URBANA, Ill. — New samples confirm the spread of the disease Phyllachora maydis, or "tar spot," to corn in additional Illinois counties, reports Suzanne Bissonnette, director of the University of Illinois Plant Diagnostic Clinic and state coordinator for the federal Integrated Pest Management (IPM) program.

"Tar spot counties confirmed in Illinois – those we have actual samples from – are Putnam, Lee, Woodford, Ogle, Winnebago, Marshall, LaSalle, DeKalb and Bureau, with new confirmations in Kendall and Grundy counties, Grundy being the furthest-south county to date," she said.

"(The disease) is mostly in our northern counties. Some of the samples sent in were very severe in the leaves, some weren’t."

The tar spot pathogen was confirmed through samples from three northern Illinois counties by the USDA Animal Plant Health Inspection Service in Beltsville, Md., in September. The samples were collected from commercial fields by Monsanto breeders and pathologists.

Since then, scouting for the disease has been active in Illinois. The distinctive symptoms of tar spot are defined by small flecks of tarry spots on corn leaves that can range from about 1/64- to 5/64-inch. The lesions are typically black, sunken oval to circular and can merge together to form an affected area of up to 3/8-inch. The aberration will feel like tiny bumps when a farmer runs their finger over them.

Tar spot is a new corn disease never before tracked in the United States, with its origins in Mexico and central South America. Since it was such a late arrival, Bissonnette said that plant pathologists aren’t overly concerned about its impact on this year’s corn harvest in Illinois and Indiana, where samples of tar spot on corn leaves were first submitted this summer.

It actually takes two separate organisms for tar spot to contribute to yield loss in corn, based on what researchers have learned from tar spot research in Latin America, she explained. "One of the most important things about this disease is that to get any significant yield loss, you actually need the interaction of two pathogens. We found the one that causes the black spot, the tar structure, but there is another fungus that moves in after the initial infection (Monographella maydis), that produces a blight surrounding the tar spot.

"When those two show up together is when, at least in Mexico, yield loss actually occurs," she explained.

Researchers, including a professor from the U of I Plant Diagnostic Clinic working with an extension cooperator, will conduct tar spot "survival studies" over the winter to see if the pathogen can withstand harsh Midwest conditions to survive in corn stover and residue. Otherwise, the disease will have to arrive anew from warmer climes to reestablish each year.

"These are just the basic, beginning questions we need to have answered about this disease," Bissonnette said. "What’s going to happen in Illinois in the future, we just don’t know. Right now we’re still pursuing that second fungus, which is why we still want to receive samples and get a better idea of distribution."

Scientists seem to concur that Hurricane Bill, which reached into America’s heartland in June after gathering steam in the Gulf of Mexico, may have deposited the tar spot pathogen into Illinois and Indiana cornfields. It is considered relatively rare for crop diseases to enter the country in such a manner, though soybean rust disease offers a notable and relatively recent exception, Bissonnette noted.

"One of the concerns about tar spot in Illinois is that it is a new disease in the U.S. That always causes some concern for plant pathologists and for regulatory agencies," she said. "They want to know why it’s here."

She said the Plant Diagnostic Clinic and extension will provide updates on their tar spot research online at www.bulletin.ipm.illinois.edu and through this winter’s scheduled events with producers, such as the U of I Corn & Soy Classic.

Farmers wishing to contribute suspected tar spot leaf samples may mail them in a suitable envelope to the U of I Plant Diagnostic Clinic located on the university’s Urbana campus. Call 217-333-0519 to learn more.