Dear Anonymous,

I don’t have good news.  This sounds like and appears to be Cytospora canker, below are the quick facts and I am including the URL for the entire fact sheet.

• Cytospora canker is caused by several species of Cytospora (sexual form Valsa and Leucostoma) fungi. The name comes from the asexual stage of the pathogen that is more commonly seen.
• The disease occurs on woody shrubs and trees or parts of plants that are slightly stressed.
• Many trees and shrubs are affected by this disease (apple, ash, aspen, birch, cottonwood, elm, maple, peach, spruce, willow).
• The canker-causing fungi cause girdling of the plant, killing the plant above the canker.
• To manage the disease, reduce stress on trees, use resistant plants, remove infected limbs, clean wounds and prune properly.

Aspen have a very difficult time at this altitude, they are basically mountain trees.  But it will be very challenging to remove the clump if you decide to do so.  Any root system left will continue to send up sprouts in your yard.

Please refer to the URL below for more information about this disease.

http://www.ext.colostate.edu/pubs/garden/02937.html

I wish I had more options for you, good luck.

Thanks,

Janene Willey

Hello! In reviewing the problem with your aspen grove, our conclusion that this is probably oystershell scale – an insect (not cytospora – a fungal issue). Your description that they can be “easily scraped off” really helped! The female insect starts out as soft-bodied, but develops a hard covering (looks like a small shell) that remains on the tree for a long time, even years. Eggs overwinter under that covering.

This fact sheet can help with management, treating the insects at the correct time in their lifecycle. http://www.ext.colostate.edu/pubs/insect/05513.html

This winter when the trees are dormant (Jan/Feb), use a horticulture oil that can kill (smother) the insects’ eggs as they overwinter.

In the spring when they hatch, before they get the protective “shell”, they are called crawlers and are exposed, making it a good time to use an insecticide (start checking for them early-to-mid-May). Look at the insecticide product labels and/or talk to a reliable nursery about which one to use on oystershell scale. The fact sheet will give you some guidelines, also.

For now, remove the dead/dying trees and branches. On healthy-looking trees that have scale, you can scrub off the scales where you can reach them (again, see the fact sheet) for 2 reasons: it removes some of the eggs, and provides a place where you can see any new infestation next spring.

Some trees (even in the same grove) can be resistant to the insect, too.

I hope this helps; do not hesitate to contact us with any questions.

Karin