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tar spot on corn leaf
SYMPTOMS: Tar spot, a relatively new corn disease in the U.S., is most recognizable by black spots covering the leaf surface.

Protect your blind spot from tar spot

Keep an eye out for this new corn leaf disease in 2019.

During the 2018 growing season, the prevalence and severity of tar spot, a foliar disease of corn, increased dramatically in the Midwest. In some areas it caused substantial yield loss. It showed up in some counties in eastern Iowa, but the infestation wasn’t widespread or severe. Still, it bears watching in 2019.

Tar spot is a new disease here and was first reported in the U.S. in 2015, says Alison Robertson, Iowa State University Extension plant pathologist. Prior to that, it was only found in Latin America. However, since 2015 tar spot has spread to several Midwest states. And disease severity has ranged from just cosmetic damage to entire fields dying down early.

Since this disease is so new, it’s important to know the best way to identify and manage tar spot “so you can put the best practices in place to protect your corn yield,” she says.

What’s known about tar spot
“Most of the information we have on tar spot comes from Latin American research,” says Eric Tedford, fungicide technical product lead at Syngenta. “The disease and pathogen may behave differently in the Midwest than it does in Latin America, but we can still learn a lot from plant pathologists in Latin America.”

Tar spot is caused by the fungus Phyllachora maydis, and prefers cool, wet climates. It is not believed to be seedborne, but the spores can be carried by wind or rain, and infect nearby corn.

“Tar spot can overwinter in the U.S.,” Tedford says. “It’ll most likely be a small population, but the P. maydis fungus can produce millions of spores under the right conditions. So a small population can cause big issues. Since tar spot was more widespread in the Midwest in 2018, it may become even more prevalent in 2019.”

For every 10% increase in disease severity, 10 to 15 bushels of corn could be lost, according to ISU research. Tar spot could easily cause yield losses of 30 to 40 bushels per acre, and this could be greater with early infection, Robertson says.

Managing tar spot
When scouting for symptoms of tar spot, look for small tar-like spots splattered on the leaf surface, Tedford says. “Occasionally, symptoms of eyespot, another leaf disease, can develop, too.”

Tedford says Syngenta recommends the following practices for effective tar spot management:

  • Plant corn hybrids that are least susceptible to tar spot. There are no known resistant hybrids to date, but some hybrids are less susceptible than others.
  • Where tar spot has occurred, rotate the field to a nonhost crop the following season.
  • Start fungicide programs early for preventive activity and consider a two-pass application if necessary.

Syngenta offers Trivapro fungicide for tar spot management. In a 2018 University of Wisconsin trial, Trivapro reduced tar spot severity by almost 10% in comparison to untreated corn, yielding 12 bushels per acre more corn.

“It is highly unlikely that tar spot will be the only fungal disease in a field,” Tedford says. “Consider all the diseases you could face, and make sure your fungicide protects your crop on all fronts. You want to be using a fungicide that delivers broad-spectrum preventive and curative activity against several of the main yield-reducing corn diseases to help maximize yield and your potential return on investment.”

Learn more about disease
When tar spot was first identified in the U.S. in 2015, it was in northern Illinois and Indiana. As of 2018, it has also been confirmed in eastern Iowa and parts of Michigan, Wisconsin, Ohio and Florida. During the 2018 growing season, the prevalence and severity of the disease increased dramatically, and in some areas of the Midwest, tar spot caused substantial yield loss.

A new publication “Tar Spot” is available in the Corn Disease Management series through the Crop Protection Network at ISU plant pathologists Alison Robertson and Daren Mueller, along with colleagues from other universities, wrote the publication. It shows diseases with similar symptoms to tar spot, and explains how to distinguish corn rusts, Physoderma brown spot and other corn leaf diseases from tar spot.

In eastern Iowa where it was detected in 2018, severity of the disease was low and was detected late in the grain fill stage of corn growth. “However, the fact that this disease was present in the U.S. for a second year suggests we may see it again in the future,” Robertson says.

In Mexico and Central America, where the disease is more common, tar spot alone does not cause economic damage. However, when tar spot is associated with another fungus, Monographella maydis, significant corn yield losses can occur. “This disease complex is known as the tar spot complex,” she says. “And M. maydis has not been detected in the U.S., not yet anyway.”


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