Ash Dieback

A fatal fungal disease of ash trees

First confirmed in the UK in 2012, ash dieback (also known as Chalara or Chalara ash dieback) is a disease of ash trees caused by a fungus called Hymenoscyphus fraxineus (formerly known as Chalara fraxinea).

This disease has spread quickly and is now affecting ash trees and woodlands in Sussex and right across the UK, leading to the death of thousands of trees. Ash dieback has already caused widespread damage in continental Europe.

Sadly there is no cure for ash dieback but some ash trees can tolerate or resist infection. Investigating this natural resistance could be the best way to secure the future of the UK's ash trees.

For the very latest updates on ash dieback, please visit the Forest Research website.

What is ash dieback?

Ash dieback is a disease that affects ash (Fraxinus) trees, caused by a fungus called Hymenoscyphus fraxineus. The fungus has two stages to its lifecycle - a sexual stage, which helps the fungus spread, and an asexual stage, which is what grows on the tree and causes damage. The fungus blocks water transport in the tree, leading to lesions in the bark, leaf loss and the dieback of the crown.

The ash dieback fungus is believed to have originated in Asia. It was first discovered in Europe in Poland in 1992, and is now found widely across the continent. The first confirmed case in the UK came in 2012 and since then it has spread across England and to Scotland, Wales, Northern Ireland, and the Isle of Man.

What are the symptoms?

The main symptoms of ash dieback are:

  • Dead branches
  • Blackening of leaves, which often hang 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 pathogens, including honey fungus, which can cause butt or root rot and lead to the tree falling.

How widespread is ash dieback in Sussex? 

The disease has been confirmed in ash trees across the majority of the county.  The latest distribution maps are available on the Forest Research website.

We have been assessing the health of ash trees on our nature reserves and planning our response to this disease.  This will include a careful programme of unavoidable tree removal work in locations where safety is a priority.

How does 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.

What are the impacts?

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).  

The Joint Nature Conservation Council (JNCC) in January 2014 published a report on the potential ecological impacts of Chalara ash dieback in the UK, and on the options for long-term monitoring of its impacts on biodiversity.

What can be done?

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.  

Following advice from woodland ecologists and plant health specialists it has been agreed 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. 

Whilst the proactive 'slash and burn' response worked to some degree with Dutch Elm Disease in Brighton and Hove, it would not be appropriate in this case for two reasons: Chalara spores are spread by the wind, so it would be extremely difficult to create an effective barrier; and ash is much more genetically diverse then English elm, so is more likely to develop a natural resistance to the disease.

Due to the risk of falling branches and collapsing trees, infected ash trees growing beside roads and footpaths are likely to pose a significant threat to public safety and will need to be removed for the safety of site users. In these cases, ecological surveys are important to check for the presence of protected species such as badgers and dormice, enabling the appropriate mitigation to be undertaken.  In general, ash trees in the middle of woodland and away from public footpaths will be left for nature to take its course. 

I think my tree is infected, what should I do?

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

My trees are fine but I would still like to help, what can I do?

  • Be vigilant - ash dieback is now present across most of Sussex but any new cases should be reported to the Forestry Commission. Their Pest Alert Factsheet summarises and illustrates the main symptoms to look out for 
  • Buy with care - only buy plants from reputable suppliers
  • Be diligent - practice good plant hygiene and biosecurity in your own gardens and woodlands to prevent the accidental spread of plant diseases. The Forestry Commission has produced posters that can be put up in woodlands to let people know if Chalara-infected trees have been found there
  • Keep up to date - check the Forest Research website regularly for updates on ash dieback

Chalara fraxinea - Life Cycle and Symptoms - Ash Dieback disease