Botryotinia calthae is an ascomycete fungus in the family Sclerotiniaceae, the sexual stage of a Botrytis-like pathogen that infects marsh marigold (Caltha palustris) in wetland habitats. It forms sclerotia in infected plant tissue that persist in soil to initiate future infections. As a necrotrophic pathogen, it kills host tissue and then feeds on the resulting dead material.