Plum tree diseases
This looks like shothole disease, but is it?
Certified GKH Gardening Expert
These are leaf galls, caused by some pest on your tree.
Specific identification may help with control and treatment.
You can take a leaf sample to your nearest County Extension Office to inquire on any specific infestations in your region.
This article has more information.
Here is a link to help you locate the nearest Extension Office.