Seeing Spots?

Tar Spot Corn Disease Catching Eyes in the Midwest

Emily Unglesbee
By  Emily Unglesbee , DTN Staff Reporter
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Tiny black spots against a brown lesion are a symptom of the tar spot complex in corn -- a new disease that is surfacing in greater quantities in the north-central Midwest this year. (Photo courtesy of Martin Chilvers, Michigan State University)

ROCKVILLE, Md. (DTN) -- If you're seeing spots in your cornfield, don't worry -- your vision is fine.

A new corn disease called tar spot is surfacing in many Midwestern cornfields this year, sometimes at levels high enough to cause standability issues or -- more rarely -- yield loss.

"We've seen it every year since 2015, but this year is the heaviest by far," said Jim Donnelly, a technical agronomist for DeKalb and Asgrow in Illinois. Donnelly estimates the disease is present in every cornfield in northern Illinois.

"It's widespread and very common; it's just a matter of what degree and what severity," he said. "Fields not treated with fungicide at tasseling are primary candidates for heavier infections, but even fields that have been treated are definitely showing some tar spot symptoms."

The disease, which has its origins in Latin America, first surfaced in the U.S. in Indiana and Illinois in 2015. Since then, it's been confirmed in four additional states -- Iowa, Florida, Michigan and Wisconsin, said Michigan State University plant pathologist Martin Chilvers.

Given its recent showing, U.S. scientists are still working to understand key components of the disease -- how it spreads, how much damage it can do and what fungicides work against it.

The disease's history in Mexico has given us some hints about it, Chilvers said. There, the fungus seems to prefer more moderate temperatures, which could explain why it has surfaced mostly in the north-central U.S. and not in more Southern states.

Most importantly, the most severe yield losses documented from the disease in Mexico and Central and South America occur when two pathogens appear together -- Phyllachora maydis, which causes the characteristic black spots, and Monographella maydis, which causes brown lesions surrounding the spots, often called "fisheyes."

So far, scientists have only confirmed that the tar spot fungal strain, Phyllachora maydis, is here. Fisheye symptoms have also been spotted in U.S. cornfields, but scientists haven't been able to isolate and identify the Monographella fungal strain yet, Chilvers said.

So what does this mean for the rapidly maturing cornfields of the north-central Midwest?

"The vast majority of fields will not experience yield loss from this pathogen, but it's heavy enough across a wide area that it is definitely opening some eyes," Donnelly said. "And there may be some situations where we could experience yield loss, and that would be a first for us."

Standability is of the greatest concern at this point in the season, he added. "There is potential for fields that may not experience yield loss to have some agronomic concerns with stalks just because of reduced leaf area while grainfill is occurring."

Scout your fields and -- if you find significant infestations -- consider harvesting them first, Chilvers and Donnelly advised.

Look for black dots, starting in the lower to mid-canopy.

"They're easy to identify, because just like the name suggests, they look and feel like flecks of tar," Chilvers said. In some cases, they may be surrounded by the light-brown "fisheye" lesions. In longstanding infestations, the tar spots can surface throughout the entire plant, even on the husk, Donnelly said.

Growers with drones or access to a plane might notice severe infestations from above, as the black spots and dead leaf tissue can darken plants enough to produce shadowy patches visible from the sky, Chilvers said.

Once you spot this disease in your fields, the chances of seeing it again are pretty good, Chilvers added.

"The tar spots on the leaf are spore-producing structures," he said. "Like head scab or white mold, spores are forcibly discharged into the air. And they appear quite capable of overwintering, as we're seeing the same fields and the same farms infected for a second year in a row."

The hybrids that Latin American breeders have identified as resistant to tar spot are not adapted to this region, and the fungicides they use against it are older ones not in use in the U.S., Chilvers noted. For now, plant pathologists don't have any recommendations on avoiding the disease, and the industry is still in fact-finding mode.

"If you find it, pull samples and give some location information to your local state Extension agent," Chilvers said. You can send samples to your local state plant diagnostic lab -- see a listing here:…

For more information, see this Michigan State University article:…

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Emily Unglesbee