Hymenoscyphus fraxineus is a fungus that primarily affects ash trees, particularly Fraxinus excelsior. It causes a disease known as ash dieback, which leads to leaf loss, crown dieback, and ultimately tree death. The fungus spreads through spores and can be transmitted by wind, rain, or infected plant material. This pathogen was first identified in Europe in the early 1990s and has since spread to other regions, including North America. Its impact on forestry and biodiversity is significant, as it threatens the health of ash tree populations and the ecosystems that depend on them.