It's really hard to say without seeing your plant, but it sounds like you have two things going on. It's unlikely the leaf and bark problems are related. For the leaves, you clearly have some type of insect eating away. I would try a general purpose insecticide since you don't know what exactly the pest is. The bark is probably more problematic. You don't say how old your tree is, but plums are rather short lived trees and often they will develop bark problems as they get older. If your tree is 15 years old or older, it could be a natural decline of the tree. "