I’ve looked for these galls for some time, and, like that saying about buses (‘you wait ages for one, then two come along at the same time’), I’ve now seen them twice in the past couple of days.

Though you’d be forgiven for not recognising them, these are the fruit of the Blackthorn or Sloe (Prunus spinosa) but they have been attacked by a fungal pathogen. That pathogen changes the shape and the colour of the fruit and, if you were to split one open (which I did), you would find that there is no stone inside (or, rather, there is a miniscule speck of brown matter that would have developed in to a stone). The gall is sometimes referred to the Pocket or Bladder plum gall and can also affect domestic plum trees.