), Johanson Taphrina pruni is a fungal plant pathogen of blackthorn (Prunus spinosa) that causes the pocket or bladder plum gall, a chemically induced distortion of the fruit (sloes), producing swollen on one side,[1] otherwise deformed and flattened fruit gall without a stone.

[4] The gall is widely distributed, under recorded in the United Kingdom, but found throughout the temperate Northern Hemisphere.

The surface of the gall becomes corrugate and coated with the fungus, showing as a white bloom of ascospore producing hyphae.

[3][6] In Britain the galls form in May and June, reach full size in July and August, and persist to September; some remain on the tree over winter, but most fall to the ground.

[6] The fungus infects the ovaries causing a pseudo-pollination and an enhanced cell division, resulting in the infested fruit being larger than the healthy one.