Ustilago filiformis is a smut fungus in the family Ustilaginaceae, an obligate biotroph that infects reed grasses (Glyceria species) and transforms their floral parts into masses of dark teliospores. It manipulates host plant development to create galls in place of normal ovaries, which then disperse the fungal spores. Smut infections by Ustilago species can significantly reduce seed production of their grass hosts.