Ash dieback is a disease of ash trees caused by a fungus called Chalara fraxinea and can affect trees of any age and the symptoms are:
- Black blotches at the base of leaves before they begin to wilt
- Legions or spots on the bark of stems and branches
- The whole tree with show signs of shoots and branches dying back
It is important to note that Chalara fraxinea does not affect mountain ash (Sorbus aucuparia)
Take a look at this video from The Food and Environment Research Agency about the Life Cycle of Chalara fraxinea and Symptoms to look out for.