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Chalara dieback of ash - RBG Kew's response

Chalara dieback of ash, a fungal disease which threatens the species, is currently receiving much press attention in the UK. Tony Kirkham, Head of Kew's Arboretum, provides a synopsis of the disease and how Kew is responding.

Background information

Chalara dieback of ash is caused by the fungus Hymenoscyphus pseudoalbidus (also known as Chalara fraxinea) and was first recorded in the UK in February 2012 in a consignment of nursery stock imported to an English nursery from Holland. It has since been found in several locations in England (Yorkshire, Durham and Kent) and west of Glasgow in Scotland. It is believed that all these sites have had imports from the continent of young ash transplants over the past five years. There have also been a small number of cases confirmed in East Anglia which have no links to the nursery trade.


Ash trees suffering with H. pseudoalbidus infection have been found widely across Europe since trees now believed to have been infected with this newly identified pathogen were 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. There are approximately 80 million ash trees in the UK and as such it poses a very serious threat. It caused widespread damage to ash populations in continental Europe, including estimated losses of between 60 and 90 per cent of Denmark’s ash trees since 2007.  Experience on the continent indicates that it kills young ash trees very quickly, while older trees tend to resist it for some time until prolonged exposure causes them to succumb as well.  

What are the symptoms?

Symptoms of H. pseudoalbidus are easier to spot on young trees and can be visible on leaves, shoots and branches of affected trees. In severe cases, the entire crown shows leaf loss and dieback and there may also be the formation of epicormic shoots on branches and the trunk.


Leaves can suffer from wilting and black-brownish discoloration at the leaf base and midrib. Dieback of shoots and twigs is also very characteristic, but can be similar to drought or frost damage.

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 cankers 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 affected trees have extensive shoot, twig and branch dieback and often have prolific epicormic shoots. 

How is the disease spread?

Details of how H. pseudoalbidus is spread are still uncertain, but local spread may be via rain splash or even transmission by insects. Over longer distances the risk of disease spread is most likely to be via the movement of diseased ash plants/nursery stock. Movement of logs or timber from infected trees may also be a pathway for the disease.

What ash species does Kew have?

We have around 520 accessioned specimens of Fraxinus growing in the collections at both Kew Gardens and Wakehurst. In the ash collection at Kew there are 227 specimens; this comprises some 43 different species of Fraxinus from Europe, Asia and North America. This Includes 18 different cultivars grown for their horticultural merit. The main ash collection is grown along either side of Princess Walk.

To date, we have found no symptoms of H. pseudoalbidus in any of these trees.

What measures are Kew taking to deal with chalara ash dieback?

We do not import Fraxinus specimens for the collections from external commercial nurseries in the UK or Europe. All staff in the Arboretum have been given the Forestry Commission Pest Alert Ash Dieback Disease guide to help with diagnosis and a toolbox talk on the subject. All staff are encouraged to be vigilant whilst working in the Gardens and look out for the symptoms on ash trees. Additionally, we have been through the collection looking for any signs of H. pseudoalbidus

I'm visiting Kew; do I need to take any precautions to help stem the spread of the disease?

Unfortunately, there is little that can be done to prevent the spread of chalara ash dieback in the UK. As the disease is fungal, it will be transmitted on boots and other footwear. If you are visiting woodland known to be carrying the disease then a deep clean of your boots before entering Kew or another woodland would help to some degree.

Can't Kew deploy a fungicide?

There is no known fungicide application that will control chalara ash dieback, but it may be possible for Kew to work with a researcher to use the collections as a potential research project.

We know that the disease affects Fraxinus excelsior and F. angustifolia, but Kew will be able to observe what other species are likely to be affected from the diverse collection that we hold, particularly Asian and North American species. This will be useful information for future plantings of ash across the UK.

What is the future of ash trees in Kew's Arboretum?

We have just started (1st November 2012) our annual tree planting programme in the Arboretum with 13 specimens of ash in the taxonomic collection (Oleaceae). These plantings include species from Asia and North America and grafted specimens from the Arboretum nursery of selected historical planting within the Arboretum.

We will implement a pro-active monitoring programme to observe when this disease arrives and the effects that it will have on the collections.

On 7 November 2012 Kew will host in partnership with the IDS a Pest & Disease conference and chalara ash dieback will be one of the conference topics.

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2 December 2012
There are 21 species of ash trees in North America. Have they tried to apply the DNA of any of these trees to the European ash tree to create a strain resistant to the fungus. That was done to the American chestnut tree, when its was attacked by the Asian bark fungus. They crossed it with the DNA of the Chinese chestnut tree. They were able to take this new strain and replant it in its former habitat.
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19 November 2012
I am curious to know what arboriculturalists knew and thought at the time about the devastation of Polish ash forests in the early 90s. Was there any response?
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6 November 2012
Thank you for this information. If there are precautions that we visitors can or should take when visiting Kew, I'm sure Kew will let us know. Let's all hope for the best.
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