Hello Rick -

Your tree has serious cankering issues. It is difficult to tell if it is Cytospora canker from the photos, but it is a common problem in Aspen trees. Information from the CSU Fact Sheet on Cytospora canker states “The fungus attacks trees or parts of trees that are injured or in a weak or stressed condition. The fungus grows in the living bark (phloem) and wood (xylem) and kills by girdling the branch or tree. The fungus can attack tree bark during the fall-winter spring seasons when temperatures are warm but the tree is dormant and cannot defend itself. Trees affected by drought, late spring frosts, insect and fungi defoliation, sunscald, herbicides, or mechanical injury are susceptible to Cytospora infection.”

Here is the Fact Sheet on Cytospora canker: http://www.ext.colostate.edu/pubs/Garden/02937.html

There is no treatment for cytospora canker and it appears that pruning would take out most of the tree. Removal may be your only option. You may want to consult with a certified arborist which can be found here:

http://www.isarmc.org/resources/find-a-certified-arborist-2/

Thank you for your question.