When walking along a narrow path between fields last week, I turned to face into the neighbouring hedgerow so that two other walkers could safely pass behind me. In so doing, I noticed these galls, which I think are Aceria campestricola (also known as Aceria ulmicola).

These growths betray the presence of the hundreds, perhaps thousands of the tiny mites that have caused these galls to form.

These particular galls can only be found on specific elm species, which in Britain include English elm and Small-leaved elm.