Introduction to Chalara dieback of Ash

Chalara dieback is a serious disease of ash trees caused by the fungus Hymenoscyphus fraxineus (the asexual stage of which is known as Chalara fraxinea). Infection by the fungus causes leaf loss and crown dieback in a range of ash species, and the disease is frequently fatal.

The disease is now widely distributed across the UK. The incidence of ash dieback can be checked on the Defra Chalara map. If the disease has not been recorded in a hectad, a report through TreeAlert with good photos can be made. This report can then be used to update the distribution map.

Symptoms of the disease can be visible on the leaves, shoots, branches or stems of affected trees. In severe cases, the entire crown shows leaf loss and dieback, which is often associated with the formation of recovery shoots on the branches and trunk. In early autumn, infected leaves typically display wilting and brownish-black discoloration which extends into the midrib and leaf stalk. Lens-shaped areas of bark death (lesions) centred on a dead side shoot are often visible on stems and branches. Wilting and dieback of shoots and branches may also be seen when lesions have girdled the branch or stem below. Occasionally, dark lesions ascending from the stem base may be found but these are unlikely to develop before crown dieback is seen on the affected trees.

In the following pages you will be asked some detailed questions concerning the trees about which you are concerned and the symptoms which they display. You will also be asked to upload digital photographs of the trees in order for us to determine whether they are likely to be infected by Hymenoscyphus fraxineus. If you cannot provide this information at present you may wish to re-inspect and photograph the trees before submitting your report.