Phragmidium is a genus of rust fungus that typically infects plant species in the family Rosaceae. It is characterised by having stalked teliospores borne on telia each having a row of four or more cells. All species have a caeoma which is a diffuse aecidium lacking a peridium. There are a number of species of Phragmidium, most of which are restricted to one or a few host species. Examples include: Possibly the most commonly encountered is P. mucronatum, found on most species of wild roses including Rosa canina and Rosa arvensis.