Tree Disease Symptoms

A guide to tree diseases and their symptoms

A healthy tree can often withstand the harshest of conditions and the worst the weather has to throw at it, but trees, like most living things, can get sick and die. Changing environmental conditions and an invasion of non-native trees and plant species can all mean tree species facing growing threats from an increasing number of diseases and pests.

Diseases like Dutch elm disease, Acute oak decline (AOD), and Sweet chestnut blight are all examples that have all left their mark on the tree landscape to varying degrees. They act as a reminder that tree disease, if left unchecked, can run rife.

Find out some of the common signs of diseased or dying trees and discover 10 common tree diseases and those trees most at risk in the UK.


So how can I tell if a tree is diseased?

If a tree is affected by a disease or pest then it may start to show signs that it is sick. One or several of the following symptoms may be present in the tree:

  • Leaf discoloration or blight. Discoloured blotches or dead areas on the tree’s leaves, particularly at a time of the season when leaves should appear healthy.
  • Defoliation. Trees losing their leaves at the wrong time of year.
  • Root decay. Broken roots or signs of fungus present in the roots.
  • Fungus spores. Appearing over the tree’s bark
  • Dead branches. Branches will appear dry and break easily.
  • Bark abnormalities. Deep cracks, splits or holes will appear in the bark. 
  • Soft, crumbly wood.Tree may effectively be decaying from the inside out. 


There are a wide variety of diseases and each species of tree is susceptible to a disease to varying degrees. Not all known tree diseases affect trees in the UK, but here are some of the most common trees diseases and their symptoms to watch out for:



1) Ash Dieback


What is ash dieback?

Ash dieback is caused by the Chalara fraxinea fungus which originated in Asia. While it doesn’t cause an issue on trees in its native countries, it has had a devastating effect on the UK’s ash species as they do not have a natural defence against it. Ash dieback can affect ash trees of all ages and younger trees will generally die off quicker, although all trees will display symptoms.


What are the symptoms of ash dieback?

  • Leaves develop dark patches in spring/early summer which then wilt and turn black.
  • Leaves may shed early.
  • Dieback of the shoots and leaves is visible in the summer.
  • Thinning of the crown (top part of tree).
  • Lesions may appear on stems and branches.
  • Inner bark looks brownish-grey under the lesions.
  • New growth from previously dormant buds further down the trunk. 


Control and treatment of Ash Dieback

Ash Dieback disease has no effective treatment but you can slow the spread of the disease by removing infected branches and gathering up and burning, burying or composting them.


2) Acute oak decline (AOD)


What is Acute Oak Decline?

Acute Oak Decline (AOD) is caused by a bacteria which can cause stress to a tree and kill it within five years. The UK has two native species of oak, both of which can be affected by AOD. Additional environmental stresses like soil conditions, pollution, drought can make oaks more susceptible to bacteria and push them into decline. The worst affected trees appear to be mature oaks over 50 years.


What are the symptoms of Acute Oak Decline?

  • Noticeable thinning of the crown. 
  • Dark liquid coming through cracks in the bark.
  • Stem bleeds that can stop and heal as the tree tries to recover.
  • As the tree becomes stressed, an increase in pest insects may become apparent.


Control and treatment of Acute Oak Decline 

There is no definitive treatment for AOD but management of affected trees is considered important both for safety reasons and to prevent spread. 


3) Dothistroma needle blight


What is Dothistroma needle blight?

Dothistroma needle blight is a fungus which pine trees to prematurely lose their needles. It is particularly troublesome to Scots pines which can lose their needle and then die. The  symptoms of Dothistroma needle blight are most visible between June and July.


What are the symptoms of Dothistroma needle blight?

  • Needles of pine trees developing yellow spots which later turn red.
  • Older needles are usually the most affected.
  • The needles might have an overall brown or reddish colour
  • Infected needles are shed within a few weeks of the tree being affected.
  • Branches have a ‘lion’s tail’ look with only the most recent needle growth remaining at the end.


Control and treatment of Dothistroma needle blight

Dothistroma needle blight can be controlled, but not cured, with fungicide treatments. The aim of spraying the tree is to break the cycle of infection in new needles. Multiple seasons of treatment are required before any noticeable results are seen.


4) Dutch elm disease


What is Dutch elm disease?

Ask anyone to name a tree disease and Dutch elm disease is likely to be the one they mention. The disease was accidentally introduced to the UK from the USA in the late 1960s on imported elm logs and has been responsible for killing millions of elm trees in the UK. Dutch elm disease is caused by the fungus Ophiostoma novo-ulmi which is spread by the elm bark beetle. It takes its name from researchers in the Netherlands who carried out research on the disease in the 1920s.


What are the symptoms of Dutch elm disease?

  • Clusters of yellow leaves that wilt or turn yellow in the summer months and then brown and fall from the tree.
  • Shoots that die back from the tip and and curl into a shape resembling a shepherd’s crook.
  • Dark brown staining underneath the bark of twigs, or dark spots and rings in the cross-section.


Control and treatment of Dutch elm disease

Dutch elm disease cannot be treated, but in some situations, injecting trees with fungicides can be an effective preventative measure for the management of Dutch elm disease (DED). Dead elm trees can be a safety hazard so should be properly felled.



5) Great spruce bark beetle


What is the great spruce bark beetle?

The great spruce bark beetle is a large beetle that damages spruce trees. The adults can be up to 8mm long and have a life cycle of up to 18 months. The pest was accidentally introduced into the UK in the early 1980s and inhabits spruce trees by tunnelling into the bark and then laying eggs which then hatch and continue the cycle of destruction. 


Symptoms of the great spruce bark beetle;

  • Poor tree health can be a sign of a beetle infestation in the tree. 
  • Browning foliage over some or all of the crown can be present.
  • Entry of female beetles into the bark of trees leads to resin bleeds and ‘resin tubes’ on the trunk as the tree tries to protect itself.
  • Feeding galleries, where the larvae bore tunnels, often occur beneath the bark. 
  • In heavily infested trees the bark may fall away and expose these feeding galleries.
  • Exit holes made by adult beetles may be present on the tree.
  • Powdery wood debris (also known as ‘frass’) can collect at the base of a tree.


Control and treatment of great spruce bark beetle

It is not possible to eradicate great spruce bark beetle through chemical means although management should involve the destruction of infested trees. A predatory beetle (Rhizophagus grandis) that feeds on the spruce bark beetle has been used as a method of biological control in some instances.



6) Horse chestnut bleeding canker


What is horse chestnut bleeding canker?

Horse chestnut bleeding canker is a bacterial infection within the water transport systems of the tree just under the bark. Over time, the bacteria causes a blockage, meaning that the tree can no longer take up water, so part of or even the whole tree can die. 


Symptoms of horse chestnut bleeding canker

  • Cracks in the bark start appearing.
  • A dark bacterial sticky substance oozes out of those cracks. 
  • This substance can dry out in winter leaving a rusty brown or black deposit.
  • Wood becomes discoloured under the bark, having patches of brown or purple (as opposed to health wood which should be white or pinkish in colour.)
  • On older cankers, the dead bark might fall away to expose the wood.


Control and treatment of horse chestnut bleeding canker

No direct treatment is available for horse chestnut bleeding canker, although there is hope that research may lead to a solution. Affected trees may have to be felled for safety reasons.



7) Horse chestnut leaf miner


What is the horse chestnut leaf miner?

The larvae of the moth Cameraria ohridella bores within horse chestnut leaves, eventually causing them to drop and harming the tree, though not necessarily killing it. The adult female moths lay up to 180 eggs on newly opened leaves, and the hatched larvae feed on the leaves.

The larvae can then pupate and overwinter in the fallen leaves until they emerge as adults in early spring to lay eggs on fresh leaves on the tree.


Symptoms of horse chestnut leaf miner

  • Tree will appear like it has turned autumnal very early
  • Tracks will appear on the leaves left by the feeding larvae.
  • Leaves will appear dry, crisp and brown leaves.


Control and treatment of horse chestnut leaf miner

It is advised to clear away fallen leaves in autumn and burn them to halt the pest. As the miner itself is not likely to cause death of a tree, there is no need to fell affected trees, and the tree can quickly recover the following spring. A number of insecticides have had some effect in controlling the miner, but their use is limited in built up areas..



8) Oak processionary moth


What is the oak processionary moth?

The oak processionary moth lives and feeds almost exclusively on oak trees. The caterpillars will munch their way through the leaves of the oak tree, leaving the tree vulnerable. There is a risk associated with the caterpillars, whose small hairs can cause health risks in humans, namely rashes and breathing difficulties.


Symptoms of oak processionary moth?

  • A procession of caterpillars on the tree will be apparent late spring and early summer. 
  • Nests of caterpillars, made of silken white webbing will appear on trunks and larger branches of oak trees. 
  • Dislodged nests may appear on the ground near oak trees.


Control and treatment of oak processionary moths

To minimise health risks you should not touch the caterpillars or their nests and should only call in professionals to deal with the infestation. Controlled treatment with an approved insecticide or bio-pesticide can be used in the spring to kill the caterpillars soon after they emerge.



9) Phytophthora ramorum


What is Phytophthora ramorum?

Phytophthora ramorum is a microscopic fungal-like organism that causes the death of a wide range of tree species, though its impact so far has been greatest on larch plantations in the UK, hence it is also known as ‘larch tree disease’. The disease can progress very quickly so whole trees will be dead within a short period of time.


Symptoms of Phytophthora ramorum;

  • Crown and branch dieback, with yellowing or ginger colour beneath the bark.
  • Withered shoot tips with blackened needles.
  • Lesions which ooze resinous fluid from infected bark.
  • Fluid can dry to a crust on the trunk.
  • Premature shedding of needles. 


Control and treatment of Phytophthora ramorum

Trees affected by Phytophthora ramorum need to be carefully controlled, and this involves the infected trees being felled and disposed of, usually by burning.



10) Sweet chestnut blight


What is sweet chestnut blight?

Sweet chestnut blight is a fungus that attacks the bark of chestnut trees. The fungus infects the trees through wounds and then grows underneath the bark. If left untreated, the infection will spread around the width of the tree and kill it.


Symptoms of sweet chestnut blight

  • Orange substance between cracks in the bark.
  • Individual branches wilting.
  • Brown wilted leaves that remain even when other leaves have fallen.
  • Cankered bark can become bright brown on younger branches instead of its usual greenish colour.


Control and treatment of sweet chestnut blight

A diseased or stressed tree could be a danger so will often be recommended for felling and burning on site. If you suspect that chestnut blight is present in your garden you should not attempt to control the disease yourself but instead contact a professional service.



If you need help with tree disease

If you think your trees are infected with any of the diseases described above or are showing signs of distress then get in touch. You cannot effectively treat a diseased tree until the disease of pest that is affecting it has properly been identified.

Even then, treatment is not always possible, although fungicides can work on some trees and shrubs. Other methods, such as pruning, fertilisation or changing watering habits can sometimes help to reduce disease.

It is  always best to seek the advice of a professional who can help identify the disease and advise on the next steps, whether that involves treatment or even felling of the tree.