Dear Client,

The symptoms of damage that you are reporting and photographing are not at all consistent to a boring insect pest. I would suspect a plant disease based on both the leaf spots and the unusual deflating of the plum. I suspect that you may be dealing with a disease call plum pockets disease (Taphrina spp.) although I cannot tell what type of plum you are growing. French and Italian prunes tend to be susceptible to this disease. There may be other fruit rot diseases that are causing this damage to your fruit. As a homeowner, the "easiest" management strategy is to pick off and destroy all infected fruit ASAP.

Good luck in your endeavors.  The good news is that if this is plum pockets, the disease will not lead to the death of the tree.

A couple of local resources that you need to review to help confirm the disease is :

http://hyg.ipm.illinois.edu/article.php?id=478

http://www.extension.umn.edu/garden/yard-garden/fruit/integrated-pest-management-for-home-stone-frui...