Ash die back is caused by a fungus, Chalara fraxinea (C. fraxinea). It is a serious, potentially fatal disease affecting ash trees.
The early symptoms are:
- Leaf loss, beginning with the leaves closest the tips of twigs and working back to larger branches and trunks;
- Crown dieback (progressive death of woody growths in the crown starting from the tips of branches and working back to the centre of the tree.