What is ash dieback?
Ash dieback is a devastating disease which is predicted to severely affect or kill over 90% of ash trees dramatically impacting wooded landscapes across the UK - including here in Avon.
The disease, also known as Chalara is caused by a fungus called Hymenoscyphus fraxineus (The fungus was previously called Chalara fraxinea, hence the name of the disease.) which affects the vascular system of ash trees, inhibiting the tree’s ability to draw nutrients up into its upper branches. Young ash trees are killed very rapidly by the disease. Older trees often resist the disease for longer periods but succumb eventually.
Ash trees are the third most common tree in Britain after oak and birch - there are 80 million ash trees in the UK.
What does it look like?
The main signs of ash dieback are:
- Dead branches
- Blackening of leaves, which often hang limply on the tree
- Discoloured stems, often with a diamond-shaped lesion where a leaf was attached
- Trees may eventually drop limbs, collapse or fall
The symptoms are often easier to spot in mid-late summer, when a healthy ash should be in full leaf. It becomes much harder in autumn, when leaves are naturally changing colour and falling.
Once a tree is infected the disease is usually fatal - but some trees are tolerant or resistant to infection.
Mature ash trees infected by ash dieback may survive for several years but often succumb to a secondary attack by other pests or diseases.
How is it spread?
The disease may spread locally (over tens of miles) by wind dispersal. The reproductive stage of the fungus grows on the previous year's fallen leaves, producing fruiting bodies that release spores between June and September. These spores are dispersed by the wind and settle on the leaves of healthy trees. If a healthy tree receives a high enough dose of spores, it too will become infected. Over longer distances, the disease may be spread by the movement of infected ash plants.
How widespread is it in Avon?
By early 2019, the disease had been recorded across almost all of Avon. Ash trees are present in all of Avon Wildlife Trust’s wooded nature reserves and dieback has been recorded in most of these reserves.
What are the impacts and what can be done?
Ash dieback is likely to cause significant damage to the UK's ash population. Experience from Europe has shown that young trees are often killed quite quickly and while older, mature trees may survive for longer, they are often brought down by secondary infection (e.g. honey fungus).
There is no cure for ash dieback but some trees are less susceptible to the disease, so investigating this natural resistance could be the best way to secure the future of the UK's ash trees.
There is agreement from woodland ecologists and scientists that ash dieback cannot be controlled in any permanent way. Scientific evidence suggests that the best way to fight this disease is to allow it to spread through the ash population and wait for trees with natural resistance to regenerate woodlands.
How we're tackling the problem
Avon Wildlife Trust is producing a strategy to mitigate the impact of ash dieback on our nature reserves. The first stage of this is identifying any affected ash trees where there could be a risk of branches or entire trees falling - for example if the trees overhang public footpaths or roads. We will fell ash trees where they would pose a significant and unavoidable threat to public safety when they become infected.
We will also tackle the wider impact for wildlife of the disease and the loss of ash trees on our nature reserves and in the wider countryside. This will be through a combination of replanting appropriate trees to replace some of those felled, as well as allowing natural regeneration to take place - where seeds from trees are left to fall and germinate in situ.
What to do about diseased ash trees in gardens or on your land?
It is vital that people who are concerned about the trees in their garden or woodland do not start to panic and simply cut them down, please get expert advice:
- Leave healthy ash trees well alone. Ash trees are a vital habitat for birds and insects. Some ash trees may have a natural resistance and seeds from surviving trees could be used for replanting schemes
- If you think a tree might be infected by Chalara, look at the videos below and visit the Forest Research website to check their symptom guides
- Check that the tree is an ash rather than a rowan (also known as mountain ash). Rowan trees are easily mistaken for ash but are not susceptible to Chalara
- If you think a tree is infected with Chalara, report it to the Forestry Commission using their Tree Alert system
- You are not required to take any particular action if you own infected ash trees, unless the Forestry Commission asks you to
- Mature ash trees will not currently be removed unless they pose a significant safety risk, as they are valuable to wildlife, take longer to die and can help us learn more about genetic strains that might be resistant to the disease
- Keep an eye on the trees' safety as the disease progresses, and prune or fell them if they or their branches threaten to cause injury or damage
- In garden situations, you can help to slow the spread of the disease by collecting up and burning (where permitted), burying or composting the fallen leaves - this breaks the fungus's life cycle
- Further advice for woodland owners and managers is available in the Forestry Commission's Operation Note 46: Managing ash in woodlands in light of ash dieback