Question
I live in Northern Utah and have a couple Quaking Aspen trees that are planted separately (not together), both are about 5 years old.  On one of the Quaking Aspen Trees, some of the branches look completely dead and the leaves have all fallen off.  The remaining branches have leaves that are all spotted black.  I have looked at the tree but do not find any sign of worms or bugs.  What could this be, it's getting worse by the day and looks like the entire tree is dying.

Answer
Sounds like leaf spot. Leaf spots are caused by fungi or bacteria. Brown or black spots and patches may be either ragged or circular, with a water soaked or yellow-edged appearance. Insects, rain, dirty garden tools, or even people can help its spread.

Prevention and Control: Remove infected leaves when the plant is dry. Leaves that collect around the base of the plant should be raked up and disposed of. For fungal leaf spots, use a recommended fungicide according to label directions. Check with your local nrusery type store fro a fungicide for leaf spots.
Here is a web site that discribes the leaf spots.
http://www.ext.colostate.edu/pubs/garden/02920.html