Ash Dieback (ADB)

Ash dieback, also known as Chalara dieback of ash, is a fungal disease that affects all species of ash tree (Fraxinus).

It is the most significant tree disease to affect the UK since Dutch elm disease. In Britain, the disease was first officially recorded in south-east England in 2012 from where it has spread west across the UK. It is now affecting all parts of Wales.

The fungus (Hymenoscyphus fraxineus) attaches itself to the leaves of ash trees and spreads through the branches where it blocks the water transport systems, causing leaf loss, lesions in the wood and on the bark.

This leads to the gradual dieback of the tree crown. Trees become brittle over time with branches breaking away from the main body of the tree. If they are not managed appropriately, trees are at risk of collapsing, presenting an immediate danger to the surrounding area.

There is no known cure or practical way to prevent the disease from spreading. ADB has the potential to kill over 90% of Ceredigion ash trees over the next 5 to 10 years.

Ash dieback affects the leaves of ash trees causing them to blacken, wilt and die from around June onwards. For very young trees this and the diamond shaped lesions (areas of discoloured bark) on the stem are the key features of early infection.

As the name suggests, the disease causes the tree to die back from the edge of its canopy. In mature trees, the first sign of the disease is bare, dead twigs at the top of the tree and ends of the branches.

As the disease progresses and the number and length of dead branches increases, the tree responds by growing new leaves closer to the main branches and trunk giving the tree a clumpy ‘pom-pom’ look. Eventually the tree will look increasingly bare and dead.

Ash dieback can also lead to serious discolouration, cracking and death of the bark at the base of the trunk. Ash trees with these symptoms have a higher risk of sudden death and collapse, so should be a priority for safety works if in a location which poses a risk to public safety.

Please note that the tree commonly referred to as mountain ash or rowan is not affected by ash dieback as it is not a member of the ash genus (Fraxinus).

There are thousands of ash trees on public land in Ceredigion and many thousands more on private land. Ash is one of the most common tree species in the county, and it contributes significantly to the local landscape and ecology.

Ceredigion County Council will only be dealing with trees on public land, such as parks, schools, or within roadside verges that it owns and manages.

The Council also has a duty of care to protect the public from dangerous trees on private land that have the potential to impact public areas such as highways.

Ash trees affected by ash dieback will be categorised into one of four health classes, based on percentage of dieback in the crown. This will help us determine which trees are in need of removal.

Trees in category 1 are either unaffected by ash dieback or show early signs of infection.

Trees in category 2 have lost 25-50 % of their canopy.

Trees in categories 3 and 4 will be heavily affected by the disease with more than 50% canopy loss and brittle branches - these trees will be a priority for crown reduction or felling, but only where they pose a risk to the public.

Some ash trees show high levels of resistance to the disease and should not be considered for removal; these trees are very important for the ecological value they retain in the environment and they may help repopulate the species in the future.

Even trees that are affected by ADB should be retained wherever it is safe to do so.

Sometimes it may be appropriate to pollard or de-limb an affected tree to make it safe, rather than fell it at ground level. Ash trees support a wide range of associated biodiversity, such as lichens, bryophytes and animals that breed, roost and shelter in the trees, and standing deadwood is a very valuable habitat.

If you have ash trees on your land that could potentially fall on neighbouring land, roads, right of way or property, it is important that the trees are assessed by a suitably qualified and experienced arboriculturist to establish their health and the level of risk they pose.

Private landowners have a duty of care under common law to ensure they do all that is reasonably practicable to prevent injury or damage to neighbours. They also have a duty of care towards visitors to their land, including trespassers, under the Occupiers Liability Acts. The Highways Act also requires landowners to ensure their trees do not endanger people on roads and footpaths.

Businesses have additional obligations under the Health and Safety at Work Act to ensure their work places are safe.

For single householders this may require you to keep checking the condition of your ash trees, for larger landowners and businesses you will need to:

  1. Identify how many ash trees you have
  2. Assess their current condition
  3. Identify where affected trees pose a risk
  4. Make safe any hazardous trees 
  5. Monitor on an annual basis

The optimum time to survey for ash dieback is between the end of June and September as the dieback will be most apparent while the tree is in full leaf. The optimum time to fell trees if necessary is from September to February inclusive as this avoids the bird nesting season.

Please note, due to the structural changes ash dieback causes to the timber, trees may become unpredictable and are unsafe to fell from ground level. It is recommended that only suitably experienced, qualified and equipped tree surgeons or contractors fell severely infected trees.

Current TPOs

As described above, it is not recommended to fell healthy trees. Applications for consent to fell both affected and unaffected trees will be judged on their own merits, and the potential for infection by ash dieback will not be a significant consideration. Felling infected trees under Statutory Plant Health Orders will be an exception.

New TPOs

Until further notice, the potential risk of infection by ash dieback will not be considered a significant justification for not making a TPO, although a confirmed case of ash dieback is likely to be a significant factor weighing against making an order.

Dead and dangerous trees exemptions

Felling required under a Plant Health Order is exempt from the need to obtain TPO approval. If a TPO tree of any species is dead or posing an immediate risk, the works to fell the tree or remove parts that are dangerous is exempt from the need to obtain the Council’s permission for the works. However, the Council must be given five days’ written notice of exempt works.

Other legal considerations

Tree felling licences

You may require a felling licence to remove ash trees on your land, even if they are infected with ash dieback. Information on felling licensing can be found here:

Protected species legislation

Nesting birds and their eggs, chicks and nests are legally protected. Undertake tree felling and pruning work outside the bird nesting season (1st March to 31st August) wherever possible.

Before felling any mature trees, these should be checked for potential to support roosting bats. Bat roosts are protected even when not occupied. You will require a licence from Natural Resources Wales to destroy a resting place or breeding site of any species of bat. For more information see: