Hymenoscyphus fraxineus is a fungal pathogen 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 Poland in the early 1990s and has since spread across Europe, causing significant ecological and economic impacts. Efforts to manage the disease include monitoring tree health, promoting resistant tree varieties, and implementing biosecurity measures to prevent further spread.