The fungus Protomyces pachydermus causes galls on the leaves or flower-stalks of Dandelions (Taraxacum).   The flower stalk or leaf midrib is swollen and twisted, initially yellowish then red or purplish.  On leaves, purple-red swelling cause distortion of the leaf. 

Some galls on midribs, stems and leaves may be caused by a fly larva. 

host

Photograph the gall on the host plant.  Ensure you have included a note of the host plant species in your comments.  Opening the gall will help to rule out galls caused by gall-midges. 

Dandelions