What are the symptoms?
The main symptom is dieback in the crown (top of the tree), including leaf loss and wilting, dead twigs and branches. Trees infected with the disease are more easily identified in the summer when they are in leaf and should have a healthy crown.
On younger trees dark lesions (diamond shaped) can be seen at branch junctions and at the base of the trunk. As the disease progresses it kills the bark and trunk resulting in the death of the tree.
Once affected by dieback, ash trees can be more vulnerable to secondary infection by, for example, honey fungus. This can speed up their decline.
Ultimately, as infected ash trees deteriorate and die they may fall over or shed large limbs.
For more information please see this guide from the Forestry Commission:
Forestry Commission: ash dieback (external website)