I have a cluster of aspen trees in my yard looking very unhealthy. Here are the symptoms I've watched this summer:

1. Some leaves with dark spots began falling from the trees in late June; eventually many of these leaves turned black. Before turning black, the leaves may also appear brown or yellow

2. The trees in the worst shape have lost about half of their leaves by early September

3. Many leaves dropped to the ground with a cluster of thick cottony fluff attached; attached to the fluff were small dark lumps that looked like seeds. This fluff appeared from June into August. I raked up and disposed of the leaves with the fluff. I'm attaching a photo of the fluff I found on the ground today - it's now become weathered, but was originally larger and whiter.

4. My aspen trunks are covered with small dark spots; on some trees, the spots are so numerous that they become a clump of blackness that can be rubbed off

5. I've just noticed that the grass around some of the trees has a white film growing on the singular blades

6. Two to three thousand aspen sprouts popped up on the grass over the summer; I pulled these sprouts on a weekly basis. Originally I assumed the heavy sprouting was due to the rain, but now I think it may be because the trees are dying.

When I look back on this problem, I realize it must have started some time ago; my aspens trees used to have beautiful yellow leaves in the fall, but last year the leaves just turned crisp and brown and fell from the trees. Summertime sprouting was heavy for the previous two summers, but almost unmanageable this summer. 

What do you think has invaded my aspens and can I do anything to save them?  They are 20-25 years old.