Question
We planted our trees 3 yrs ago and have never had any issues before. Last year we had a huge infestatiom of aphids but we lowered the numbers with lady bugs. Anyhow the first tree has these weird bumps on it and when you turn the leaves over the bumps are hollow on the other side, they almost look like blisters on the leaves. It is only on the leaves and the whole 1st tree is covered with them. The 2nd and 3rd tree have them too but not as much. Here is pics of the front and back of 2 leaves from the first tree
http://imageshack.us/photo/my-images/694/p1110758s.jpg/
and
http://imageshack.us/photo/my-images/16/p1110763d.jpg/
The 2nd tree has black spots on some of the leaves. They dont seem dry but just discolored. Not powdery either. Here is pics of 2 leaves from the 2nd tree
http://imageshack.us/photo/my-images/26/p1110764h.jpg/
and
http://imageshack.us/photo/my-images/35/p1110765.jpg/

The 3rd tree isnt as bad as the first two but the 2nd tree is the only one with the back spots on the leaves. All 3 aspens have the bubbles/blister things on them and it seems worse on the leaves lower on the tree. All 3 trees are planted beside eachother about 3 feet apart from eachother. If aphids are not the SURE suspect I would also like to hear what other problem it could be so please dont say yep the aphids did it if you arent sure.

Thank you so much!!

Answer
Genna:

This may be a fungus disease called yellow leaf blister, caused by the fungus called Taphrina.  It would need to be confirmed in a lab, but based on your description I would say that it is a strong candidate.  You may try to "Google" this disease and look for pix to compare.  There is quite a bit of information from Washington and New York on this disease.  It can also affect cottonwood.  This disease is mostly considered aesthetic, thus it doesn't pose a health risk.  It mainly affects the appearance of the leaves and is favored by cool wet springs. Spraying the tree is not usually recommended unless it is a high value tree, then spraying needs to start in the spring BEFORE buds break.  Spraying now is not going to be very effective.  The spray chemical needs to cover the entire tree.  Multiple applications may be necessary (2-3) with a material called Daconil. Otherwise, rake up and destroy the leaves that fall.  Be sure to do this before springtime.

The black spots on the leaves I am not sure, but do not think it is a bona fide disease, but rather an environmental effect.  Be sure to water your tree during hot/dry summer conditions to promote vigor.

Regards

Steve