The larvae of the gall midge Physemocecis ulmi produce yellowish, blister-like galls on the leaves of Elm species. The galls contain a white larva.

Other species produce small pocks pouches or dimples on elm.  Not all blemishes on elm leaves are galls. 

Gall Adult

Gall: Unless identified by a recognised expert, a photo is required. If the photo doesn't show the key ID features then in the comments box describe the size and identifying characters you have observed.  Check there is a  larva inside

Where Elm, the larval host plant, is present.

Mines may be found when Elm is in leaf.

Single generation per year. Pupation in the ground.

Only recently recorded in Britain but seems to be establishing.

Rarely recorded in Leicestershire and Rutland, although it may be moving into our area.