Ash dieback is a serious tree disease caused by the fungus Hymenoscyphus fraxineus. It primarily affects ash trees, leading to leaf loss, crown dieback, and eventually tree death. The disease spreads through spores that can be carried by wind, rain, or animals. First identified in Europe in the 1990s, ash dieback has since spread to many countries, including the United Kingdom. Efforts to manage the disease include monitoring affected areas and promoting the growth of resistant tree species. The decline of ash trees poses ecological challenges, as they play a vital role in many forest ecosystems.