Chalara fraxinea is a fungal pathogen that primarily affects ash trees (genus Fraxinus). It causes a disease known as ash dieback, which leads to leaf loss, crown dieback, and ultimately tree death. The fungus spreads through wind, rain, and infected plant material, making it a significant threat to forests and woodland ecosystems. First identified in Poland in the early 1990s, Chalara fraxinea has since spread across Europe and into parts of North America. Efforts to manage the disease include monitoring tree health, promoting resistant tree varieties, and implementing biosecurity measures to prevent further spread.