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Foresters search for cause of tree deaths in Mankato area

Associated Press

MANKATO, Minn. — Dozens of trees that line boulevards in the Mankato area are dying, and local foresters aren’t sure what’s to blame.

Under threat is the black ash tree, and the only thing foresters can say for sure is what’s not causing the trees to die — ash borers, the target of an aggressive campaign by the Department of Agriculture to keep the insect parasites out of Minnesota, are not the cause.

The death of the trees "has us puzzled. I can’t put my finger on it," said Brian Hagberg, Mankato city forester.

"It was fairly bad last year and it’s worse this year," said Russ Johnson, his counterpart in North Mankato.

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A major factor is likely the lack of precipitation in recent years, especially a lack of snow the last few winters that’s caused soil to get too cold. Other suspects include a fungus called verticillium wilt, a disease called ash yellows and an insect called the cottony ash phsyllid.

But the biggest culprit may be overplanting. Hagberg said the city planted a lot of black ash trees in the past decade, as a replacement for elm trees felled by Dutch elm. Overuse of a single tree can make it more vulnerable and can endanger the entire urban forest if it disappears.

The cities will likely have to remove dead or dying ash trees and replace them with different varieties.

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