Ash Dieback

The chances are you’ve heard or seen something about ‘Ash Dieback’, but how much do you really know about this issue? Our Woodland Conservation Officer and tree specialist Doug Lloyd explains more about this serious threat.

So what is Ash Dieback?

Ash Dieback is caused by a fungus, apparently new to science in 2006 and given the name Chalara fraxinea, this being a stage in the life cycle of a cup fungus, Hymenoscyphus albidus, previously thought of as a harmless and common fungi of fallen Ash leaves. This is not the end of the story though, as in 2010 it was discovered that Chalara fraxinea is actually a stage in the life cycle of a similar but previously unknown fungus, named at the time as Hymenoscyphus pseudoalbidus. This fungus is indistinguishable from albidus by eye and can only be identified from its differing genetics hence the name, pseudo-albidus.

Once the fungus is present in an Ash tree it causes an infection, and this moves along a twig. Often it stops at a junction between twig and branch but sometimes continues past this union, spreading through and thus weakening the tree. At the same time, it also creates a poison, Virodol which is toxic to Ash Trees, further weakening them. This weakening can mean that secondary fungal pathogens such as Honey fungus can attack the tree and this can sometimes lead to its eventual death.

A recent Forestry Commission survey which split the UK into 10km grid squares found infections had been confirmed across 80% of Wales

Where did it come from?

It was widely publicised at the time of outbreak in Britain (around 2011/12) that it came from eastern Europe and probably before that had come from Asia, possibly China. There was no real evidence for the latter, and only some evidence for the former in that is was first noticed in Latvia, Baltic states and later in Poland in the 1990s. Although importation of Ash trees from nurseries in Europe definitely hastened and probably started the infection in the UK. Interestingly though, analysis of Ash leaf specimens in a collection in Switzerland has shown that pseudoalbidus (the fungus causing Ash Dieback) was already present in these collected specimen leaves in the 1960s. Furthermore, Oliver Rackham, woodland ecologist and historian notes that after the droughts of 1975/76 there was a similar looking disease of Ash Trees in the Midlands, this was never investigated but the trees recovered.


What does Ash Dieback look like?

The symptoms of Ash Dieback on younger trees have been widely publicised and include black lesions on twigs, leaves dying off, twigs and branches going brown, orange and purple and dying off and then seeming to recover with growth of new leaves in a bunched form. These symptoms are good indicators of Ash Dieback on young trees but it isn’t these that are a concern.

Symptoms on mature Ash are a little more difficult to spot, partly because they are often 10-20m above the ground! The most obvious signs that Ash are infected are late to very late leafing (as happened this year) and when looking at the tree from a distance the canopy looks distinctly sparse with protruding and bare twigs and branches

Monitoring Ash trees

Young infected Ash trees are never going to pose a problem but mature Ash, especially if they are roadside or adjacent to buildings, should be monitored by the owner.

The easiest way to do this is to take a photos of the tree at a specified points in the year, (mid summer is good as it should be in full leaf) and if the canopy is fairly healthy then nothing needs to be done. The photographs show approximate percentage cover of canopies as a quick survey method as employed by various local authorities. If the canopy is above 50% then it can be considered reasonably safe, below 50% then advice sought, below 25% then it is likely to be dangerous.

Photographing of Ash trees should be carried out again the following year and any changes noted – if there is a significant decline in the canopy cover and it has the ability to fall onto a target such as a road or a house then the owner should consider taking action, such as felling. But please bear in mind that a dying or holey Ash tree may support important wildlife such as bats, which of course have legal protection. Advice should be sought from Natural Resources Wales. You can do this via their website at and searching under the business sector ‘forestry’ and then ‘tree health and biosecurity’.

It should also be noted that if a tree poses little risk to people or property e.g. an Ash tree that has dieback but is in a field with little or no public access, or in a large garden where it cannot hit any targets, then it could be left. Some of these trees will die quickly, some more slowly and importantly, some will show levels of tolerance to the disease. In Europe approximately 10-15% of Ash have shown moderate levels of tolerance to Chalara with 1-2% having high levels of tolerance, so whilst there will be a large number of Ash that will die in the coming years, it is likely that some will survive.