So sorry about your sad looking plums but I do need a bit more info to be sure of what is happening here. I suspect the tree may be getting too much water or the branches get wet from a sprinkler system? Plums usually bear well for about 15 years then grow into mature trees with more problems. Try to keep the area dry and collect all fallen fruit in the fall and clean up under tree to stop the disease spores from overwintering. It looks like maybe mildew on the fruit so a winter dormant spray or fungicide spray might give some relief. Keep growing, Marianne Binetti