Aspen tree and oozing
My 20 year old aspen has some orange colorado oozing. What should I do?
Larimer County Colorado trees and shrubs
I'm not sure what you mean by "Colorado" oozing, but the ooze is likely from either cytospora canker or bacterial wetwood.
Cytospora tends to effect weakened or stressed trees...aspens planted below 8,000 feet tend to become inflicted with a variety of things, simply because this is not the elevation (nor temperature) they prefer to grow at. Unfortunately, there is no control for cytospora, but you can reduce stress to the tree with proper watering (including in fall and winter and during dry periods in summer). If you prune these areas, make proper pruning cuts. Here is additional information: http://www.ext.colostate.edu/pubs/garden/02937.html
The other possibility is bacterial wetwood (also known as slime flux). This is characterized by a foul-smelling odor (kind of like yeast). Like cytospora, there isn't anything to control this, but reducing stress to the tree and providing water during dry times is important. Additional information: http://www.ext.colostate.edu/pubs/garden/02910.html
It's also important to remember that aspen tend to be shorter-lived species. A 20-year-old tree is reaching its maturity, especially at lower elevations. If you want to continue to have aspen in your landscape, you may consider training one of the root suckers that could replace the older trees in time.