By DOUG SCHMITZ

Iowa Correspondent

AMES, Iowa — With Iowa and Midwest soybean farmers still reeling from discovering a new pest – soybean gall midge, which was confirmed in three other neighboring states last fall – corn growers last fall faced a new malady of their own: Tar spot.

“Tar spot was first observed in Iowa in 2016 in a couple of eastern counties,” said Alison Robertson, Iowa State University associate professor of plant pathology and microbiology. “In 2018, it was reported from 15 counties in eastern Iowa.”

Tar spot was first reported in the United States in Indiana in 2015, and was also confirmed later that season in Illinois. In 2016, the disease had again been found in Indiana and Illinois, with additional confirmations in Michigan, Ohio, Wisconsin, and Florida.

“It’s becoming a widespread issue across northern Illinois, northern Indiana, and Wisconsin,” said Steve Crafton, technical agronomist for LG Seeds in Westfield, Ind. “Universities in each of these states are trying to evaluate and come up with some best management practices on how to combat this disease.”

In 2018, it became much more severe, with significant outbreaks reported in Illinois, Indiana, Wisconsin, Iowa, Ohio, and Michigan, according to DuPont Pioneer in Johnston, Iowa.

Crafton said researchers speculate tar spot wind-driven storms from Mexico distributed spores into those counties. The disease overwinters in soil debris and spores can be distributed further by storms.

He noted tar spot is favored in cool conditions with high humidity. Tar spot symptoms on the corn leaves are round raised black spots that feel “bumpy” when you touch them. Moreover, tar spot alone is mainly a “cosmetic” disease with little known yield loss.

“Fellow LG Seeds agronomists and I have been scouting plots and trying to come up with some answers as well,” he said. “We are all in the ‘research phase’ of this new disease, and I will try to sum up what we know to date.”

In the fall of 2018, ISU’s Plant and Insect Diagnostic Clinic received samples of corn leaves, with symptoms of tar spot from Jones County in eastern Iowa. Robertson said the disease damages the leaves and consequently reduces photosynthesis for grain fill, adding that “severe infections may lead to premature senescence.”

According to Agriland FS, Inc. in Winterset, Iowa, symptoms of tar spot begin as oval to irregular, bleached to brown lesions on leaves in which black spores are formed. Mark Licht, ISU assistant professor of agronomy and extension cropping systems specialist, said tar spot (caused by the fungus Phyllachora maydis) was also found throughout east-central Iowa, Wisconsin, and Michigan in 2018.

He said since tar spot was first identified in Illinois and Indiana in 2015, “it appears to be spreading slowly.” But much more research needs to be done to determine yield impacts and ideal conditions for growth and spreading.

Robertson said each species appears to be host-specific; thus the tar spot that occurs on corn will not cause disease on other plants. At this point, she said there’s not much farmers can do to avoid or control tar spot.

“Not much is known about the pathogen/disease cycle,” she explained. “We suspect it overwinters in infested corn residues – consequently, fields that had tar spot in 2018. (They) should be rotated to soybean in 2019.”

She said data from Michigan and Wisconsin indicate fungicides will reduce disease, but timing of application (at disease onset) is important. “Therefore, fields should be scouted regularly for tar spot, and a fungicide applied if the disease is present. Not all fungicides are labeled for tar spot, so remember to check the label before applying a product.”

When asked if tar spot would eventually spread to other parts of Iowa, Robertson said, “Probably. A recent paper suggests that all of Iowa is at high risk for the disease.”

She and Edward Zaworski, ISU plant diagnostician, who are both tracking the disease, said it is unlikely tar spot will cause any yield loss in eastern Iowa where it was detected.

“Severity of the disease was very low and was detected late in grain fill. However, the fact that the disease is present in the U.S. for a second year suggests that we may see it again in the future,” they noted.

Crafton agreed with Robertson concerning lack of pest control, saying tar spot is a new disease with a lot of “unknowns” on best management practices.

“There will be a learning curve the next few years, but there are some management practices that will help combat this new disease,” he said. “Keeping that plant happy and healthy as long into the season as we can will help fend off severe infection.”

He added crop rotation, healthy hybrids, fungicides, and a good fertility program, with late-season nitrogen application, should all help to prevent severe tar spot infections.

“We have seen cases of the same hybrid planted on rotated ground, good fertility and management with hardly any tar spot, and five miles down the road, the same hybrid suffering from poor fertility and corn-after-corn with severe tar spot infection,” he pointed out.