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Tar spot is here, so start scouting

The first confirmed case of tar spot this year is in Lancaster County, Pa.

Chris Torres, Editor, American Agriculturist

August 1, 2022

3 Min Read
tar spot on corn leaf
LOOK FOR SPOTS: Tar spot appears as small, raised black spots scattered across the upper and lower leaf surfaces of corn. It’s easiest to see on green tissue, but can also be found on dried leaves and fodder. Chris Torres

Almost two years after the first confirmed case of tar spot in Lancaster County, Pa., the disease has already been confirmed in a field in the county.

Penn State Extension recently sent out a Field Crops Alert confirming the first known case of tar spot this season.

Paul Esker, associate professor of epidemiology and field crop pathology at Penn State, says that based on the symptoms, it is estimated that infection likely occurred in the affected field at the end of June, before the widespread and intense heat that hit the region in July.

The best conditions for tar spot development are temperatures between 59 and 70 degrees F, relative humidity of at least 85%, and more than seven hours of leaf wetness.

Alyssa Collins, field crops plant pathologist with Penn State Extension, says the field was in continuous corn. Tar spot is caused by the pathogen Phyllachora maydis and is believed to overwinter and spread in crop residue. Experts recommend rotating away from corn for at least a year to break the disease cycle, especially in a field that has had tar spot before.

Under favorable conditions, tar spot can cause severe yield losses. According to the online Crop Protection Network, tar spot severity on ear leaves at R5 (dent stage) can exceed 50% in susceptible hybrids when conditions are right for disease development. The leaves of infected plants will prematurely die when severity is about 30% or more.

Tar spot appears as small, raised black spots scattered across the upper and lower leaf surfaces of corn. It’s easiest to see on green tissue, but can also be found on dried leaves and fodder.

The tar spot pathogen is believed to have come from Central America and Mexico, where conditions are ripe for its development and spread. It was first identified in the U.S. in 2015, and in 2018, a major outbreak in northern Indiana spread to surrounding states.

It has since been identified in Illinois, Iowa, Michigan, Minnesota, Wisconsin, Pennsylvania and western New York.

map of cases of tar spot of corn

Growers are often told to rotate away from corn the following year to allow the residue to decompose, or to incorporate tillage to bury the inoculum.

In Latin America, growers use resistant corn hybrids to control its spread.

Craig Austin, a Syngenta agronomist based in Lancaster County, says there have been several false positive suspected cases of tar spot this season around the county that have turned out to be something else.

Still, he suspects tar spot spores have already spread and will turn up in bigger numbers once weather conditions favor its development. He says growers should scout and report anything unusual to their agronomist or Extension agent.

“There are lots of diseases that are yield robbing, and this is one of them now,” Austin says. “Grey leaf spot, northern corn leaf blight, now tar spot. This is part of the puzzle.”

The fact that there have been false positives is a good thing, Collins says.

“It means people are on the lookout for the disease and are more likely to detect it early if it appears on their corn,” she says.

The Crop Protection Network has a list of potential fungicides that could work to control tar spot and other corn diseases, but make sure to read the label for harvest restrictions.

Penn State has an online reporting system for anyone who suspects a field with tar spot in it. 

About the Author(s)

Chris Torres

Editor, American Agriculturist

Chris Torres, editor of American Agriculturist, previously worked at Lancaster Farming, where he started in 2006 as a staff writer and later became regional editor. Torres is a seven-time winner of the Keystone Press Awards, handed out by the Pennsylvania Press Association, and he is a Pennsylvania State University graduate.

Torres says he wants American Agriculturist to be farmers' "go-to product, continuing the legacy and high standard (former American Agriculturist editor) John Vogel has set." Torres succeeds Vogel, who retired after 47 years with Farm Progress and its related publications.

"The news business is a challenging job," Torres says. "It makes you think outside your small box, and you have to formulate what the reader wants to see from the overall product. It's rewarding to see a nice product in the end."

Torres' family is based in Lebanon County, Pa. His wife grew up on a small farm in Berks County, Pa., where they raised corn, soybeans, feeder cattle and more. Torres and his wife are parents to three young boys.

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