Last year most of the fruit was inedible. There were white slugs inside, although they looked ok from the outside. Would a band around the tree help and when should it be applied?
Certified GKH Gardening Expert
Please take a look at the photo on this page - did the slug look like this?
If so, your tree was infested with the plum curculio beetle pest. The following article will give you more information and prescribe a course of treatment.
If that wasn't it, let me know and I will go back to the drawing board!