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How to identify Ash Dieback Disease

The first signs of Ash Dieback 

The leaves will start to develop dark patches during the summer months (July to September). These leaves will wilt and turn from brown to black.  Often you may notice dead and blackened leaves hanging amongst the live foliage. The bark of live shoots and twigs turn darker, often with a purple tinge.

Diamond shaped lesion on ash tree    Leaf die back

The disease will cause diamond shaped lesions where older twigs and branches join the stem or trunk. Lesions are areas of discoloured, sunken and dead bark. These lesions can eventually wrap around and kill the affected branch, stem or trunk. Often trees will respond by producing a growth of new shoots and leaves beneath the lesion.

Please note: care needs to be taken when identifying the disease, as other diseases and conditions may cause similar symptoms. 

Online guides to help identify the disease have been produced by:

  • The Forestry Commission
  • Woodland Trust
  • Arboricultural Association
  • the Tree Council 

We have found these two guidance documents particularly useful:

This video about how to identify ash dieback in the field from the Forestry Commission is also really helpful.

How the disease develops 

All ages of trees can be affected. Younger trees can decline and die in one or two growing seasons.  Mature trees may decline over a number of growing seasons.

Crown DiebackAs the disease progresses the leaf cover will become sparse. Dead twigs and small branches will become visible at the edge of the canopy. Lesions will start to become increasingly obvious upon larger branches and stems. In later growing seasons larger dead branches and dead stems will become visible. You will often see a flush of new growth below the dead stem. Mature trees may survive for many years with a smaller canopy. 

As branches and stems die back there will be a greater possibility of falling deadwood. The longer the deadwood is present, the greater the chances of it failing. This presents an increasing risk to the public and property. 

The loss of the leaf canopy places trees under stress. This can make them more vulnerable to other diseases and decay fungus. It is often fungus and other diseases in combination with ash dieback that will cause the death of more mature trees.

Trees can also be infected at the base of their trunk when they are surrounded by many diseased trees and diseased leaf litter. Lesions will be visible at the base but there may be no obvious signs of infection in the leaf canopy. When combined with the effects of other decay fungi, the tree can become unstable over time, leading to tree failure. (See your responsibility as a landowner page). 

Disease identification in winter

Look out for sunken and dark lesions on the branches and trunk, and young shoots or twigs with a dark purplish tinge. There may also be dead twigs and branches in the upper crown of the tree. Please note it is advisable to wait until the tree has come back into leaf to confirm the presence of the disease. Some similar symptoms can be caused by other factors, such as insect attack, other diseases or old age.

How the disease spreads

Ash dieback is caused by a vascular wilt fungus. This is a fungus that affects the water transport system of trees, which is just beneath the bark. 

The fungus (formally known as Chalara fraxinea) produces small white mushroom-like growths (Hymenoscyphus pseudofraxineus) between July and October. The fungus develops upon leaf stalks found within the leaf litter from the previous autumn's leaf fall. These release numerous spores into the air. Winds can then disperse the spores many miles from the original site. Trees need a high ‘dose’ of spores to become infected. This means that isolated ash trees are often slower to be affected by the disease.

The spores land on the leaves, which are then penetrated by the fungus and the disease spreads through the stems. Ash trees that are surrounded by diseased trees and diseased leaf litter can also become infected at the base of the tree. Please note that there is a very low likelihood of spread by clothing or from animals or birds.  As the disease is spread by winds it is not possible to protect trees from the fungus.

Ash Dieback fruiting spores

This photo shows the fruiting bodies (mushroom like growths) of ash dieback upon a leaf stem (photo: Forest Research).

The disease originated within Asia. Both the Manchurian ash (Fraxinus mandschurica) and Chinese ash (Fraxinus chinensis)  have resistance to the disease. Our native ash (Fraxinus excelsior) has not had the benefit of evolving with the fungus and so has very little or no resistance to it.  The disease is also known to affect other ash species. These include Manna ash (Fraxinus ornus), Black ash (Fraxinus nigra), Green and Red ash (Fraxinus pennsylvanica). 

It is very unlikely that other trees species or plants will be affected. There is also no evidence of harm to animals or wildlife apart from the considerable loss of ash tree habitat. 

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