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Steamboat Springs — A fungus is cutting a path of black flecks and dead leaves through Routt County's aspens, Extension Agent C.J. Mucklow said.

The culprit, Marssonina blight, is not fatal, he said. It causes leaves to fall off early, however, and could mean less vibrant fall colors for the area.

"It's worse when we have a wet, windy, cool, damp spring," Mucklow said.

To reduce the spread of the fungus, people who notice it on their trees could rake up and dispose of dead leaves, he said. The fungus survives the winter in fallen leaves, he said. It can be treated with a fungicide in spring, but most people don't take that step because the blight isn't fatal, Mucklow said.

Marssonina can, however, make trees more susceptible to other diseases and winter injury, according to information from Colorado State University. The fungus infects trees in spring when spores are released and carried to developing leaves.

The fungus is different from sudden aspen decline, which is killing about 35 percent of aspens in western Colorado, Mucklow said. Trees that die from that affliction normally don't have the dark leaf spots visible on those affected by Marssonina blight, he added. The leaves just die.

Sudden aspen decline affected 358,000 acres of the state's aspen trees in 2007, according to information from the U.S. Forest Service.

The southwestern and northwestern regions of the state are taking the biggest hit, according to the report. The Forest Service attributed sudden aspen decline to factors including drought from 2000 to 2005, insects and disease.

"The good news about aspens in Colorado showing SAD is that we see principally a lot of regeneration," Mucklow said, meaning new aspen trees are replacing those that die.