Richard,

Thank you for contacting Ask Extension. My name is Kat and I’ll be helping you today.

To confirm that you have cytospora canker, can you please send some pictures of the affected areas?

If it is cytospora canker, attached below are some fact sheets about this disease.

As the fact sheets indicate, unfortunately, there are no sprays or other such treatments for the disease.

If the canker is not too severe, the diseased areas can be removed and burnt, buried, or otherwise disposed of.

If a tree is heavily infected, it may be best to cut it down. Just be sure to remove & dispose of the infected parts so the cytospora doesn’t stay in the ground,

And, since healthy trees are rarely infected with cytospora, take stock of the care the aspens have been getting. 

Were they planted properly, have they been getting too much or too little water? Were they being watered through the winter when there was a week or more with no snow or rainfall?

CSU Fact Sheet 2.937 – Cytospora Canker
https://extension.colostate.edu/topic-areas/yard-garden/cytospora-canker-2-937/
Gilpin County Extension – Cytospora in Aspen
https://gilpin.extension.colostate.edu/programs/natu/canker/

Please review this information and if you have any questions or this or other issues, don’t hesitate to contact Ask Extension.

Thanks,

Kat