Ash dieback

How to identify diseased ash trees and why we must help them heal

A canopy of tree branches over blue sky
This is a guest blog post, written by our colleagues at RBG Kew.

A beautiful and established feature of the European landscape, ash trees are a vital component of our ecosystem. Several species of insect and plant depend on them for survival. Tragically, over the past 10 years, ash trees have been overtaken by the disease: ‘ash dieback’. Caused by an invasive fungus, ash dieback is killing healthy trees and posing a threat to the fragile biodiversity of our planet.  

However, all hope is not lost. Although their numbers are small, tolerant ash trees do exist. Identifying and studying them is the key to fighting this disease. Read on to learn how to spot an ash tree and how to tell whether it has ash dieback. 

A sprig of green leaves, covered in water droplets
Vincent Ryan/ iStock

What are ash trees and how do you spot one? 

According to Forest Research, European ash (Fraxinus excelsior) accounts for 12% of woodland in Great Britain. It is the third most common tree species in British woodlands, following the much loved English oak (Quercus rober) and Silver birch (Betula pendula). 

To spot an ash tree, look out for these signs:

In winter, you can recognise an ash tree by its distinctive grey/green smooth twigs and contrasting black buds. Don’t you think they look a bit like deer’s hooves?

Three small twigs lying on a piece of white paper next to a pair of secateurs
Isabel Negri/ RBG Kew

An ash tree’s flowers emerge before its leaves in spring, if the tree is mature enough. They can be male, female or hermaphroditic (male and female) depending on the individual ash tree. The flowers look like purple spiked clusters and can be tricky to spot. This is what a hermaphroditic flower looks like under the microscope. 

Spikey black buds on a green plant
Isabel Negri/ RBG Kew

Ash leaves are more noticeable. They are ‘compound’, which means they are formed of several different leaf segments, or ‘leaflets’. Usually, ash leaves comprise of 3-6 opposite pairs of leaflets, plus one ‘terminal’ leaflet, attached at the at the end of the same stalk.  

An ash leaf, with 4 pairs of leaflets and a terminal leaf at the tip
filmfoto/ iStock

Ash seeds (also called keys) are described as ‘winged’, because the seed itself is encased in a papery wing-like structure. They are found in clusters on the tree. 

Ash leaves and seeds
Marina Troynich/ iStock

What is ash dieback and why must we combat it? 

Ash dieback is a disease caused by the invasive fungus Hymenoscyphus fraxineus, which was first reported in the UK in 2012. In its native habitat of Asia, it coexists with other ash species without causing them significant damage. However, it has the ability to kill European ash trees by blocking the movement of water and nutrients within their vascular systems. Only a very small percentage of European ash trees are currently displaying any tolerance to this disease.  

If ash disappears from our landscape, so might many other species which depend on it, such as the aptly named Ash Bud Moth (Prays fraxinella), the beautiful Dusky Thorn Moth (Ennomos fuscantaria) and the rare Warty Wax Lichen (Thelenella modesta, it doesn’t look as bad as it sounds!) It is therefore a priority to find, protect and propagate these resistant trees in order to conserve both the species and the ecosystem. 


Lichen growing on a tree trunk
A healthy ash tree, covered in Lichen: Vicky Philpott/ RBG Kew
An orange moth sitting on a dark green leaf
Dusky thorn moth, Ennomos fuscantaria: Ian_Redding/ iStock

What are the signs of ash dieback? 

Some indicators of ash dieback infection are: 

  • Blackened or shrivelled leaves 

  • Diamond-shaped black lesions (which look like bruises) on the bark 

  • Dead external branches with new growth towards the centre of the tree. This is known as ‘epicormic growth’. It is where a branch or shoot grows out of a previously dormant tree bud that has been lying beneath the bark. It is often a result of severe stress. 

A healthy ash tree, assessed as being potentially tolerant:

A green tree standing on a gentle slope
Vicky Philpott/ RBG Kew

A diseased tree with only epicormic growth remaining:

A black branch growing out of a leafy tree
Vicky Philpott/ RBG Kew

A blackened shoot tip likely caused by the ash dieback fungus:

Black, drooping leaves on a plant shoot
Vicky Philpott/ RBG Kew

How you can help to save our ash trees 

Identifying ash trees that are ‘tolerant’ to (i.e. able to survive) ash dieback is crucial if we are to rescue the species. Now that you know how to recognise ash trees and assess their health, you can help with this. To find out how, head to our blog post about the amazing projects that are underway to source and study healthy ash trees. 


With thanks to Vicky Philpott, Ash Collecting Project Field Officer, and Russell Croft and James Pumfrey, Wakehurst Arboretum Manager and Lead for throwline training.   

Many thanks to everyone from the arboriculture and horticulture teams who helped out with the Living Ash Project, and to Chloe Bradbrooke and the National Trust team at Nymans

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