Question
My plum tree leaves have green spikes on them.  What is the problem and what do I do about it.

Answer
Don:
Difficult to say w/o seeing these, but it may be a type of spindle gall that is formed by a type of mite. If so, these 8 legged critters usually are more cosmetic than anything else, thus we don't attempt control.  You are welcome to send me a closeup photo or collect a few affected leaves and carry them by your local garden center or nursery for those folks to see.

You might also want to check out Google Images and search for spindle galls and see some different types.

Steve