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Know different types of tar spotKnow different types of tar spot

Farm and Garden: Tar spot on corn is not the same as the tar spot we often see on maple trees.

Curt Arens

October 30, 2023

2 Min Read
Tar spot on maple leaves
DIFFERENT TAR SPOT: Tar spot on maple trees causes dark black spots on leaves, but as unsightly as it is, it is usually relatively harmless. It is caused by a different organism than the tar spot that is affecting cornfields. MihailTsvetkov/Getty Images

Can tar spot in corn spread to become tar spot on my maple trees? No. It is not the same thing. The tar spot we see spreading to cornfields around the country is caused by Phyllachora maydis. Tar spot in maple trees is caused by Rhytisma acerinum.

There are also different tar spot diseases that will infect lima beans and soybeans. They are all commonly called tar spot, but they are not caused by the same organism. As one source put it, plant pathologists like using common names such as “tar spot” to describe diseases that may be quite different and infect different plants.

Tar spot in corn has spread to counties as far west in the U.S. as the eastern third of Nebraska and the northeast corner of Kansas, along with a couple of counties in the southeast corner of South Dakota. First reported in the Midwest in 2015, the disease continues to spread across the Corn Belt — with the capability of causing considerable yield loss in corn.

Unsightly but harmless

On the other hand, tar spot on maple trees is unsightly, but it is relatively harmless to the trees. Several different fungi in the genus infect the leaves of maples, causing raised black spots to form on the upper-leaf surfaces.

While heavy infestations can cause early leaf drop, it is rarely more than a cosmetic condition. Usually, the first symptoms show up in mid-June as small pale-yellow spots. Over time, the spots enlarge, and the yellow color intensifies, eventually becoming a black spot later in the season.

The fungus that causes tar spot in maple trees generally overwinters on infected leaves that fall to the ground. The following spring, as new leaves are becoming prevalent, the fungal tissue of the spots on the old leaves ripens, splits and allows tiny spores to escape and infect new susceptible hosts.

The only way to manage tar spot in trees is through sanitation. That involves raking and destroying leaves from affected trees in the fall, which reduces the number of overwintering fungal structures that can reproduce the next spring.

It is difficult to control tar spot on maples with fungicides because complete coverage of the leaf surfaces is necessary and not usually a viable option, especially for mature trees.

Get more details at plantclinic.cornell.edu.

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Tar Spot

About the Author(s)

Curt Arens

Editor, Nebraska Farmer

Curt Arens began writing about Nebraska’s farm families when he was in high school. Before joining Farm Progress as a field editor in April 2010, he had worked as a freelance farm writer for 27 years, first for newspapers and then for farm magazines, including Nebraska Farmer.

His real full-time career, however, during that same period was farming his family’s fourth generation land in northeast Nebraska. He also operated his Christmas tree farm and grew black oil sunflowers for wild birdseed. Curt continues to raise corn, soybeans and alfalfa and runs a cow-calf herd.

Curt and his wife Donna have four children, Lauren, Taylor, Zachary and Benjamin. They are active in their church and St. Rose School in Crofton, where Donna teaches and their children attend classes.

Previously, the 1986 University of Nebraska animal science graduate wrote a weekly rural life column, developed a farm radio program and wrote books about farm direct marketing and farmers markets. He received media honors from the Nebraska Forest Service, Center for Rural Affairs and Northeast Nebraska Experimental Farm Association.

He wrote about the spiritual side of farming in his 2008 book, “Down to Earth: Celebrating a Blessed Life on the Land,” garnering a Catholic Press Association award.

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