Spotting a problem before a tree starts visibly declining is what leaves you with real options.
The pests and diseases below are the most common ones affecting UK trees, along with the physical signs to check for and what to do when you find them.
Why tree health matters for your grounds – Advice by tree removal experts
Pest attacks and fungal disease rarely stay contained to a single tree.
Across woodland and estate grounds, a confirmed infestation or disease can spread to neighbouring trees and different plant species if it isn’t identified and managed early.
A dead or dying tree also creates a safety liability.
Crown dieback weakens branches, root rot destabilises the base of the tree, and once a tree becomes structurally unsafe, removal is usually the only option.
For landowners and estate managers, regular monitoring and a clear management plan are the most practical form of tree pest control available.
Spotting a problem at the infestation stage costs far less than dealing with established disease or significant damage to a mature tree.
Ash dieback, bark beetles and other established UK tree pests
Tree diseases can be caused by fungi, bacteria, water moulds, and the physical damage of insect attack.
The type of disease and pest you’re dealing with determines what control measures are available and how urgently the tree needs a professional assessment.
Ash dieback
Ash dieback is caused by the fungus Hymenoscyphus fraxineus and has spread across ash trees throughout the UK since it was first confirmed here in 2012.
No chemical or biological treatment halts ash dieback once a tree is infected, and the fungus travels through spores released from the dead leaf stalks of infected ash trees.
Early signs include diamond-shaped lesions on the bark, wilting at the tips of twigs, and discoloured or dead leaves that remain attached past the point they should have fallen.
Crown dieback marks the visible stage when dieback has moved from the outer branches inward toward the main stem, and by that point the disease is well established in the tree’s structure.
Ash trees showing signs of ash dieback should be assessed by certified arborists promptly. Management decisions depend on how far the disease has progressed, the tree’s proximity to people and structures, and whether monitoring over time is safe and practical.
Oak processionary moth
Oak processionary moth (OPM) is a regulated tree pest, currently established mainly across London and parts of south-east England. It was accidentally introduced to England in 2005.
The caterpillars defoliate oak trees by feeding on the leaves in late spring and early summer, and their white silken nests become visible on tree trunks and in the canopy from May onwards.
OPM caterpillar hairs cause skin rashes, eye irritation, and breathing difficulties. Anyone working on or near affected trees should treat both the caterpillars and their nests as a direct health risk.
Larvae hatch from egg masses laid on the upper branches of oak trees, and by the time nests are visible at the base of the tree, the colony is already well established.
Reporting OPM sightings to the Forestry Commission is a legal requirement in parts of England. Do not attempt to remove nests without specialist equipment.
If OPM is suspected on your grounds, contact a tree care service with pest management experience immediately.
Horse chestnut, sweet chestnut and associated diseases
Horse chestnut trees are affected by both a bacterial disease and a leaf-mining moth. Horse chestnut bleeding canker, caused by the bacterium Pseudomonas syringae, produces dark sticky patches or bleeding lesions at the base of the tree and further up the trunk.
The canker gradually girdles the trunk. Once the bark is fully girdled, water and nutrients can no longer move through the tree, and crown dieback follows as the tree dies back above the infection point.
The horse chestnut leaf miner moth causes premature leaf drop and browning of leaf margins from mid-summer. Repeated infestations weaken the tree over successive seasons and increase its vulnerability to further pest attacks and blight.
Sweet chestnut blight, caused by the fungus Cryphonectria parasitica, has been recorded in the UK and is considered a serious biosecurity risk to sweet chestnut populations.
Lesions form on the bark, and the canker spreads around the trunk until it kills the tree above the infection point.
Bark beetles in UK trees
Several bark beetle species cause significant damage to UK trees.
The eight-toothed spruce bark beetle (Ips typographus) has been a growing concern in south-east England since outbreaks were confirmed in 2018.
Spruce is the primary host, but bark beetles as a group attack a wide range of trees and shrubs, including elm, pine, birch, and juniper. A beetle infestation spreads quickly once a tree is weakened by drought, disease, or environmental conditions that reduce its natural defences.
Signs of bark beetle attack
Bark beetle infestation leaves physical evidence on and around the tree.
D-shaped holes in the bark are left by adult beetles emerging from inside the wood. Frass, the fine sawdust-like material produced as larvae bore into the wood, collects at the base of the tree or in bark crevices.
Pitch tubes are small blobs of resin that appear on the outer bark of pine and spruce where the tree is actively pushing back against invading beetles.
Their presence alongside holes in the bark is a strong indicator of active beetle infestation.
Inside the bark, larvae hatch and bore galleries through the phloem and cambium layers.
Once those galleries are cut through, the tree can no longer move water and nutrients across that section. By the time the canopy shows it, the beetle infestation is usually well advanced.
Phytophthora root diseases
Phytophthora is a group of water moulds that attack the root systems of many species, including oak, alder, beech, and larch across the UK.
Environmental conditions that keep soil wet for extended periods increase the risk considerably.
Symptoms include wilting, foliage discolouration, and progressive dieback spreading from the outer canopy inward. Root decay at the base of the tree often precedes any visible canopy symptoms by several months.
Aphids and gall insects on UK trees
Aphid infestation is common on many UK tree species, particularly in spring when new growth is soft. Heavy infestations cause leaf curl, sticky honeydew residue on the bark, and in some cases defoliate the tree when populations build unchecked.
Gall-forming insects, including several gall wasp species on oak trees, produce visible growths on leaves, twigs, and buds.
Gall formation affects many species including oak, mountain ash, and willow, and is one of the more frequent reasons landowners contact a tree service when they notice something unusual on a tree.
For a healthy tree, galls very rarely threaten its survival on their own. The presence of galls alongside other symptoms on the same tree is a reason to call for a professional check rather than treating the two issues separately.
Biosecurity threats on the UK watch list
Asian longhorn beetle
The Asian longhorn beetle (Anoplophora glabripennis) attacks a wide range of broadleaved trees and is one of the most watched biosecurity threats to UK woodland.
Host plants include maple, willow, elm, and birch, and the beetle has been intercepted in the UK on multiple occasions.
Infested trees cannot be saved once the beetle has established. Confirmed outbreaks require felling of all host trees within a defined radius to prevent spread, and the scale of that response reflects how seriously the risk is taken.
Emerald ash borer and other biosecurity threats
The emerald ash borer (EAB) has destroyed ash tree populations across North America and parts of Europe and is considered one of the most serious biosecurity threats to UK ash trees if it reaches these shores.
The bronze birch borer is another borer species on the radar, though it is not yet present in the UK.
Other species on the watch list include the pine processionary moth, tent caterpillar species, and tussock moth larvae that have caused widespread defoliation across parts of continental Europe.
Non-native pests represent a growing threat to our trees, and early reporting alongside regular checks on imported plant material are the main tools available to prevent spread into established woodland. The Woodland Trust publishes identification guides for UK tree pests and diseases that can help you confirm what you’re looking at. Anything you believe to be a notifiable pest should be reported to the Animal and Plant Health Agency (APHA).
What to do if you find a tree pest: reporting and treatment options
Some tree pests and diseases are notifiable, meaning you are legally required to report them to the relevant authority.
In England, notifiable tree pests must be reported to the Forestry Commission or APHA. The preferred route for most sightings is Tree Alert (treealert.forestresearch.gov.uk). In Scotland, reports go to Scottish Forestry.
Photograph any suspected notifiable pest before reporting via Tree Alert.
Try taking a clear photo is required to submit a report. Do not attempt removal or treatment of pests like OPM; contact the Forestry Commission for guidance.
Tree pests in the UK: common questions
What is the most common tree disease in the UK?
Ash dieback is the most widespread tree disease affecting UK trees by volume, with a high proportion of ash trees across England, Wales, and Scotland showing symptoms since 2012. Phytophthora root diseases affecting oak and other broadleaved species are also widespread across the UK, along with horse chestnut bleeding canker. Tree diseases and pests often interact, with fungal disease weakening trees and making them more vulnerable to subsequent beetle infestation.
What insects live in trees in the UK?
UK trees host a very wide range of insects, the majority of which cause no significant damage. Bark beetles, including the eight-toothed spruce bark beetle, are a growing concern for forestry and estate owners.
How do you identify tree pests?
Look for physical signs at the bark surface, in the canopy, and at the base of the tree. Exit holes, frass, pitch tubes, egg masses, unusual gall growth, and abnormal leaf drop are all indicators that need closer attention. Our tree survey service covers all tree species on a site and produces a written condition report with any pest or disease findings clearly recorded.
Get expert tree care for your grounds with GGM Landscaping
Get for signs your tree is unhealthy to protect your species with GGM Landscaping.
Annual tree surveys and active monitoring across your site are the most practical way to protect mature trees before a pest or disease forces the issue. GGM’s award-winning, certified arborists cover central Scotland and Glasgow, carrying out tree surveys, pest and disease assessments, and tree sugery for private, commercial, and estate clients.
Call 07946 322717 to arrange a survey for your site, or get in touch online to discuss your grounds.