Chalara dieback of ash, also known as Chalara or ash dieback, is a disease of ash trees caused by a fungus called Hymenoscyphus fraxineus. (The fungus was previously called Chalara fraxinea, hence the name of the disease. See ‘The Science’ below for an explanation of the name change.)
Chalara causes leaf loss, crown dieback and bark lesions in affected trees. Once a tree is infected the disease is usually fatal, either directly, or indirectly by weakening the tree to the point where it succumbs more readily to attacks by other pests or pathogens, especially Armillaria fungi, or honey fungus.
The first signs of Chalara in Britain were found in a nursery in Buckinghamshire in February 2012. Improved monitoring techniques continue to uncover new finds. As we continue to monitor the situation around the country we expect to continue to find more new cases.
We don’t yet know what the full impact of Chalara will be in Britain. Evidence from continental Europe suggests that older, mature ash trees can survive infection and continue to provide their landscape and wildlife benefits for some time.
The best hope for the long-term future of Britain’s ash trees lies in identifying the genetic factors which enable some ash trees to tolerate or resist infection, and using these to breed new generations of tolerant ash trees for the future. Government scientists, including our Forest Research agency, are working hard on this in partnership with a range of other respected scientific research institutions.
We do know that Chalara dieback of ash has potential to cause significant damage to the UK’s ash population. It has caused widespread damage to ash populations in continental Europe, where experience indicates that it can kill young ash trees quite quickly, while older trees can resist it for some time until prolonged exposure, or another pest or pathogen attacking them in their weakened state, eventually causes them to succumb.
The Joint Nature Conservation Council (JNCC) in January 2014 published reports of studies into the potential ecological impact of Chalara ash dieback in the UK, and on the options for long-term monitoring of its impacts on biodiversity.
Chalara dieback of ash is especially destructive of common or European ash (Fraxinus excelsior), including its ‘Pendula’ ornamental variety. Narrow-leaved ash (Fraxinus angustifolia) is also susceptible, and both species have been affected in the UK.
Local spread, up to some tens of miles, may be by wind. Over longer distances the risk of disease spread is most likely to be through the movement of diseased ash plants. Movement of logs or unsawn wood from infected trees might also be a pathway for the disease, although this is considered to be a low risk.
Ash trees suffering with the infection have been found widely across Europe since trees were first reported dying in large numbers in Poland in 1992. These have included forest trees, trees in urban areas such as parks and gardens, and also young trees in nurseries.
It was first confirmed in the UK in February 2012 when it was found in a consignment of infected trees sent from a nursery in the Netherlands to a nursery in Buckinghamshire, England.
In October 2012, Food & Environment Research Agency scientists confirmed a small number of cases in Norfolk and Suffolk in ash trees at sites in the wider natural environment, including established woodland, which did not appear to have any association with recently supplied nursery stock. Further finds in trees in the wider environment have since been confirmed in a number of places, mostly on the eastern side of England and Scotland, and mostly concentrated in the east and south-east of England.
In May 2013 the first wider-environment case was found in south-west Wales, which is the farthest west site in Britain that a wider-environment case has been confirmed.
The agent which causes Chalara dieback of ash is treated as a quarantine organism under national emergency measures and any suspected sighting must be reported.