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What Do You Do If You Have Ash Dieback?

Updated: Mar 11, 2023

There is no known cure for Ash Dieback, so instead the focus must be on identifying the disease, slowing the spread and ensuring that the very small percentage of Ash trees that are resistant to dieback are preserved.

As tree surgeons we frequently get asked to look at trees suspected of suffering from Ash dieback. So what causes this highly infectious disease, what are the signs to look out for and what can be done about it?

An Ash tree infected with Ash dieback

What is Ash dieback?

Ash dieback is caused by a fungus called Hymenoscyphus fraxineus. It originated in Asia, however the native Ash trees there evolved with the fungus and so it is of little concern there. Unfortunately in Europe the Ash trees evolved without the presence of this fungus and so they didn’t develop a natural defence against it. It was first observed in Poland in the 1990’s and rapidly spread across Europe. It was first officially recorded in Britain in 2012 and has since spread across much of the UK.

Why should we care about Ash dieback?

It is expected that between 80-90% of the UK’s Ash trees could be destroyed by this disease and given Ash trees account for 22% of woodland in Devon, the impact here could be devastating – both to wildlife and the overall character of Devon’s countryside, as well as posing a significant threat to public safety in some instances.

How does Ash Dieback spread?

The fungal spores travel in the wind, landing on Ash leaves which then germinate and infect the tissue. Once the infection spreads and the fungus grows inside the tree it damages the vascular tissue and blocks its’ water and nutrient supply, eventually causing branches to ‘die back’. Ash dieback certainly can kill a tree on its own, however it is often the case with mature Ash trees that it puts the tree under such stress that it then becomes vulnerable to other diseases, which it ultimately succumbs to. Unfortunately the airborne nature of the fungus means there is little that can be done to prevent the spread, however the Forestry Commission does advise on ways to slow the spread such as by collecting and burning (where permitted), burying or deep composting fallen ash leaves.

What does Ash dieback look like?

Typically the first signs a tree is suffering from Ash dieback will be dark patches on the leaves, these will then wilt and turn black. You may also notice lesions on shoots,

Diamond shaped lesions on the trunk of an Ash tree suffering from Ash dieback
Diamond shaped lesions are one of the signs of Ash dieback

diamond shaped lesions on the stems and where the disease is advanced, there will be noticeable dieback / loss on the crown and dead branches will become visible at the edge of the canopy. As branches ‘dieback’ the greater the possibility of falling deadwood, in turn posing an increasing risk to the public and property.

The Tree Council has produced a very useful guide including photos to help you identify if your Ash tree has any of the above symptoms.

It is important to consult a tree surgeon if you think Ash dieback might be a possibility as other diseases may cause similar symptoms and so care needs to be taken when diagnosing and considering the best course of action.

Can Ash dieback be treated?

Unfortunately there is no cure for Ash dieback, however there is a glimmer of hope for the future of Ash trees in Devon and indeed the rest of the UK as it is thought that between 1-5% of Ash trees have a natural tolerance to Ash dieback. This is why it is important to first get the opinion of a tree surgeon to avoid the possibility of felling one of these precious few trees, which could produce the next generation of Ash trees.

When is the right time to do something about a tree with Ash dieback?

The Forestry Commission states that where more than 50% of the crown is infected removal is recommended; it is now seriously failing and will be producing a large volume of spores putting other nearby Ash trees at risk. Where less than 50% of the crown is infected the tree should be regularly monitored and tree owners should also check for secondary pathogens such as the honey fungus, which could kill it.

It is however important to highlight that whatever stage your tree may be at safety should come first and owners should be aware of their responsibilities and liabilities should the tree or part of it fall causing injury or damage to third parties.

Reporting Ash dieback

Although Ash dieback is already prevalent across much of the UK, the forest authorities still want to receive reports of ash dieback where not already recorded. If you think you have spotted ash dieback, the advice is to check the latest distribution map and report it using Tree Alert

Addition useful resources:

If you would like to learn more about Ash dieback, the below resources might be of interest:

Forest Research on pests and diseases: Ash Dieback (part of the Forestry Commission)

Woodland Trust on pests and diseases: Ash Dieback


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