Pocket Plum 

Taphrina pruni

Pocket Plums are blackthorn fruits (sloes) that have been deformed by the ascomycete fungus Taphrina pruni. They are called 'Pocket Plums' because the fungus destroys the developing stone and seed, creating a small pocket in each fruit. Initially green, the galls change to brown or grey as the spore-bearing asci develop on the exterior.