Ash dieback is a serious disease of ash trees, caused by a fungus now called Hymenoscyphus fraxineus. The fungus was described as a new fungal species in 2006 as the cause of ash (Fraxinus excelsior) mortality in European countries during the previous ten years.
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 was first confirmed in trees growing in nurseries or on recently planted ash trees. However, many cases have now been confirmed in the wider environment in the UK and the disease is widely distributed. The latest distribution maps for cases of the disease in the wider environment can be found on the Forestry Commission website.
The disease is also 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.
In 2018 ash dieback has been found infecting three new ornamental tree and shrub species in the UK. These species; mock privet (Phillyrea latifolia), narrow-leaved mock privet (Phillyrea angustifolia) and white fringe tree (Chionanthus virginicus) are in the same family as ash (Oleaceae). These findings are unlikely to have a big impact on the environment as these plants are not native or widespread in the UK.
Symptoms of ash dieback include;
If you find a suspected case of ash dieback in an area where it has not previously been reported (see the distribution map on the Forestry Commission website) you should report your suspicions to the relevant plant health authority by submitting a report via TreeAlert. Sightings in Northern Ireland should be reported via TreeCheck.
Collecting leaves and leafmould
If composting ash leaves in an area where ash dieback is known to be present, the Forestry Commission recommends covering them with with a 10cm (4-inch) layer of soil or a 15-30cm (6-12 inches) layer of other plant material, and leaving the heap undisturbed for a year (other than covering it with more material). This is likely to prevent any spore dispersal and may help to slow the spread of the disease in an affected area.
There is no chemical control available to gardeners for this disease.
Hymenoscyphus fraxineus 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 (stalks) 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 not believed to be infectious and can only spread over short distances by water splash. When it is producing asexual spores the fungus is known as Chalara fraxinea, and the disease is therefore sometimes called Chalara dieback or just Chalara.
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 a ban was applied on all movement of ash trees and seeds in October 2012, high volumes of ash (F. excelsior) were imported every year either for forestry or non-forestry purposes; therefore the potential for entry of the pathogen to the UK was very high.
However, the theory that spores wind-blown from the continent are a common source of entry is now widely accepted, as cases recorded in the wider environment were initially located in the eastern parts of the country. The density of wider environment infections is still greatest in the east but there have now also been cases recorded in many other areas. Because the disease is now so widespread the movement ban on ash within the UK and from EU countries has now been lifted.
The fungus is also seed-borne.
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 Hymenoscyphus fraxineus. Cankers caused by the fungus Neonectria ditissima 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 and removal of bark at the base of shoots, sometimes leading to shoot killing.