Hymenoscyphus is a genus of fungi belonging to the family Helotiaceae. These fungi are typically found on decaying wood and other organic matter, playing a crucial role in the decomposition process. They are characterized by their small, cup-shaped fruiting bodies, which can vary in color and size. Some species within the Hymenoscyphus genus are known to be associated with plant diseases, such as Hymenoscyphus fraxineus, which causes ash dieback in Fraxinus trees. This has raised concerns about the health of ash populations in various regions, highlighting the ecological impact of these fungi.