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First confirmed in Britain in 2012, Chalara dieback of ash, also known as ‘Chalara’, ash dieback or Chalara 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.
However, some ash trees appear to be able to tolerate or resist infection, and scientists are studying the genetic factors which make this possible so that tolerant ash trees can be bred for the future.
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.
We do know that the disease has potential to cause significant damage to the UK’s ash population. It has caused widespread damage 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.
Ash can grow in a variety of soils and climatic conditions. It is one of our most useful and versatile native tree species, providing valuable habitat for a wide range of dependent species as well as strong, durable, flexible and attractive timber with a wide range of practical and decorative uses.
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.
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.
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. It has not been reported infecting any non-ash species.
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.
According to the European Plant Protection Organization (EPPO), Austria, Belgium, the Czech Republic, Finland, France, Germany, Hungary, Italy, Lithuania, the Netherlands, Norway, Poland, Slovenia and Sweden have confirmed its presence. On the basis of symptoms, the disease has also been observed in Denmark, Estonia, Latvia and Switzerland.
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, a small number of cases were confirmed in Norfolk and Suffolk on ash trees at sites in the wider natural environment, including established woodland, which did not appear to have any association with recently supplied nursery plants. Further finds on trees in the wider environment have since been confirmed in most regions of the UK and Ireland, and it was first confirmed on the Isle of Man in 2017.
Hymenoscyphus fraxineus, the fungus which causes Chalara dieback of ash, is treated as a quarantine organism under national emergency measures, and any suspected sighting must be reported.
Upon discovery of infected ash trees in the wider environment in October 2012, we and others undertook a great Britain-wide survey of about a thousand sites which had received saplings from nurseries where Chalara dieback had been found to establish an understanding of the distribution of the disease.
Article courtesy of Forestry Commission England
T: 0300 067 4000
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