Taphrina crataegi

This is a fungus which causes pale green, yellowish or reddish galls on Hawthorn.  The galls are blisters, folds or bulges in the leaf, and often several leaves together are affected.  The galls are often flushed pink.

Several species of Dysaphis aphids cause similar bulges, puckers or folds on hawthorn leaves in late spring/early summer

Check the underside - if waxy grey aphids are present it is probably a Dysaphis

Photograph underside and upperside

Hawthorn leaves.

Spring and summer.

Status in Leicestershire and Rutland not known.