Please tell me this isn't Plum Pox!
Even better, please tell me this is some sort of innocuous nutrient deficiency or pest on this Japanese plum.
The tree seems to be doing okay. It was grafted in 2012 onto a sucker of St. Julien that I transplanted. It has two varieties of Japanese plum grafted onto it plus some St. Julien growing. The scions were from an exchange.
I see ring shaped discoloration on several leaves (there are also holes, but those alone wouldn't concern me). The ring outlines are tinged reddish which is different from the images I've seen of plum pox.
The varieties may be Santa Rosa and AU producer (I'm going on fuzzy memory but they are labeled if it matters). I I don't recall if both grafted varieties are affected or just one, but the St. Julien doesn't seem to show these symtoms.