Chalara dieback of ash is a serious disease of ash trees caused by a fungus called Chalara fraxinea (C. fraxinea), including its sexual stage,Hymenoscyphus pseudoalbidus (H. pseudoalbidus).
The disease causes leaf loss and crown dieback in affected trees, and usually leads to tree death. Visit the Forestry Commission website for more information on identifying affected trees.
The Forestry Commission have also produced a free Smartphone App (iOS or Android) that allows you to report affected trees