CHALARA FRAXINEA - a disease coming to an ash tree near you

(c) Steve Colin - Norfolk Wildlife Trust

Over the last couple of years an insignificant fungus has become a household name and has started to attack one of our most common woodland and hedgerow trees - the Ash (Fraxinus excelsior).

1. What is ash dieback disease, what are the symptoms and how can we spot it?

Ash dieback is a disease which has started to infect British ash trees. Often the first signs that a tree is infected are dead and dying leaves in early summer. By this time, however, the tree may already be badly infected internally and lesions may also be seen on the bark, characteristically in a diamond shape around a side branch. See examples here:

The disease is caused by a fungus which goes by the name of Hymenoscyphus fraxineus. This fungus was formerly called Chalara fraxinea but was subject to a name change because it was discovered that the fungus has two separate stages to its life cycle. So you may often hear people talk about the disease simply as Chalara. Whatever we call it, it has the potential to devastate the nation’s ash trees. Some of our ancient woodlands here in Gloucestershire may be comprised of up to 70% ash, and it is also a common tree in hedgerows and open green spaces. So the implications of this disease are significant.

2. When was it first confirmed in the UK/ and in Gloucestershire?

The first signs of Chalara in Britain were found in a nursery in Buckinghamshire in February 2012. Since then it has spread throughout eastern England with some counties, notably Essex, Suffolk and Norfolk, being particularly badly affected. As at May 2016 it was confirmed in nearly 800 10x10 km squares across the UK which amounts to just under a third of the UK now showing signs of the disease.

3. Where did it come from?

There is some indication that the disease originated in Asia where ash trees are allegedly immune to the disease. How it got to Europe is uncertain but it was first identified in Poland in 1992 and it has since spread across the continent.

4. Why is it so prevalent/ how does it spread?

Like all fungus species its spores are distributed by the wind and with ash being such a common and widespread tree it is not surprising that the disease has spread so rapidly. However, controversially, the disease is first thought to have arrived in the UK on infected trees imported from commercial nurseries in continental Europe. While this raised concern about the levels of biosecurity involved, it was only a matter of time before the disease arrived naturally from windborne spores.

5. How much of a problem is it in Gloucestershire?

As at May 2016 Ash Dieback was confirmed in 9, 10x10 km squares across the county, but this is probably an under-estimate. Woodlands around Cirencester and west of Cheltenham are known to be infected as are woodlands around Tetbury, north of Bristol, the Lower Wye Gorge and Cinderford. This is almost certainly an under-estimate as these are mainly areas of commercial forestry where foresters are always on the look-out for such infections. As ash trees come into full leaf in June it is likely that many more infected sites will be found. While I was answering these questions I received an email from a member in Box with a photo attached showing an ash sapling showing signs of the disease.

5. Is there hope for Gloucestershire’s ash trees?

It is difficult to say how badly Gloucestershire’s ash trees will be affected by the disease. Almost certainly many of our trees will succumb to the disease, but the good news is that (unlike Dutch Elm disease in the 1970’s where UK elm trees lacked any significant genetic variation) ash are known to be very genetically diverse so natural resistance is likely to be the key to the survival of our ash trees. The important thing we need to do in the county is to follow the national strategy produced by the Forestry Commission (1) and have trying to find ash trees which appear resistant to the disease as our major action. Landowners shouldn’t start to fell ash trees in the hope that this may slow the spread as this is likely to be no help at all.

6. Which reserves are currently affected by ash dieback?

At the time of writing no ash trees have been reported as being infected on any Trust reserve – but this may well be the calm before the storm!

7. How will it affect our reserves in the long-term and what does it mean for our ecosystems if we lose too many ash trees.

There is no doubt that our woodland nature reserves will change – but there will be a natural response from other tree and shrub species as they fill the gaps left by the dying ash trees. This will be a slow process as the trees may take some time to die completely. But with more light eventually reaching the woodland floor there is likely to be an explosion of growth from other species usually suppressed, such as bramble. Observations from Forestry Commission suggest that young ash growth (e.g. saplings and regrowth from coppice stools) may be more susceptible, so we may see the older trees lingering while the understorey is replaced by different tree species seeding in, such as oak and beech which may do better in the absence of the ever-present ash. Ecologically this is going to be significant and at the same time fascinating. Ash supports a large number of invertebrates, mosses and lichens so undoubtedly there will be an inevitable decline in some taxa. On the other hand deadwood invertebrates – a conservation priority – are likely to see a resurgence. And the rare lesser spotted woodpecker may see its fortunes change, so there will be winners and losers, but nature will not leave the gap left by dead ash trees empty!

8. How many trees have we lost so far in the county because of it?

This is not being recorded as there are simply too many. The Forestry Commission website simply shows a presence or absence at the 10 x 10 km square level

9. Does it affect other trees?

As far as we know it only affects 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.

10. What is GWT doing to help prevent ash dieback and what can members do to help?

There is nothing that can be done to prevent the disease from spreading. Biosecurity measures such as washing footwear and vehicles tyres is unlikely to have any effect as the spores are overwhelmingly windborne. We have information on our website about ash dieback and we are pointing people to the Forestry Commission website if they want to report a sighting – link here:

After a while as trees start to succumb to the disease, dying ash trees will start to become very apparent. Even more obvious should be ash trees which are still healthy and not visibly affected by the disease. If these remain healthy over time we will need to know where they are as they may be resistant varieties.


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Ash Dieback - GWT policy V4 (August 2016).pdf88.79 KB