Chalara fraxinea causes a lethal disease of ash and represents a substantial threat to the UK’s forests and amenity trees growing in parks and gardens. Detected at a number of sites in the UK for the first time in 2012, the fungus is a notifiable pathogen and suspected cases of the disease must be reported to the relevant plant health authority.
What is ash dieback?
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.
Symptoms of ash dieback include;
- On leaves: Black blotches appear, often at the leaf base and midrib. Affected leaves wilt
- On stems: Small lens-shaped lesions or necrotic spots appear on the bark of stems and branches and enlarge to form perennial cankers. The infection may girdle the stem and kill it in a single season. If the bark is peeled, the wood underneath has a brownish to grey discolouration. This discolouration extends beyond the bark necrosis
- On whole tree: Affected trees show extensive dieback of shoot, twig and branches. Trees have often prolific epicormic shoots
If you suspect that Chalara fraxinea is present in your garden you should not attempt to control the disease yourself. You should report your suspicions immediately to the relevant plant health authority;
Collecting leaves and leafmould
Where there is no reason to suppose that nearby ash trees are infected with ash dieback it is safe to incorporate fallen leaves into compost and leafmould bins. Where ash dieback is suspected it should be reported to the relevant authorities (see above) and they will advise on how to deal with leaf litter.
There is no chemical control available to gardeners for this disease.
Chalara fraxinea has been isolated from the roots of symptomatic trees, as well as from leaves, shoots and branch/stem lesions.
The infectious spores (sexual) of the fungus are produced by fruiting bodies (apothecia) and can be wind-blown over long distances (20-30 km). The apothecia are produced from June to October on ash leaf petioles and rachises from the previous year in the leaf litter. The spores land on leaves or other parts of the trees. From the leaves, the fungus makes its way down the petioles, rachises and stems. The fungus can also produce asexual spores but these are believed to be non infectious and can only spread over short distances by water splash.
The fungus has several pathways of spread over long distances. It can be spread through the movement of diseased ash plants and logs or unsawn wood from infected trees. Until the ban was applied on all movement of ash trees and seeds on 29 October 2012, high volume of ash (F. excelsior) were imported every year either for forestry and non-forestry purposes therefore the potential for entry of the pathogen to the UK was very high. The theory that spores wind-blown from the continent was the primary source of entry is now widely accepted as cases recorded in the wider environment are located on the east part of the country. The fungus is also seed-borne.
The soil phase of the fungus is not known. Long-lived spores that can persist in the soil have not been observed.
Other pests and diseases of ash
Dieback on ash can also be the result of an infection by several wood decay fungi and also by the root pathogen honey fungus. These fungi can also affect trees that are already suffering from Chalara fraxinea. Cankers caused by the fungus Neonectria galligena and the bacterium Pseudomonas savastanoi pv. fraxini are also associated with dieback on ash. The latter disease has only been confirmed on Fraxinus excelsior.
The pest ash bud moth (Prays fraxinella) affects Fraxinus excelsior causing hollowing out of buds, removal of bark at the base of shoots sometimes leading to shoot killing. The damage is usually seen in May.
Image: © Forestry Commission Picture Library