Harrow's trees

Ash Dieback (Hymenoscyphus fraxineus)

Ash Dieback will kill around 80% of Ash trees across the UK. This is likely to have devastating effects, both changing the landscape and threatening many species which rely on Ash.

Find information below on the disease, how to identify it, and what you can do to help stop it spread.

What does Ash Dieback look like?

Ash Dieback can affect Ash trees of all ages. Younger trees succumb to the disease quicker but in general, these are the first signs of Ash Dieback:

An example 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. The bark of live shoots and twigs turn darker, often with a purple tinge. Often you may notice dead and blackened leaves hanging amongst the live foliage.

The disease will cause diamond shaped lesions where older twigs and branches join the stem or trunk

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:

Other helpful guidance for identifying the disease:

The Forestry Commission have made a video about How To Identify Ash Dieback In The Field

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.

As 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.

What happens to the tree?

The fungus overwinters in leaf litter on the ground, particularly on Ash leaf stalks. It produces small white fruiting bodies between July and October which release spores into the surrounding atmosphere.

These spores can blow tens of miles away. They land on leaves, stick to and then penetrate into the leaf and beyond. The fungus then grows inside the tree, eventually blocking its water transport systems, causing it to die. The tree can fight back, but year-on-year infections will eventually kill it.

In the UK, Ash Dieback has had the most impact in the south-east of England. This is where it was first recorded in the UK in 2012. It is not known how long the fungus was in the area before symptoms became apparent - perhaps some years.

There is evidence of the disease throughout the UK, however the full impact is not known yet, since we are at the beginning of the epidemic.

How did Ash Dieback get here?

The spores of this fungus can travel in the wind so it is possible that it arrived in the UK naturally however it was also inadvertently imported on Ash saplings. The UK was importing thousands of Ash plants from infected parts of Europe until a ban came into place in 2012. This undoubtedly sped up the spread of the disease within the UK because the disease was able to spread from areas of new planting via wind to mature trees.

What to do if you spot Ash Dieback

If you think you've spotted the signs and symptoms, you should Report Ash Dieback to Forest Research

Gardeners and managers of parks and other sites with Ash trees can help stop the local spread of Ash Dieback by collecting the fallen Ash leaves and burning, burying or deep composting them. This disrupts the fungus's lifecycle.