Corn Disease Alert

Tar Spot in Corn Surfaces Again in Illinois

Emily Unglesbee
By  Emily Unglesbee , DTN Staff Reporter
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Tar spot produces small, black bumps on corn leaves, which do not rub off the way some late-season fungal spots do. (Photo courtesy of Russ Higgins, University of Illinois Extension)

ROCKVILLE, Md. (DTN) -- Russ Higgins wasn't even looking for diseases when he stumbled across an exotic tropical corn fungus in a northern Illinois corn field this year.

The University of Illinois Extension educator was hunting for rootworm beetles in early September in LaSalle County when he noticed the distinctive black specks of tar spot, a corn disease hailing from the high valleys of Mexico.

His discovery marks the second year in a row that this tropical fungus, known formally as Phyllachora maydis, has surfaced in the U.S. In 2015, it was found in 13 counties in Illinois and seven counties in Indiana. In its native Mexico and parts of Central and South America, the disease often occurs alongside an additional pathogen, Monographella maydis, and can cause significant damage. However, that additional pathogen has not been found in the U.S. and no yield losses have been associated with tar spot here, said Suzanne Bissonnette, assistant dean of Agriculture and Natural Resources at the University of Illinois.

The sudden and isolated appearance of tar spot in the northern Midwest, with no documented pathway up from the South, has scientists scratching their heads a bit, said Bissonnette, who also heads up the university's plant diagnostic clinic.

"How it traveled here is not completely known," she said. "The presumption last year was that it moved in on a weather event."

Researchers collected samples in 2015 and then ran tests last winter to see if the disease could overwinter in Illinois. "We buried some samples to emulate tillage and left some on the surface," Higgins explained. When they examined the samples in the spring, they couldn't find any live fruiting structures, and it appeared the fungus was not up to the rigors of a Great Lakes winter after all, Higgins said.

"So we just assumed it was an oddity in 2015," he said. "It normally likes cool and humid conditions, and though we get plenty of humidity, our summers are far from cool."

After his surprise finding of tar spot in LaSalle County in the second week of September this year, Higgins began looking for the disease in earnest. He "easily" found more likely cases of the disease in DeKalb County and another researcher spotted some suspect leaves in Kendall County, as well, he said. The latter two counties' findings haven't been confirmed yet, but the disease is very distinctive, Higgins and Bissonnette both noted.

"It really looks exactly as if someone had flicked a bunch of tar on the leaf," Bissonnette said. "This fungus produces little fruiting structures that look like charcoal black spots and stick up from the leaf a little bit -- they feel like a little bump."

As corn leaves are dying down toward the end of the season, many different types of fungi can create small, black spots on leaves, but those will typically rub off with a finger swipe, Bissonnette added. Tar spot lesions are firm and won't rub away.

Based on research on the disease in Mexico, tar spot behaves like most other fungal leaf blights, Bissonnette said. Infected debris produces spores in the springtime, which are splashed or blown on to additional leaves, where lesions begin to develop.

"We know from research done in Mexico that the same fungicides we use to control our common leaf blights here are also what control this pathogen, so we can't know for sure when it would normally show up here," Bissonnette said. That also makes it hard to know how long it has actually been in the U.S. and how widespread it really is, she added.

When tar spot occurs with the second pathogen, Monographella maydis, the black spots are surrounded by a light-colored ring, and the entire lesion resembles an eye, Bissonnette said. That ring produces spores as well, and in those cases, the two pathogens have been shown to cause up to 30% yield loss, she said.

"But we don't consider tar spot a serious pathogen when it occurs on its own," Bissonnette said. The second pathogen has not yet been found in the U.S., but researchers are looking for it, she added.

For now, growers are encouraged to look for tar spot lesions on their maturing corn crop and send any suspected samples to their closest plant diagnostic lab. You can find more details on the disease from Purdue University here:… and the University of Illinois here:….

There's no immediate reason to be concerned about the disease, and farmers of northern Illinois don't seem to be, Higgins noted.

He actually contacted the farmer of the LaSalle County field where he found those first tar spot lesions and told him he had identified a surprising new disease in his corn.

"He just wanted to know when he could start combining the field," Higgins recalled. "He asked if I checked the moisture. And he wanted to know how wet the field was."

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Emily Unglesbee