Looks like some heavy duty chomping plus leaf spots of some kind.

Seldom considered to be a problem at this time of year because the leaves will soon drop anyway. Rake and discard.

Hawk:

I grow Stanley prune plums. Even though I spray with Imidan & Captan in my orchard, I still have lousy looking leaves on my plum. The tree grows fine, yields good and the plums are insect free. I have come to the conclusion that plum leaves must be the absolute favorite for insects as mine always have damage on them. Everything else in my orchard has clean leaves but not my plum. Since it yielded two bushels of fruit last week, I cannot complain.

If not too severe, I would learn to live with it.