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Disease in the trees

Fungus attacking

Jay Whaley holds the branch from a cotton wood that has been infected by marssonina leaf spot fungus. The fungus is impacting as many as 30 percent of cotton wood trees in Routt County, but will not kill trees.
John F. Russell

— Leaves are falling off cottonwood trees across Routt County, but not because it’s almost that time of year.

A naturally occurring fungus called marssonina leaf spot is causing leaves to fall off cottonwoods prematurely, said C.J. Mucklow, director of the Routt County Cooperative Extension Office. Mucklow said Thursday that as many as 30 percent of cottonwoods in Routt County may be infected with the fungus, which creates black, brown or yellow spots on leaves. The fungus does not kill the trees.

“This disease is real common in aspens, but I’ve never seen it effect the cottonwoods this way,” Mucklow said. “It really went after the cottonwoods this year.”



Mucklow said he first noticed the fungus on local cottonwoods about three weeks ago, especially along the Elk River and in the Blue Sage Circle area near Fish Creek Falls.

The airborne fungus is spread by wind and rain. Although marssonina leaf spot can impact a tree’s ability to take in nutrients, Mucklow said cottonwoods are in no real danger.



“Because the summer is almost over, the disease’s effect on a tree’s health is minimal,” he said.

Healthy cottonwood leaves turn a golden color in the fall, Mucklow said.

For landowners concerned about their cottonwood trees, the situation is not dire.

“It’s not going to kill the trees,” Mucklow reiterated about marssonina.

Whether the fungus comes back next year depends on weather patterns, Mucklow said.

The fungus “winters” on fallen leaves, meaning removal this fall can prevent more fungus next spring.

“Make sure to rake up the leaves,” Mucklow said, referring to landowners who may have infected cottonwoods on their property.

Applying fungicide in the spring, at “bud break,” can also help, but Mucklow said he has never seen a disease in cottonwoods reach a level necessitating fungicide. He has worked for the county extension office since 1989.

“We’ve never recommended fungicide for cottonwood trees,” Mucklow said. “But we could.”

– To reach Mike Lawrence, call 871-4203 or e-mail mlawrence@steamboatpilot.com


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