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Tar Spot, a New Corn Fungus, Blows Into U.S.


Mesh bags filled with black-speckled corn leaves are buried throughout test locations for the University of Illinois Plant Clinic.

Clinic Coordinator Suzanne Bissonnette hopes those tiny black fungus pods indicative of tar spot fungus don’t last through an Illinois winter.

“When was the last time we had a new disease show up in corn? It’s been decades and decades. Why now,” she challenged the audience Jan. 20 at the Illinois Crop Management Conference. “That’s the big concern. Is it weather? It is the climate change? We don’t know what it is.”

The U of I Extension plant pathologist explained a likely route for this disease known only in Mexico and never seen in the United States until it was first found in early September in Grundy County. It was soon also found in nine other north central Illinois counties, as well as in six counties in northwest Indiana.

Bissonnette said the tar spot was found across several corn varieties exhibiting low to severe infection.

Besides knowing next to nothing about the disease, it looks much like corn rust does in debris, making it easy to misidentify, she said.

It spreads by spores that tend to develop after seven hours of wet conditions, low temperatures and high humidity. The spots feel rough and bumpy to touch.

Bissonnette did trace some information to Mexico and learned tar spot thrives in high elevations — “that’s really not anywhere in Illinois.”

“My thought is that this likely blew in from Mexico and Central American from Hurricane Bill and infected fields that had not been sprayed yet because of the wet weather,” she said. “But we don’t have a good idea when it showed up in the state.”

Tar spot tends to lead to two fungi infestations — phyllachora maydis and monographella mandis. While the phyllachora is not much of an issue, Bissonnette said the monographelle is not yet found here.

The Mexican records indicate when the two fungi appear together, corn yields are reduced.

Disease finds are required to be reported to the U.S. Environmental Protection Agency. Bissonnette said she received “an alarming reaction from the EPA, which wanted to quarantine at first.”

Now, it is awaiting results of the overwintering samples before taking steps.

“We’re just waiting to see if the disease survives,” Bissonnette said. “It seems to be on the rust pathway.”

Source: Karen Binder, AgriNews

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