Coleroa robertiani is an ascomycete fungus in the family Phacidiaceae, specialized as a parasite or saprotrophic organism on herb-Robert (Geranium robertianum) and related geraniums (family Geraniaceae). The species produces small, dark apothecia on senescent or infected leaf tissue, releasing ascospores that perpetuate the fungal life cycle. Like other Coleroa species, it forms characteristic setose (bristle-bearing) fruiting bodies that anchor to host plant surfaces. This fungus is recorded primarily from temperate European regions where Geranium robertianum grows in shaded, moist habitats such as hedgerows, woodland edges, and rocky slopes. Its host specificity to Geraniaceae reflects co-evolutionary relationships between fungal lineages and particular plant families. Coleroa robertiani is generally not considered an economically significant pathogen given the weed or naturalized status of its host plants in many areas, but it plays a role in regulating wild Geranium populations and cycling nutrients through decomposition of infected tissue. As with many microfungi on minor hosts, detailed ecological and distributional data are sparse, and molecular phylogenetic studies continue to refine its classification within the broader Rhytismatales.