Friday, July 9, 2021

Keeping Your Aspen Trees Healthy

By Eric McPhail, Gunnison County CSU Extension

Everyone enjoys watching the aspen leaves as they glitter in the wind and change colors. But aspens do have enemies. Two of the most common are cankers and Eriophyid mites.  


The term canker describes an area of dead cambium (living cells just beneath the bark) and bark, usually on the tree trunk and usually elliptical in shape. Aspen cankers display great variety.  They can kill individual twigs, branches, and portions of trucks when it succeeds in girdling those parts. They kill many aspen each year in Colorado, and affect all sizes and ages of trees.

Cytospora is probably the most common canker-causing fungus. Look for orange-stained areas of bleeding bark, small orange tendrils of a jellylike material on the bark, and large patches of dead bark. Sooty-bark cankers give the trunk a “barber pole” appearance. On close inspection, the stripes or areas of dead, black bark crumble in one’s hand to a sooty powder. Black cankers are large, black swollen or flared areas on the trunk that contain concentric rings. This feature gives them the nickname of “target cankers.” 

Black canker

Canker fungi travel from tree to tree in interesting ways—some have their spores disseminated by wind, others by rain, still others by insects—and humans do a good job as by well by not cleaning their pruning shears between cuts!

Prevention is much more profitable than attempts at control. Avoid wounding the aspens by carving on them which means almost certain death to the tree! Sufficient watering also keeps the trees from being under stress, which makes it more susceptible to insects and disease.

If a small canker is found on a branch, prune it off. Although pruning is preferred during the dormant season, if you must prune during the growing season, be sure to sterilize the pruning shears by soaking them in a 10% Clorox® solution, by spraying the blades with Lysol® disinfectant, or by using some other alcohol product after each cut

Small trunk cankers can sometimes be cleaned up by removing affected tissue down to sound, unstained wood. Seek help from a professional arborist if unsure of how to perform this surgery. Do not attempt to remove large trunk cankers. Currently, research by the Colorado State University Forest Service shows the usefulness of fungicidal sprays and wound paints to be in question. 


Mites feed on all parts of the trees. They are narrow, very tiny (1/100 inch long), translucent, and have four legs toward the front of their bodies. Adults appear only as a speck through a hand lens. Many mites cause woody swellings or galls, especially around the buds of aspens. Infestations usually occur in early summer, immediately after tree buds open. Under ideal conditions, development from eggs to adult takes 14 days.

Although galls are conspicuous and unattractive, they rarely do any real damage to plants. Furthermore, once galls start, formation is largely irreversible. Under most circumstances, control is not recommended. Heavy infestations that occur repeatedly over several seasons may slow the growth of the plant or make the appearance unattractive.

Most galls are produced by insects that move to the trees as new growth develops in the spring. They can be controlled only with sprays, such as Sevin or Kelthane, that cover the leaves during the egg-laying period. Repeat applications often are needed.

For more information, contact your local Colorado State University Extension Master Gardener Program or visit Colorado State Forest Service - insects and diseases at

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