Trees are one of the most valuable assets on any piece of land ecologically, aesthetically and sometimes financially. But they are also living organisms, subject to a wide range of diseases, decay processes and structural failures that can develop silently over many years before becoming visible.
For tree owners and developers alike, understanding how decay and disease affect trees and what the implications are for safety, planning and land management, is essential.
This guide covers the most significant common tree diseases and wood decay processes affecting UK trees, what the warning signs are, what the legal obligations are for tree owners and what developers need to know when trees on a site are affected.
How Wood Decay Works
Wood decay in trees is caused primarily by fungi, which break down the structural tissues of the tree over time. There are two principal types:
White rot fungi break down both lignin and cellulose, the main structural components of wood, leaving the affected timber pale, spongy and fibrous. Common white rot fungi found in UK trees include Ganoderma species, which produce distinctive bracket-shaped fruiting bodies at the base of the trunk or on major roots, and Meripilus giganteus (giant polypore), which typically colonises the root system of beech and other broadleaved trees.
Brown rot fungi break down cellulose while leaving lignin largely intact, producing wood that is dark, crumbly and prone to splitting into characteristic cuboidal fragments. Laetiporus sulphureus (chicken of the woods) is a common brown rot fungus in the UK, producing vivid yellow and orange brackets on oak, sweet chestnut and other species.
Decay typically enters a tree through wounds such as pruning cuts, storm damage, branch failures where there are infected branches, construction damage to roots and spreads internally, often without any visible sign on the outside of the tree until the decay is well advanced. This is why professional inspection and, where necessary, specialist diagnostic techniques such as sonic tomography or resistance drilling are valuable tools in accurately assessing the extent of internal decay.
Soft rot fungi primarily degrade cellulose and hemicellulose within the wood cell wall, forming characteristic cavities while leaving much of the lignin matrix relatively intact. This results in timber that is typically brittle, often darker in colour and prone to surface checking or a cracked, blocky texture rather than the fibrous decay associated with white rot. In UK trees, Kretzschmaria deusta (brittle cinder) is a key example associated with a soft rot–type decay, most commonly affecting the buttress and root flare of broadleaved species such as beech and lime.
Colonisation leads to a progressive loss of structural integrity with minimal external symptoms, often resulting in a brittle fracture of the stem at or near ground level. This form of decay is particularly significant in arboriculture due to its association with sudden, unpredictable basal failure under load, even where the crown appears physiologically functional.
The Most Significant Tree Diseases in the UK
The UK’s tree population faces an unprecedented range of disease threats, many of them introduced through imported plant material or facilitated by climate change. The following common tree diseases are most relevant to tree owners and developers.
Ash Dieback (Hymenoscyphus fraxineus)
Ash dieback is now the most widespread and damaging tree disease in the UK and early detection is important. Caused by a fungal pathogen originally from Asia, it affects all species of ash (Fraxinus) and is now present across virtually the entire country. Symptoms include wilting and discolouration of leaves, diamond-shaped bark lesions on stems at the base of dead shoots, crown dieback progressing from the tips inward, and the development of epicormic shoots (stress shoots) on the trunk as the tree attempts to compensate for crown loss.
For tree owners, ash dieback creates a serious and ongoing duty of care challenge. As affected trees decline, they become structurally unpredictable: the wood becomes brittle and branches can fail without warning, often in calm conditions. For developers, Fraxinus species on or adjacent to a site require careful assessment, as the timeline for decline is difficult to predict and a tree that appears structurally sound today may deteriorate rapidly within a single growing season.
Phytophthora Root and Collar Rots
Phytophthora is a genus of water mould that affects a wide range of tree species in the UK, including alder, beech, oak, larch and many ornamental species. Several species are of particular concern:
Phytophthora alni is the principal cause of alder decline across the UK, causing blackening and death of roots and collar tissue, bleeding cankers on the trunk and a progressive dieback of the crown. Infected trees are a common sight along watercourses, where the disease spreads readily through water.
Phytophthora ramorum (sudden oak death) affects a wide range of hosts including Japanese larch, rhododendron, viburnum and sweet chestnut. On larch, it causes rapid and widespread mortality and has resulted in the felling of millions of trees across Wales, Scotland and the south-west of England.
Phytophthora cinnamomi is increasingly implicated in the decline of mature trees in urban and amenity settings, particularly following periods of soil waterlogging.
For developers, Phytophthora is significant because infected material including soil, water, plant material and footwear can spread the pathogen to new sites. Biosecurity measures during site investigation and construction are increasingly expected by local authorities on sites where Phytophthora is known or suspected to be present.
Oak Processionary Moth (Thaumetopoea processionea)
Oak processionary moth (OPM) is an invasive pest now established across London and the south-east of England, with an expanding range. Its caterpillars feed exclusively on oak foliage and repeated defoliation weakens trees significantly, making them more susceptible to secondary pathogens and decay. The hairs of OPM caterpillars are a significant human health hazard, causing skin rashes, eye irritation and respiratory problems.
For developers working on sites with oak trees in the affected zone, OPM must be considered in the tree survey and any works programme. The Forestry Commission operates a management zone for OPM and movement of oak material from affected areas is subject to biosecurity controls.
Acute Oak Decline (AOD)
Acute oak decline affects both pedunculate (English) oak and sessile oak across parts of England and Wales. It is characterised by bleeding lesions producing dark, sticky fluid, splitting bark and rapidly progressive crown decline. The condition involves a complex of bacterial pathogens and bark-boring beetles and can kill mature trees within three to five years of first symptoms appearing.
Dutch Elm Disease (Ophiostoma novo-ulmi)
Dutch elm disease destroyed the majority of the UK’s mature elm trees in the 1970s and remains present in the environment. It is spread by the elm bark beetle and causes rapid wilting and death of shoots (flagging), progressing to full crown death. While large mature elm trees are now rare outside of Brighton and a few other managed populations, elm regenerates readily from root suckers and young elm growth is widespread in hedgerows where it is periodically reinfected as it approaches maturity.
Bleeding Canker of Horse Chestnut (Pseudomonas syringae pv. aesculi)
Bleeding canker is now widespread in horse chestnut trees across the UK, producing dark, sticky bleeding patches on the bark that indicate necrosis of the underlying tissue. Severely affected trees can suffer significant crown dieback and structural instability, and the bacterial canker has led to the removal of large numbers of roadside and parkland horse chestnut trees.
What Tree Owners Need to Know
Tree owners have a legal duty of care to ensure that their trees do not pose an unacceptable risk to people or property. This duty applies regardless of whether the tree is protected by a Tree Preservation Order (TPO) or is located within a conservation area: protection status does not remove the obligation to manage risk.
In practice, this means:
- Carrying out regular inspections of your trees, particularly after storms, high winds causing significant damage or periods of dry weather to check tree health.
- Commissioning a professional tree risk assessment where trees are large, over areas such as paths, roads or buildings, or where visible signs of decay or disease are present.
- Acting on the recommendations of a professional arborist and keeping records of inspections and works carried out.
- Notifying the local planning authority before carrying out any works on a TPO tree or any tree within a conservation area, even works that are clearly necessary for safety reasons.
Where internal decay is suspected, a standard visual inspection may not be sufficient. Specialist diagnostic tools including sonic tomography and resistance drilling can identify the extent and location of internal decay without causing additional damage to the tree, allowing informed decisions to be made about retention, remedial works or safe removal.
What Developers Need to Know
For anyone considering developing land where trees are present, decay and disease have direct implications for both the planning process and the programme.
Under BS5837:2012 (Trees in Relation to Design, Demolition and Construction), all trees on or adjacent to a development site must be surveyed and assigned a retention category based on their condition, life expectancy and amenity value. A tree survey will identify trees with significant decay, disease or a reduced life expectancy that are more likely to be classified as Category C (low quality) or Category U (unsuitable for retention), which can make their removal more straightforward to justify in the planning application.
However, this cuts both ways. A tree that appears structurally sound and is assigned a high retention category at survey stage may subsequently be found to have significant internal decay when more detailed investigation is carried out. Discovering this late in the design or construction process can require costly redesigns to avoid root protection areas or to accommodate the felling of a tree that was originally intended to be retained.
Key considerations for developers:
- Commission a BS5837 tree survey to assess tree health at the earliest possible stage, ideally before site acquisition or at pre-application stage, so that any decay or disease issues are factored into the design from the outset.
- Where trees with suspected decay are present, instruct additional expert diagnosis alongside the standard survey to reduce the risk of late surprises from finding other diseases.
- Consider the management liability that comes with retaining diseased or declining trees on a development: trees retained through the planning process become the developer’s responsibility, and a tree in declining health today may become a safety and liability issue within the development’s design life.
- Be aware of biosecurity obligations: moving soil, timber or plant material from a site where notifiable diseases such as Phytophthora ramorum or ash dieback are present without following the relevant biosecurity protocols can result in enforcement action and the spread of disease to neighbouring land.
When to Call a Professional
Any of the following should prompt a professional inspection by a qualified arboriculturist:
- Fungal fruiting bodies (brackets, crusts, toadstools) at the base of established trees or on trunks or roots.
- Bleeding or weeping patches on the bark.
- Crown dieback, wilting or unseasonable leaf loss.
- Cracks, splits or cavities in the trunk or major branches.
- Leaning that has recently developed or increased.
- Trees that have recently been affected by construction activity, soil compaction or waterlogging.
- Any tree within falling distance of a building, road, footpath or occupied space.
Arbtech’s qualified arboriculturists carry out BS5837 tree surveys, VALID tree risk assessments and specialist decay investigations across the whole of the UK. Whether you are a homeowner with a single tree of concern, a landowner managing a large estate, or a developer needing a full pre-application tree survey, contact our team for a fast, professional and LPA-accepted report.
