This gall is actually an abnormally distorted catkin, and is probably caused by a virus or phytoplasma ( a type of Bacteria), but the precise causer has not yet been identified. It used to be thought that it was caused by the mite Stenacis triradiatus but it now seems that the mites just choose to live in and around these galls. The galls can be very large and variable in shape and degree of distortion of the catkin - often the gall forms a green straggly mass, or it can be more of a woody mass.

They may persist on the willows into winter.

Gall

On various willows.

Summer growths persist into winter when they can be particularly prominent.

Status in Britain not known.

Status in Leicestershire and Rutland not known.