plum leaf disease (with pics) Help please

I have a plum tree, Canadian Plum I think, that has a funny leaf disease. The leafs develop little, protruding growths on them. The leafs stay attached and don't seem to die any sooner than the rest come fall. In spring, the tree blooms nicely and the leafs bud out and look good for a couple weeks then the growths start and the leaves shrivel some. Any ideas what is causing this and what I can do to fix it?

Thanks in advance

 

its difficult to see from the pictures, close up of the leaves during the day might help.

 

Here are some close ups during the day, Any ideas? treatments? Chain Saw?. What ever they are they grow from the underside of the leaves only

 

You may want to google "leaf galls".

 

Ok so it looks like a leaf gall, any better identification, Should I treat it?

And when is a good time to prune the tree?

 

Resembles "Spindle gall Mites" I believe their called. There are no effective chemical treatments at this stage from what I know of. Other than being unsightly, this infestation is not commonly known to hurt the tree. Dormant oil can be applied during the dormant season to try to smother over wintering eggs.
From my expierence galls seem to appear out of nowhere one year, and the following year may not be present at all.
I am sorry I have no expierence with pruning, and can not answer when is a good time to prune. Regards Jim.