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Brian Korthals/Daily Globe City of Worthington forester Scott Rosenberg examines the leaves of an anthracnose infected ash tree in Ludlow Park Thursday afternoon in Worthington.

Disease hits local ash trees

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Disease hits local ash trees
Worthington Minnesota 300 11th Street / P.O. Box 639 56187

WORTHINGTON -- Ash trees across the city and throughout the area continue to drop their leaves as a result of a fungal disease sparked by recent cool and damp weather.


Worthington Park Supervisor and City Forester Scott Rosenberg said area residents shouldn't be too concerned about anthracnose of ash trees -- the fungal leaf spot mainly hampers aesthetics and does not cause the trees to die.

"What we've been seeing in Worthington is only affecting the ash trees," said Rosenberg, adding that the leaves of affected trees contain little brown spots. He began receiving calls last weekend from residents concerned about defoliation of their ash trees.

This is the first time Rosenberg has seen anthracnose to such a great extent. It can cause permanent damage if the same tree gets infected year after year for a span of four or five years, he said.

"If it's the first time the tree has ever had it or ... doesn't get it every year, it's probably no more than an aesthetic problem within the tree," Rosenberg explained. "It's just not going to look as full -- it's going to be thinner for this growing season."

It is possible, because it is still fairly early in the growing season, the tree may generate new leaf growth.

Though people don't need to treat the trees in most instances, Rosenberg encourages residents with affected ash trees to rake up and dispose of the fallen leaves.

"Raking it and hauling it away will help to reduce recurring infections in trees," he said. The spores hold on the leaves and can start the cycle again, he added.

The anthracnose fungus is just one of the issues affecting the city's tree populations in recent years. Rosenberg said tar spot of maple trees has been around for the past four or five years in the community.

"Most maple trees have had moderate to severe infections," he said, adding the disease shows up as irregular-shaped blotches of tar on the leaves. Unlike the anthracnose of ash disease, tar spot of maple does not cause defoliation.

Rosenberg recommends the same response for tar spot -- thoroughly raking leaf litter and removing it to stop the spread of infection.

Varieties of maple and ash make up a majority of the publicly owned trees in Worthington, with ash accounting for approximately one-third of the trees in city parks and on boulevards, Rosenberg said.

Decades ago, Dutch elm disease took a lot of trees in the community. With approximately 80 elm trees on public land in the city limits at this time, Rosenberg said the disease is still a concern. His department has been planting Dutch elm-resistant trees for the past five years.

"This one will kill an elm tree if it becomes infected and is not resistant to Dutch elm disease," he said. The disease is spread by the elm bark beetle, and typically shows up in mid- to late June through the end of the growing season.

Symptoms of Dutch elm disease include the wilting, yellowing and browning of leaves throughout the tree's canopy.

"This is one disease where there is a management plan," Rosenberg said. "Privately and publicly owned trees are required to be removed if infected or chemically treated to kill the disease."

Ash borer reminder

With the summer camping and cook-out season right around the corner, Rosenberg again reminds people not to transport firewood in hopes of avoiding the spread of the Emerald Ash Borer.

"Emerald Ash Borer still has not been reported any closer to us than it was last year at this time," he said. "We haven't found it and hopefully we don't find it."

EAB is deadly in ash trees, and one of the reasons it has spread to so many areas of the U.S. is because firewood has been transported, Rosenberg said.

"When it gets into the trees, it will kill the ash tree," he added. "There are some chemical treatments that can be used to try to kill the bug -- it's a continual treatment."

Julie Buntjer
Julie Buntjer joined the Daily Globe newsroom in December 2003, after working more than nine years for weekly newspapers. A native of Worthington and graduate of Worthington High School, then-Worthington Community College and South Dakota State University, she has a bachelor's degree in agriculture journalism. At the Daily Globe, Julie covers the agricultural beat, as well as Nobles County government, watersheds, community news and feature stories. In her spare time, she enjoys needlework (cross-stitch and hardanger embroidery), reading, travel, fishing and spending time with family. Find more of her stories of farm life, family and various other tidbits at
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