Ramularia rubella is a plant pathogen in Ascomycota that infects Rumex species.

Infection produces reddish spots on leaves.

The red color is caused by the production of rubellin, a photodynamic anthraquinone-derived phytotoxin.

R. rubella was originally described from Rumex aquaticus in Germany; it has a wide geographic range on Rumex species.

It is being investigated as a biological control agent of Rumex obtusifolius.