Content ID


Tar Spot Continues to Surface in Corn

This fungal disease – first found in 2015 – is spreading into states like Iowa.

There’s a new corn disease that’s continuing to spread across the Corn Belt this year. 

Tar spot first surfaced in Illinois and Indiana in 2015 and has been found in northern Illinois each year since then. The fungal disease was first reported in eastern Iowa in 2016, the same year it was detected in Michigan and Florida. This year In Iowa, the fungal disease has been recently reported in 12 Iowa counties: Jones, Jackson, Johnson, Muscatine, Fayette, Clayton, Black Hawk, Buchanan, Delaware, Dubuque, Clinton, and Scott counties. These reports have been received late in grain fill, and severity in most fields is low. That’s also been the case in other parts where it has surfaced. So far, the disease is mostly considered an oddity, notes Nathan Kleczewski, University of Illinois Extension field crop pathologist. 

As the disease reports grow, though, plant pathologists note this suggests the fungus is overwintering in the Midwest. 


Tar spot symptoms include small, raised, black, irregular-shape spots scattered across the leaf surface, note Alison Robertson, Iowa State University (ISU) Extension plant pathologist and Ed Zaworski, ISU plant pathologist. These spots are fruiting structures of the fungus, known as ascomata P. maydis, say the Iowa State University plant pathologists. If a small section of the ascomatum is viewed under a microscope, hundreds of sausage-shape asci filled with ascospores are visible. 

As with most diseases, tar spot does have look-alikes; namely, common and Southern rust. At the end of the growing season, both rust fungi switch from producing orange-red uredinospores to black teliospores. Rust pustules filled with teliospores can be mistaken for tar spot ascomata. Remember that rust spores burst through the epidermis and the spores can be scraped away from the pustules with a fingernail. Tar spots cannot be scraped off the leaf tissue.

Researchers postulate that the pathogen is spread via wind and rain water. It has been proposed that spores of the pathogen arrived in the U.S. in a storm that originated in Mexico and Latin America.

Started in Latin America 

Latin America is where tar spot got its start. There, Kleczewski notes the disease is known as tar spot complex. It has severely cut yields there. In this case, two pathogens are involved: One fungus produces the black tar spots typically seen in Corn Belt outbreaks, and another produces toxins that can cause varying degrees of foliar blight and necrosis.

 Kleczewski says colleagues at CIMMYT in Mexico are currently working on identifying the toxins involved and how they may relate to virulence. He says it is important to note that there is very little known about tar spot complex, how the pathogens interact with one another, the epidemiology of the disease, and how the pathogens interact with their corn host.  

In addition, it is possible that this disease may act differently in Midwest production systems, as hybrid genetics, production practices, and environments differ from those in Latin America, says Kleczewski. 

Hybrid Interaction

In Latin America, hybrid resistance limits its impact, note Kleczewski and Damon Smith, University of Wisconsin Extension field crop pathologist. They reported on a subset of hybrids contained within a corn variety trial naturally infected with P. maydis in DeKalb, Illinois, that was rated for tar spot response on August 27. Plots were 20 feet long and 10 feet wide. Six ear leaves were rated for tar spot severity per 20×10-foot plot. Statistically analyzed data indicated a significant hybrid response to tar spot, ranging from 2.5% to 44% severity.

Kleczewski and Smith say the data illustrates that hybrid may play a role in the severity of tar spot disease development and impact on corn. They encourage those in industry and farmers to assess their hybrids for tar spot severity. They will continue to rate local variety trials the rest of this month in order to form 2019 management recommendations. 

Who to Contact

If you have observed corn with tar spot symptoms, notify Extension plant pathologists in your state. In Iowa, you may contact Robertson or Zaworski at Iowa farmers may also tweet at @isu_ipm, with a photo (if possible) and the name of the county in which the disease was found. Please include the county of origin, whether a fungicide was applied, and the hybrid, if possible.

Illinois farmers may send samples of leaves with tar spot symptoms – particularly those with necrosis associated with the lesions – to Dianne Plewa at the University of Illinois Plant Disease Clinic at this address.

In addition, Kleczewski and others in Illinois are working to assess potential variety response and yield impacts of this disease. Illinois farmers who want to be part of this may contact Kleczewski at 217/300-3253 or email him at He also can be reached on Twitter @ILplantdoc.

Read more about

Crop Talk