Ash dieback is a serious disease of ash trees caused by a fungus called Chalara fraxinea. The fungus was described as a new fungal species in 2006 as the cause of ash (Fraxinus excelsior) mortality in European countries during the last ten years. In 2008, studies initially concluded that C. fraxinea was the anamorph (asexual stage) of an already described species, Hymenoscyphus albidus. This latter species is considered as non-pathogenic, native, and widespread in Europe so it was not clear why an outbreak of the disease occurred. However, in 2011 it was demonstrated by molecular studies that C. fraxinea was in fact the anamorph of new species called Hymenoscyphus pseudoalbidus. Both species are almost indistinguishable morphologically.
The disease affects trees of all ages. Young trees can be killed in one season and older trees tend to succumb after several seasons of infection. It has spread rapidly in continental Europe. In the UK, the disease had initially only been confirmed in trees growing in nurseries or on recently planted ash trees. However, cases have now unfortunately been confirmed in the wider environment in the UK. A surveillance programme completed in November 2012 found the disease in 115 sites including 15 in nurseries, 39 in new planting sites and 61 in the wider environment (forests and woodlands). It is thought that the disease has been in the country for at least 2 years.
The disease is already established in many other European countries, where it has had devastating effects. The natural host range of the fungus includes F. excelsior, F. angustifolia, F. ornus, F. nigra, F. pennsylvanica, F. americana and F. mandschurica. The least susceptible species are F. americana and F. mandschurica.
Note: Ash dieback does not affect mountain ash (Sorbus aucuparia). Ash (Fraxinus excelsior and other species of Fraxinus) can be recognised by the following features;
- Opposing buds and branches
- Non-waxy grey/brown bark
- Noticeably black dormant winter buds
- Compound leaves which may be smooth or have finely toothed edges
- 8-12 leaflets depending on species
Useful images of both ash and ash dieback disease can be found on the FRAXBACK website.