Taphrina pruni

Taphrina pruni is a fungal plant pathogen of blackthorn that causes pocket plum, also called bladder plum, a chemically induced distortion of the fruit. Infected sloes become swollen on one side and otherwise deformed and flattened, developing without a stone. The twigs of infected plants can also become deformed, growing small strap shaped leaves. This description is adapted from Wikipedia contributors under CC BY-SA 4.0; changes were made. https://creativecommons.org/licenses/by-sa/4.0/