The mite Eriophyes similis causes galls to form on the leaves of Prunus species, usually Blackthorn, but also on plums and damsons. The galls are elongated ovals, usually heaviest around the leaf margins.  The galls are usually with a large protrusion on the underside the leaf and a smaller protrusion with an elongated slit opening on the upperside.  However they can be found in a reversed position with the larger protrusion on the upperside.   

Eriophyes homophyllus

Gall  Adult

State host, and provide photos of upper and underside of leaves

Around Blackthorn.

Galls appear in May and are usually mature in August or September.

Very common in many localities throughout England.

Thought to be common in Leicestershire and Rutland.

The taxonomy of gall mites on Blackthorn and Plums is confusing and uncertain.  Similar galls on these hosts are chiefly along the midrib and upperside of the leaf, and have been named Eriophyes homophyllus. Redfern & Shirley (2023) advise that the mites need to be identified by an expert.  This is not currently feasible. 

See Roskam, J.C. & Carbonnelle, S. (2023) Plant galls of  the Netherlands, Belgium and Luxembourg. KNNV. and Heal, J (2020) Eriophyes homophyllus (Nalepa) (Eriophyidae) on Prunus spinosa in Britain?  Cecidology 35(2) 38-42