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Ash dieback

What is Ash dieback?

Ash dieback, also known as Chalara, is a disease that affects ash (Fraxinus excelsior) and other trees of the species Fraxinus.  The disease is caused by the fungal pathogen Hymenoscyphus fraxineus which arrived in Europe from Asia in the 1990s and rapidly spread across Europe.  It was first recorded in the UK in 2012 but it has since been established that ash dieback has been killing ash trees in the UK since at least 2004.

Ash trees colonised by the fungus display a range of symptoms including foliar leaf spots, dead leaves and shoots, sparse thinning crowns, branch dieback, dead wood, bark lesions and tree death.  The fact is that once infected the majority of trees will die!

The speed of decline once a tree is infected is not possible to accurately predict as conditions specific to the tree and the site will have a significant bearing but studies in Devon, an area where ash dieback is in an advanced stage, suggest that a 10%-15% decline in canopy during a single season is typical.  It is possible for individual trees (depending on their health and condition) to decline much more rapidly and secondary pathogens such as honey fungus (Armillaria sp.) can increase the rate of decline and the severity of decay.


Spores are produced by fruiting bodies of the fungi which take the form of small white mushrooms on the central stem of the previous year’s fallen ash leaves, the spores are spread by the wind and infect healthy trees by landing on the leaves. 

As the fungus grows it destroys the vessels (phloem and xylem) which the tree depends on to transport water and nutrients around its structure resulting in leaf death and branch die back.  Over time the lack of leaves available for photosynthesis results in a depletion of energy reserves which weaken the trees natural defences allowing exploitation by secondary pathogens, increasing the severity of decay.  The reduction in moisture content leads to brittle branches prone to failure, eventually the tree fails due to basal decay by secondary pathogens or dies from the ash dieback.   

Where basal decay is exacerbated by secondary pathogens such as honey fungus the decay can advance to a failure criteria before there are obvious symptoms of ash dieback in the canopy making the identification of ‘dangerous’ ash trees more difficult.


Ash dieback was first recorded at a nursery in Buckinghamshire in February 2012 and was rapidly identified in further locations primarily, but not exclusively, to the east of the UK.

By May 2018 ash dieback was found widely throughout the UK and it is now evident in 44.8% of UK 10km squares and in nearly two-thirds of England’s 10km squares, more information on the rate of infection can be found at (http://chalaramap.fera.defra.gov.uk/).

The fact that some areas of the UK have been severely impacted by ash dieback ahead of other areas that are currently relatively unaffected, provides a great deal of information on how to plan for, manage and eventually recover from the disease based on the experience of local Planning Authorities that are actively managing severely affected tree populations.

It is vital to understand that ash dieback will not be ‘business as usual’.  Ash dieback is either already in an area or is likely to be in the next few years with potentially serious practical and financial impacts to many areas and organisations.

Identification of infected trees

The disease can affect ash trees of all shapes and sizes, the symptoms are distinctive and relatively easy to identify in young trees, they are often harder to identify in more mature trees.  There is a significant on-line resource available to assist with the identification of ash dieback including the following links:

Forest research




The Tree Council



Managing infected trees

Once ash trees have been identified as infected by ash dieback it is important not to overreact and fell all ash trees immediately, it is also important not to underreact and wait for trees to start failing.  Ash populations should be monitored through surveys during the summer months when symptoms are apparent. 

Four classes or stages of decline have been identified which can be used to inform management decisions.  The four stages are:

  1. 100%-75% remaining canopy
  2. 75%-50% remaining canopy
  3. 50%-25% remaining canopy
  4. 25%-0% remaining canopy

The categories provide the opportunity to set a point where trees will be removed, likely stage 4 but possibly stage 3 depending on the criteria set out in a site specific Ash Dieback Action Plan.

What will the impact be on tree owners and managers?

Health and safety

The principle concern for the majority of tree owners/managers will be the health and safety issues that arise from having dying trees in areas with a value target area (the area that will be impacted by a trees failure) such as roads, railway lines, schools, adjacent properties, footpaths, utility infrastructure etc.

The potential for deaths as a result of tree failures due to ash dieback is significant as is the increased likelihood of litigation as a result of failures.

Of secondary concern are trees that may damage property such as fences, gates signs, stores etc.


There will be a significant increase in expenditure identifying ash populations, monitoring the extent and rates of infection and removing trees when they become dangerous.  The issue may be exacerbated by the fact that the trees in high use areas such as road sides may be more difficult to remove requiring road closures, specialist contractors for complex dismantling operations and possibly specialist plant and equipment.

Woodland owners that manage trees for commercial markets will experience significantly reduced timber yields whist possibly having to absorb the costs of managing the health and safety aspects at the same time. 

Some forecasts indicate that the market competition associated with a limited skilled labour resource may result in price increases amongst tree work contractors.

It is possible that a rise in property damage and personal injury claims relating to ash dieback will result in an increase in landowner insurance premiums.

The recovery stage will require tree planting in large numbers.

Administration resources to deal with instructing contractors, obtaining felling licences, making Conservation Area notifications and TPO applications, obtaining road closures leasing with utility companies etc. and managing public perception.


Landscape and amenity impact from the loss of ash trees, loss of carbon sequestration, impact on biodiversity, 955 species have been identified that use ash trees of which 45 only occur on ash, risk to protected species, flood risk, bank destabilisation.

What should tree owners/mangers do now?

Owners and managers responsible for ash tree populations which are not yet impacted by ash dieback have the luxury of having time to plan, how much time however is not known, it would be wise to use that time to develop an Ash Dieback Action Plan (ADAP). An ADAP is an overarching plan that will identify, communicate and address the specific risks of ash dieback to an organisation.

The basis of an ADAP is a baseline survey in order to establish the number, size and location of ash trees within the ownership of a given site.  This will provide a picture of how many trees are going to eventually need to be removed, how many are in areas with a high frequency of use, how many are large, how many will need specialist equipment and measures to remove.

This baseline information will inform budget forecasts, corporate risk registers and enable an organisation to begin strategic planning regarding resources, equipment, training, the appointment of external advisors and eventually a recovery plan.

Regular surveys are going to be required to monitor the spread and development of the disease within ash populations. 

Ash dieback disease is something that owners and/or managers of ash trees are going to have to deal with at some point.  Lessons can be learnt from other areas of the country that did not have the luxury of time to plan. 

If you have any concerns regarding how ash dieback may impact your organisation or need advice on the preparation of an Ash dieback action plan then please contact us on 01761 492 633 or .


Institure of Chartered Foresters Registered Consultant - Professional Member of the Arboricultural Association QTRA Registered User