Posted: 16/06/2013 at 17:52
what I had in mind was one of the Taphrina fungi, T. pruni or padi but the gall produced is hollow as a rule. Google Taphrina pruni and see if you recognise anything. A lot of them are sort of long, tongue shaped but some are round.
If not that I have no idea, my interest is in the galls, i'm not a fruit grower. I first met T.padi on my bird cherry. Took a long time to ID it in those early daysof interweb