appears to be the pattern. The spots produce spores that overwinter in corn residue; however, the disease does not appear to do well without a corn host.

There are several fungicide products labeled for an application at tassel to prevent infection. If suspect you have tar spot in a field, contact your local extension office or your nearest plant pathology lab to get more information on collecting and submitting samples for analysis.

Important: Always read and follow label instructions. Some crop protection products may not be registered for sale or use in all states or counties.

Please consult with an independent agronomist and consider your specific field conditions (e.g,. soil type and texture, weed pressure, and rotational factors) before making a chemical planning or purchasing decisions. You are solely responsible for complying strictly with the label and the laws in your jurisdiction and for your intended application. FBN Inputs, LLC is not a licensed pest control advisor or consultant, a Certified Crop Advisor (CCA) or Certified Professional Agronomist (CPAg) and this chemical list should not be used in states where a license may be required, including, but not limited to: California, Louisiana, Maryland, Mississippi, North Carolina, Oregon, and Washington. Sources:

Steve Sick

Sep 27, 2018