ASH DIEBACK  Chalara fraxinea


Chalara dieback of ashᅠis a very serious disease of ash trees caused by a fungus calledᅠChalara fraxinea,ᅠincluding its sexual stage, Hymenoscyphus pseudoalbidus.ᅠThe disease causes leaf loss and crown dieback in affected trees, and will usually lead to the trees death. With this newly identified pathogen being reported it produces a very serious risk to our native ash tree population. Infected trees have been found in forests, trees in urban areas such as parks and gardens, and also young trees in nurseries.

In February 2012ᅠit wasᅠfound in a consignment of infected trees sent from a nursery in the Netherlands to a nursery inᅠBuckinghamshire. Since then it has been found in a number and variety of locationsᅠin Great Britain, includingᅠurban landscaping schemes, newly planted woodlands, and even more nurseries.

Ash treesᅠhave many genetic variants in the UK, which come into leaf at different times.ᅠIn general, they come into leaf later in spring than many other trees, often as late as the end of May. So if an ash tree does not have any leaves on it in April andᅠMay, it does not necessarily mean that it is diseased or dying, but by mid-June all healthy ash should be in full leaf.



The leaf symptoms of Chalaraᅠareᅠbest observedᅠin August and September, before the onset of autumn, because in autumn infected leaves can be confused with leaves that are naturally changing colour. Some shoots on ash trees will fail to flush altogether, while others will flush normally before showing signs of ill-health or dieback later. These events might mean that the trees are damaged in some way, but shoot death and dieback in ash trees can have a number of other causes. Chalara fraxinea can be visible on leaves, shoots and branches of affected trees. In severe cases, the entire crown will show leaf loss and dieback and there may also be the formation of epicormic shoots on branches and the main stem.





Leaves can suffer from wilting and black-brownish discolouration at the leaf base and midrib. Dieback of shoots and twigs is also very characteristic.


Branches and stems

Small lens-shaped lesions or necrotic spots appear on the bark of stems and branches and enlarge to form perennial cankers these cause wilting and dieback of shoots and branches, particularly in the upper crown. Underneath the bark lesions, the wood has a brownish to grey discoloration which often extends longitudinally beyond the bark necrosis.


Whole tree

Trees with withered tops and shoots are very characteristic, heavily effected trees will have extensive shoot, twig and branch dieback and often have high number of epicormic shoots.


Reporting Infected Trees

Chalara fraxinea, is now being treated as a quarantine pest under national emergency measures and any suspected sightings should be reported.


Chalara helpline: 08459 33 55 77 (8am - 6pm daily)


Or email :

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