Fungus on aspen trees not deadly

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A wet spring has made a black-spotted fungus on aspen trees more noticeable this summer.

Routt County Agriculture Extension Agent C.J. Mucklow said he has received dozens of phone calls in the past few weeks from residents concerned about the health of their trees. He said the black spots -- known as marssonina blight -- are not likely to kill trees, but the wet spring made the fungus worse this year than in past years.

"I haven't seen it at this level in about 10 years," Mucklow said. "If we have another wet spring, it will be back again next year. For a dry spring, it won't be so noticeable."

Most of the calls Mucklow has received have come from Steamboat Springs residents who noticed the black spots on the leaves of aspen trees.

Along with the leaves turning black, they will fall off earlier than usual, Mucklow said.

Signs of an infected tree are dark brown to black flecks on the leaf surfaces. Spots will enlarge during the summer and can grow together to form blotches, which usually have a yellow border.

Infected leaves are often re----duced in size. From a distance, trees may take on a bronze appearance.

The damage of marssonina blight is mostly aesthetic, and tree death is rare unless the fungus causes several consecutive years of defoliation.

"If you see this, your tree is not going to die," Mucklow said.

Leaf blight is a cyclic disease, in which the worst years coincide with wet weather cycles.

The fungus can survive in the winter in ground leaf debris. In the spring, infection occurs when spores are released and carried by wind and rain to developing leaves.

Secondary infection occurs in the summer when the disease is transmitted to adjacent leaves.

Mucklow said the disease could be found in a lone tree or a group of trees. Dense groups of trees are especially susceptible.

Prevention and control can be difficult. Fungicides for the disease are applied in the spring, when leaves are budding. The spreading of the disease typically slows as the weather becomes drier.

The best practice is to rake up and dispose affected leaves in the fall. Tree owners also should avoid sprinkling water on lower branches of small leaves growing near lawn areas.

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