Are Your Aspen Trees Healthy?

By Marilyn Loser: Are Your Aspen Trees Healthy?

San Luis Valley trees are starting to display seeds (Siberian Elms), pink blossoms (Crabapples), and shiny, light green leaves (cottonwoods).  Homeowners wander their yards this time of the year and notice changes. Sometimes the changes aren’t positive. 

I had a call from a lady concerned about her aspen trees. We Coloradoans love aspen. However, aspens have a tough time on the floor of the San Luis Valley.

I visited her yard and she pointed out liquid oozing from wounds on branches. Many of the branches looked like they had been eaten away.  I did some research on the web before heading out and also took my copy of “Insects and Diseases of Woody Plants of the Central Rockies” published by Colorado State University Cooperative Extension (CSUCE).  It seemed there was a good chance it was Cytospora canker, but we weren’t sure.

I emailed photos to Colorado State Forester Adam Moore and he confirmed our suspicions. According to the CSUCE website, “Cytospora Canker is a fungal disease that causes cankers on branches and trunks of several species of trees and shrubs, including aspen, cottonwood and other poplars, spruce, apple, cherry, birch and silver maple.” Cankers are most noticeable on aspens since the orange/black ooze contrasts with the white trunk. Some of the trees I looked at had infected areas in which the cankers were halfway through the limb (see photos at

Cytospora Canker is pretty common in our area, according to Moore.  He said, “Best prevention is keeping good care of the tree.  Most aspen in town have this to some extent. This is why some people don’t like aspen out of the mountains.  The added stress of being in a naturally unsuited area makes them more likely to get the canker.”

This canker disease usually occurs on a weakened host, so the best method of control is to prevent tree stress.  “Drought and oxygen starvation of roots by flooding soil with water are the two most common stresses that predispose trees to Cytospora infection,” reports the CSUCE website. I wonder if compacted soil due to vehicle traffic could add to the oxygen starvation problem. I think of aspen roots as staying near the soil surface and extending at least 2 to 3 times beyond the canopy once trees are established.

Sadly, there is no magic bullet for ridding trees of Cytospora Canker. Again, according to CSUCE, “Once infection occurs, the best treatment is to increase plant vigor and sanitation. Remove all infected limbs and other areas. When removing branches, make a smooth cut at the base of the limb, as near the trunk as possible, without damaging the branch collar (swollen area at base of branch).”

It’s important to clean tools and disinfect after each cut with ethyl alcohol, Lysol or another disinfectant. And don’t apply a wound dressing as they often only worsen the situation says the CSUCE website. The best method to prevent infection or decay is to allow the cleaned tissue to dry out. By the way, avoiding wound dressing is important for all pruning cuts.

Smooth bumps on aspens are caused by the Poplar Twiggal Fly. “We start having people ask us about them this time of year every year,” said Moore. “I guess the first time people are in the yard for the year they notice them.”  My understanding is that this usually isn’t a huge problem here unless the trees are consistently overwatered.  However, if you are concerned take a look at

Deer love to browse the bark of young aspen trees and scrape their antlers on them in the fall.  The aspens near the intersection of Hwy 160 and Hwy 285 are surrounded by wire fencing to prevent deer from destroying the trees. 

I don’t have many aspen left in my yard and was sad to notice a large vertical crack running up most of the trunk on the south side of the largest one. The crack wasn’t there last fall. Chances are extreme fluctuations in daily temperatures caused liquids in the tree’s cells to freeze and burst causing the crack.

Many years ago, before I knew much about trees, I planted a small copse in a corner of the yard.  For several years I would sit under them and be transported to the mountains. Unfortunately, after a few years they blew over.  I hadn’t watered them all the way around so all their roots grew only to the north.  There was nothing to anchor them on the south so spring winds took them out.

Here’s hoping your aspens are healthy and happy (can trees be happy?) For more information visit the CSUCE Cytospora Canker webpage at:

To plant trees is to give body and life to one's dreams of a better world.” Russell Page