About ash dieback

Ash dieback (also referred to as ‘Chalara’) is a highly infectious fungal disease that is threatening to wipe out over 90% of our native ash trees and most other non-native members of the ash family.

Find out below about how the disease spreads, which species are affected, the origins and distribution of the disease, and how to recognise symptoms and report the disease.

How the disease spreads

In simple terms the disease affects the host tree by spreading through the tree’s network of conducting vessels, rapidly reducing the tree’s ability to complete its vital functions of water and nutrient movement and gaseous exchange, until the vessel becomes completely blocked and the branch girdled and the part above eventually dies. To recognise the symptoms of Ash dieback, see below.

The disease is likely to be fatal to the majority of infected trees. This may be due to the direct actions of the disease or the combined effect of other diseases such as Honey fungus which are now able to take advantage of the tree’s weakened state and reduced internal defences.

In some instances a small percentage of ash trees appear to be tolerant of the disease, the genetic markers of these are being studied where observed in an attempt to breed tolerant ash trees for the future.

Ash species affected

Whilst our single native species of ash (Fraxinus excelsior) and its varieties is particularly at risk it is also known that Narrow-leaved ash is also susceptible and the RHS note the following non-native trees as also being vulnerable

  • Manna Ash
  • Black Ash, Green or Red Ash
  • White Ash
  • Manchurian Ash

Of the above list the White ash and Manchurian Ash are considered to be the least susceptible.

Mountain Ash trees (Sorbus aucuparia sp) are in a fact botanical separate species and not affected.


The origins of Chalara are believed to be in Asia where its co-evolution with the varied Asian species of ash trees has bred resistance to the disease and therefore it is not considered to be a major problem.

The disease was first observed and confirmed to be present in mainland Europe in Poland and Lithuania in the early 1990’s, and it wasn’t until 2012 that the first recorded outbreak within the UK was confirmed. This was within a nursery in Buckinghamshire in February 2012 in a consignment of trees imported from the Netherlands. In October 2012 it was discovered to have spread to a wider environment setting in Norfolk and Suffolk.

Given that the disease has been present for a much longer period in Europe, the main body of research and information has emerged from there in recent years. Estimates of the percentage of ash trees that will eventually succumb to the disease in Europe vary from author to author and country to country dependent upon the sampling criteria used. Ranges vary from up to 5% of the ash population remaining unaffected to as low as 1% in the European example. Estimates for the UK are not yet available.

Distribution of the Disease

The situation is ever changing, but the latest Forestry Commission national mapping shows that Ash dieback has taken hold over much of the UK, including Devon.

In 2012, Devon County Council undertook a sample survey of the highways and properties it manages. Key findings were that

  • Ash dieback is present along highways in all districts
  • An epi-centre of diseased trees centres around Bickleigh Bridge in Mid Devon
  • Disease emergence continues to be patchy and no discernible pattern for outbreaks can be as yet established
  • Planted schemes showed incidences of outbreak reflecting the pattern of nursery stock promulgation of the disease
  • Ash dieback was not observed on any DCC properties sampled for survey however given the survey approach this may not be a true picture.

For more information about the survey and its findings see key facts about ash trees alongside roads

Recognising Ash dieback infection

The Forestry Commission has published several useful guides aimed at helping to identify Ash dieback (Chalara) at various times in the year including videos to aid identification. See Forest Research on Chalara ash dieback.

The key things to look out for are:

Crown dieback. This is one of the most easily observable indicators of a tree having being affected by ash dieback. Foliage in the crown of the tree becomes very sparse, of low vigour or even wholly absent. This can often be accompanied by vigorous epicormic growth (suckers or sprouts emerging from dormant buds on the trunk or branches) forming in bundles creating almost ‘pom-pom’ like clumps of crowded foliage. Such growth is not normal for Ash.

Small blackened hanging branches. These may be seen within the crown having been girdled by the disease but still retained. (Care must be taken to not confuse the clumps of often dark ash keys with hanging dead foliage).

Elongated diamond shaped lesions. These may be observed at branch junctions or leaf scars, often sunken into surrounding tissues as the tree attempts to heal the wound. These lesions are often more evident on basal shoots in mature trees or in the crowns of younger trees where the canopy can be more easily observed.

Diagnosis can be tricky as other disorders of ash trees may lead to a similar set of symptoms being displayed.

Reporting the disease

The Forestry Commission’s disease reporting tool Tree Alert should be used to inform of a suspected outbreak only where it is not already in a confirmed infected area. You can check where the disease has already been recorded on the Forestry Commission national mapping.

Movement of felled ash

There is no restriction on the movement of felled ash. The Forestry Commission’s Research pages has more information on Chalara Ash Dieback.