Taphrina crataegi

Taphrina crataegi 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

host

Photograph the galls on the host plant, on both sides of leaf (check the underside - if waxy grey aphids are present it is probably a Dysaphis). Ensure you have noted the host species in your comments

Hawthorn leaves.