Plums – this year the fruit rotted on the tree. Why?
I had a good crop of Victoria plums but before ripening the fruit started to decay.
Certified GKH Gardening Expert
It sounds like the tree may have a fungus. Some fungus that get into the fruit can be treated, while others cannot. Your best bet would be to treat the tree with a fungicide. It may stop the fungus from spreading. Even if it does not, you can stop the fungus from returning next year by making sure that all of the leaves in the fall are properly disposed of.