Disease causing early defoliation of Aspen cottonwood trees, city warns | AspenTimes.com

Disease causing early defoliation of Aspen cottonwood trees, city warns

Staff report

The city of Aspen announced Tuesday an increase in early leaf drop of some cottonwood trees due to a surge in fungal diseases.

The primary fungus causing damage is Septoria, which is a common foliar disease mainly found on cottonwoods in urban areas of Colorado, according to the city.

“The most significant impact we’re seeing is in town,” said Ben Carlsen, city forester for Aspen. Carlsen said the primary infections are in cottonwood trees, though there are “several diseases to other trees and shrubs.” Cottonwood trees downtown, especially in the West End, are experiencing defoliation as a result, a statement from the city said.

“Because of the moist weather from the spring, the trees were infected early and stayed moist,” Carlsen said.

With the exception of a short dry period in June, Carlsen said it has still been a fairly moist summer.

“Because (the leaves) were infected early, the disease was able to develop sooner. That’s why the leaves are dropping earlier this year,” he said.

Symptoms on the leaves may be identified by irregular brown to black spots that conjoin into large areas. Earlier in the summer, these symptoms appeared more like a “distinct tan circular spot with black margins and small black fruiting bodies in the center.”

The best management strategy is sanitation, according to the city. Raking and disposing of leaves this summer and fall will reduce the likelihood of spread next spring. The statement noted that fungicide sprays are “impractical” on large trees and that spraying will not cure leaves already infected, though it may help prevent new infections.