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Serving: IA
small dots of tar splattered on a corn leaf
TAR SPOT: Symptoms of this disease are black dots resembling tar splattered on a corn leaf. The hard, black bumps won’t rub off. That’s one way to distinguish tar spot from other diseases that look similar.

Tar spot marches west in Iowa

Corn Source: A relatively new disease in the Midwest, tar spot expands into more counties in Iowa.

Tar spot, a relatively new foliar disease in corn in Iowa and the Midwest, made its appearance again in 2019. In 2018, tar spot was observed in 12 counties in eastern Iowa. This year, it has been observed in 59 counties, mainly in northeast, east-central and central Iowa. It has been observed as far west as Crawford and Shelby counties.

This disease first made its appearance in the U.S. in 2015. With it being a relatively new disease, there is much to learn.

What we know

Tar spot is a fungal disease caused by the fungal pathogen Phyllachora maydis. Prior to being found in the U.S., this disease was only found in Latin America. So far in the U.S., this disease has been confirmed in Florida, Illinois, Indiana, Iowa, Ohio, Michigan, Wisconsin and most recently in Minnesota.

Understanding the disease triangle for tar spot provides insight into what we are seeing in the field. Plant pathologists use the disease triangle to help understand what it takes to see a disease show up. In order to have a disease infesting a crop, we need to have a susceptible host, the pathogen and a favorable environment.

A susceptible host is not considered a limiting factor for tar spot. No current hybrid is resistant to the disease. In addition, corn is susceptible for infection at any development stage. Tar spot has been observed on corn in the U.S. as early as the third leaf (V3) growth stage. In Iowa, it has been observed later in the growing season during mid- to late grain fill (R3 to R6 growth stages).

Plant pathologists believe that the pathogen overwinters in corn residue. When favorable conditions are observed, spores are produced, and those spores can be dispersed either via rain splash or wind.

A favorable environment will likely drive disease development each year once the pathogen becomes established in an area. Cooler (60 degrees to 70degrees F) and wet conditions (free moisture on leaves and relative humidity over 75%) favor infection and tar spot development. We didn’t see these favorable conditions until later in the season. Signs of tar spot take about two to three weeks to develop.

Potential impact of disease

The impact tar spot has on yield is relative to its onset (when it shows- up) and the growth stage of the corn. The earlier it shows up in the growing season, the greater the risk for yield loss compared to it arriving later in the growing season. 

In Iowa, this disease has generally come in late in the growing season. However, other states have seen it come in earlier during the grain-fill period of the growing season. Preliminary data from Illinois, Indiana, Michigan and Wisconsin show that hybrids vary in their susceptibility to tar spot. Also, preliminary data from those states show tar spot may cause a loss per acre of 0.32 to 1.36 bushels for every 1% increase in tar spot severity.  

What to look for

When tar spot infects corn, small round to semi-circular and irregular-shaped, raised black spots appear scattered across the corn leaves. These spots have also been observed on leaf sheaths and husks. It basically looks like tar was splattered on the plant. These spots can be observed on green and senesced leaf tissue. Occasionally, tan to brown lesions with dark borders may form around the tar spots. These are sometimes called fisheye lesions.


tan to brown lesions with dark borders  around the tar spots onplant
BROWN LESIONS: Sometimes tan to brown lesions with dark borders may form around the tar spots, particularly later in the season.

Rusts (common and southern) will sometimes be confused as tar spot, particularly at the end of the growing season when rusts change from reddish or orange to looking darker brown or black in appearance. For this reason, rusts can be mistaken for tar spot. To tell the difference, rust spores will burst through the epidermis of the corn leaf and can easily be scraped away with a fingernail. Tar spot will not scrape off the leaf. 

Common and southern rust on leafNOT TAR SPOT: Common and southern rust can be confused with tar spot. Rust spores on a corn leaf can be easily scraped away with your fingernail; tar spot will not scrape off the leaf.                

Research is being conducted to better understand this new disease and how to best manage it. Residue management and crop rotation may help to reduce the inoculum present in the field. However, residue management may not be an option in some fields.

Hybrids do vary in their susceptibility to this disease. Noting what corn hybrids you saw tar spot in this year, and visiting with your seed dealer, can help you select hybrids that are more resistant to this disease. Since tar spot is a fungal disease, a fungicide may also help, depending upon when the disease is observed in the growing season and timing of the application. However, the data available is limited.

Damon Smith, plant pathologist at the University of Wisconsin, is working on developing and validating a tar spot prediction model called the Tar Spotter App to better predict the risk of seeing tar spot show up in a field. He is working with other university Extension and industry professionals to help validate the tool this year.

For more information, refer to the Crop Protection Network Tar Spot publication.

Vittetoe is an ISU Extension field agronomist covering southeast and south-central Iowa. Contact her at


TAGS: Corn
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