Taphrina pruni is a fungal plant pathogen that gives rise to the Pocket or Bladder Plum gall.  The fruit is swollen and deformed. Affected fruits have no stone.  The shoots may also be swollen and stunted, and reddish or yellowed. 

Similar galls are found on Bird Cherry

host

Photograph the gall on the host; ensure you have noted the host species in your comments

On Blackthorn and domestic Plum.

Frequent in Leicestershire and Rutland, though its abundance may vary from year to year.