Corn leaf samples from three northern Illinois counties have been confirmed positive for the fungus Phyllachora maydis or "tar spot," according to University of Illinois Plant Clinic director Suzanne Bissonnette.

The Illinois Department of Agriculture announced news of the disease last week. USDA's Animal Plant Health Inspection Service confirmed positive samples of the disease in LaSalle, DeKalb, and Bureau counties.

The samples were collected from commercial fields by Monsanto breeders and pathologists and sent to Kiersten Wise of Purdue University in response to her inquiry for samples and distribution information of the tar spot pathogen.

Corn leaf samples from three northern Illinois counties have been confirmed positive for the fungus Phyllachora maydis. (Stock photo)

Wise and Purdue Plant Clinic director Gail Ruhl initially identified the pathogen, which is new to the United States, earlier this summer and submitted confirmation samples to the USDA.

Bissonnette said upon receipt of the Illinois samples, they diagnosed the fungus, contacted the University of Illinois Plant Clinic, and submitted the Illinois samples to the USDA for confirmation.

Corn tar spot symptoms
Tar spot has distinctive symptoms. The fungal fruiting body, called an ascomata, looks like a spot of tar on the leaf. Lesions are black and sunken oval or circular. They can be small flecks of about 1/64 of an inch up to about 5/64 of an inch. The lesions can merge together to produce an affected area up to 3/8 of an inch.

"If you run your finger across the leaf you will feel tiny bumps," Bissonnette said.

Prior to the Indiana finding, tar spot was known to occur only in cool humid areas at high elevations in Latin America.

Tar spot, USDA ARS photo

"There are actually two fungi that cause tar spot disease on corn: Phyllachora maydis and Monographella maydis," Bissonnette said.

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While Monographella maydis is known to be able to cause economic yield losses in Latin America, Phyllachora maydis is not known to significantly reduce yield.

Other pathogens may be confused with tar spot, especially the overwintering teliospore or black phase of corn rust. Also, there are many fungi, called saprophytes that feed on dead corn tissue and form black splotches on the leaves.

To date only one of the pathogens, Phyllachora maydis, has been found in Indiana and Illinois.

Growers who suspect tar spot are encouraged to submit a sample to the University of Illinois Plant Clinic.

"We would like to get a comprehensive idea of distribution in the state," Bissonnette said.

For more information on tar spot of corn, please see the USDA-ARS Diagnostic Fact Sheet.