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Ash dieback

Visual of what ash dieback is

What ash dieback is

Ash dieback is caused by the fungus Hymenoscyphus fraxineus. This fungus was previously known as Chalara fraxinea, which is why you may see or hear the disease referred to as ‘Chalara ash dieback’. It blocks the water transport system in the tree causing the leaves to wilt and die. This is followed by the development of lesions (discolorations) on the bark and the dieback of branches within the crown of the tree.

This disease was first described in Poland in 1992 and has since moved westwards throughout Europe. It was first identified in Britain in 2012 with the first confirmed sighting on the Isle of Man in 2017. Since then the fungus has rapidly spread throughout the island.

Young trees are particularly vulnerable and die back quickly once they are infected. Dieback rates in older trees are more variable but tend to be slower.

The symptoms are:

  • dark long thin and diamond shaped lesions on the main stem at the base of dead side shoots
  • shoot tips blackened and shrivelled
  • leaves blackened and dead
  • saplings have dead tops and side shoots
  • dieback of branches in the crown of mature trees, often accompanied by new growth further down the stem
  • in late summer and early autumn (July to October), small white fruiting bodies can be found on the blackened leaf stalks

Mold on tree

Source: Forestry Commission

The UK Forestry Commission has an identification video that you may find useful.

Further photographic and pictoral identification guides are also available: 


What DEFA is doing about it

The disease is spread by tiny spores released from the fruiting bodies of the fungus, which grow on the dead leaves of infected ash. There are two factors which make it very difficult to control the spread of the disease. The first is the practical problem of collecting all the dead leaves which might end up producing spores. The second is that the spores disperse naturally via wind over tens of kilometres. Because of these issues DEFA currently have no plans to execute a disease control programme. UK and European research into control methods has so far been unsuccessful, so the focus has shifted towards producing a disease resistant ash species, utilising existing genetic diversity and naturally resistant phenotypes.

The Department of Environment, Food and Agriculture (DEFA) will continue to monitor the disease and will be working with UK and European agencies to keep up-to-date with developments.

What to do if your ash tree looks infected

Canopy declineAll of the visual symptoms of the disease can also be associated with other plant health problems. You should firstly try and establish whether the symptoms you can see are being caused by Chalara ash dieback. Using the identification guides cited above have a close look at your trees and see if the symptoms are consistent with those of Chalara ash dieback; just because your trees do not have a full, healthy crown does not mean that they are infected with this disease.

Don’t worry if you’re not sure. The disease can be difficult to diagnose, particularly in mature trees. It is possible that there could be a complex interaction of biotic and abiotic factors which are causing the symptoms you’re observing. Note also, that there is difference between dormant and dead twigs; ash has a natural cycle of reduced vitality following previous growing seasons which produced a high volume of seed.

The size, age and vitality of the tree will affect the rate of decline. The Department’s tree officers do not provide safety advice or risk assessments and are not available to provide general advice or confirm a diagnosis in individual cases. If you are worried about the condition of the tree, you should contact a professional arborist for advice. View our list of accredited contractors.

The image next to this section is a guide to assessing the extent of canopy dieback. 

Requirements of the Island's Plant Health Act

The Plant Health Act 1983 only restricts the importation of ash trees to the Island and does not require you to fell a diseased ash tree.

Notifying DEFA

There is no requirement to notify DEFA of sightings of ash dieback. The UK Forestry Commission encourages members of the public to use their Tree Alert website to aid in their research.

Duty of care

You still have a duty of care to manage your trees to minimise the risk to the public. A guide for a common sense risk management of trees can be found on the National Tree Safety Group's website.

Felling trees that have ash dieback

You are not legally required to take any particular action if you own infected ash trees. We advise a general presumption against felling living ash trees, whether infected or not. This is because there is good evidence that a small proportion will be able to tolerate H. fraxineus infection. There is also the possibility that a proportion of ash trees can become diseased, but then recover to good health.

Public safety must be the priority, so 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.

Payment responsibility for the felling of infected ash trees

The cost of felling or pruning a diseased tree lies with the tree owner.

Using the infected wood for timber or fuel

There is a low risk of the disease spreading via infected logs. Whilst we would encourage you to avoid moving timber from infected trees away from the site of felling, there is no legal restriction on the movement of diseased ash. Its use on site as fuel is not a problem.

Preventing infection in healthy ash trees

The disease is spread through spores released from fungal fruiting bodies which grow on fallen leaves, so collecting and burning as much ash leaves as possible may help reduce repeat infections. If the disease is present in trees adjacent to your garden, however, this is unlikely to be effective because the spores may be carried by the wind in to your garden. There is no effective treatment for infected trees, and current research suggests that infection will be fatal in 90% of cases.

The Tree Preservation Act 1993

In accordance with the Tree Preservation Act (1993) the removal of living ash trees, or the pruning of living branches from registered trees, will still require a licence. The removal of dead trees or dead branches from living registered trees does not require a licence. Further information on the restrictions of the Tree Preservation Act, including guidance on how to apply, is available on the Tree Protection webpage.

The Wildlife Act 1990

Trees provide important habitats for wildlife, such as nesting birds and roosting bats. You should be aware that both are protected under the Wildlife Act (1990) and it is an offence to damage or disturb bats or their roosts, or to damage active bird nests.

When considering the felling or limbing of a tree, you are reminded that checks should be made to see if the tree contains active bird nests (especially from March to July) or holes/cracks used by bats to roost or hibernate in. Reckless activities which result in the disturbance of nesting birds or bats may result in prosecution.

We don’t necessarily advise felling; safety permitting, trees with deadwood should be left, as they provide valuable deadwood habitat for wildlife.

Further information about the Wildlife Act can be found on the Wildlife, Biodiversity and Protected Sites webpage.

What to plant instead of ash

Native trees such as ash provide a variety of benefits to our environment, society and economy. At present, however, we wouldn’t recommend planting any species of ash (Fraxinus sp).

If you want to plant a replacement tree which will provide similar benefits for wildlife we recommend you consider, alder, poplar or field maple.

In a garden setting it is best to seek advice from a tree nursery to ensure you select a tree that will be suited to the local environment.

Promote good Biosecurity (Keep it Clean)

Keep it clean logoChalara ash dieback is already widespread across the island but it presents a good opportunity to remind the public that visitors to woods, forests, parks and public gardens can help to minimise the spread of other plant diseases. They can do this by brushing soil, mud, twigs, leaves and other plant debris off their footwear and wheels - including the wheels of cars, bicycles, mountain bikes, baby buggies and wheelchairs - before leaving the site. 

Further information

Forestry Commission’s Forest Research has more detailed information on the current status and developments on their website.

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