Tree Pests and Diseases

A great deal of our British wildlife relies on trees for a place to live or feed. Trees provide homes for many insects, birds and mammals within their roots, bark and branches. During the lifespan of a tree – which may be many hundreds of years – some species can cause problems. However, these tend to be alien species (those that are not native to our environment) or those whose populations may be unnaturally high.


Some species of insect have leaf-eating larvae capable of completely defoliating a tree; the caterpillars of the oak processionary moth are a good example. Native to southern Europe, it has been found in recent years in southern parts of the UK. The caterpillars can completely strip the leaves from an oak tree, but usually, the tree is able to recover the following year. The hairs of these caterpillars are toxic to humans and can cause significant human health problems. Do not handle them or disturb their nests.

Members can visit the QJF for more information on pests and specific information and advice on the oak processionary moth.


Gall wasp larvae can cause problems for trees, particularly oaks. Tiny gall wasps lay eggs on bark, leaves, fruits or flowers (depending on the wasp species), causing plant tissue to swell up and form a gall. The gall provides the larvae with protection from the elements and a nutritious food supply. The wasp Biorhiza pallid lays its eggs on oak leaves, forming spherical, knobbly growths called ‘apple galls’, which cause little lasting damage. Others, such as the wasp Andricus quercuscalicis, can destroy a whole seed crop by causing distorted ‘knopper galls’ to grow on acorns.

Grey squirrels

The common grey squirrel was introduced from North America over 100 years ago, and they are now found throughout much of Britain. Grey squirrels have developed the habit of stripping bark from broadleaved trees, and in some cases, they have been known to kill mature trees. For this reason, grey squirrels are humanely controlled in many woods to keep damage to an acceptable level.

More on grey squirrels can be viewed in the QJF Eastern grey squirrel and management and red squirrel.


Where population density is high, deer can have a serious impact on trees and woodlands. Deer can damage trees by browsing them, stripping the bark or fraying the trees with their antlers. Over the long term, this can prevent tree regeneration, change the canopy structure and alter the characteristic ground flora of ancient woodland.

In Britain, our two native species of deer have large populations: 350,000 red deer and 800,000 roe deer. In addition, we have the long-established fallow deer and three other recently introduced species: the muntjac, sika and Chinese water deer, which add a further 400,000 to these figures. The Government has reported that deer numbers are now higher than they have been for 1000 years and therefore support sustainable management, including humane control.

Become a member to view more on deer management in woodlands and the impacts of deer grazing on coppicing in our QJF, or visit the home page of the Deer Initiative.

Other mammals

Other mammals such as field voles and rabbits may also damage trees and saplings by gnawing bark around the stem. This can happen when trees are located in fields or orchards and surrounded by long grass.

Invasive plants

Some invasive plant species can cause problems for trees; the most notable of these is Rhododendron ponticum. Dense thickets of rhododendron can prevent the regeneration of young trees and, in woodland, only those trees which manage to grow above the level of rhododendron can survive. Rhododendron is also host to the disease organism Phytophthora ramorum, which has been implicated as a causal agent in ‘sudden oak death’.

For more information, visit the QJF invasive species article, or if you are concerned about an invasive species, please also visit



Like all living things, trees are susceptible to disease. A tree needs a good supply of light, water, carbon dioxide and nutrients from the environment for optimum growth. A lack of one or more of these may lead to reduced growth and put the tree under stress. If a tree is stressed, then it may not have the energy required to manufacture important defences and can become vulnerable to disease.

If a diseased tree eventually dies, it can sometimes be difficult to diagnose the original cause because problems are often complex and cumulative. For example, a tree could be weakened by drought and then become the victim of a fungal attack; environmental factors such as temperature, storm wounds or pollution may play a part; or sometimes more than one disease may be present.

Tree disease can stem from fungal, bacterial or viral sources.

Fungal decay

The main types of fungal decay are brown rots and white rots.

Brown rots will attack the cellulose and hemicellulose in wood, leaving only the lignin. The decayed wood becomes brown and cracked in a brick-like form, timber value is lost, and the tree may become brittle and unstable.

White rots attack all parts of the wood, turning it into a pale spongy mass.

Fungal diseases are often only detected once the fungus’s fruiting body is visible, by which time it may be too late to act. Honey fungus (Armillaria spp.) is an example of a white rot fungus that causes the roots and butts of live trees to rot.

However, it is important to note that not all fungi damage trees. Many types of fungi enjoy a mutually beneficial (symbiotic) relationship with trees. The fungi obtain energy from the tree sugars made during photosynthesis. The tree benefits from the absorption of additional nitrogen and phosphorous due to the fungal action in the soil.

Bacterial and viral infections

Various kinds of bacteria can also cause disease in trees. Oak decline is a complex disorder or syndrome in which bacteria and other damaging agents such as insect infestation or weather damage interact to bring about a serious decline in tree condition. There are two kinds of decline: acute and chronic.

Acute oak decline affects mature oaks, and bacteria is thought to cause symptoms of stem bleeding where dark, sticky fluid oozes from cracks in the tree trunk. Both of Britain’s native oak species – pedunculate oak and sessile oak – are affected.

Chronic oak decline may take many years to kill a tree. Early symptoms include deterioration of the foliage; leaves may be smaller than normal, pale or yellowish. In some cases, the foliage may be sparse over the entire crown and the death of twigs and branches follow.

The two most important notifiable diseases in Britain today are sudden oak death caused by the pathogen Phytophthora ramorum and red band needle blight caused by the fungus Dothistroma septosporum, which affects Corsican pine.

Information on bacterial bleeding can be viewed in the QJF.

Ash dieback

Ash dieback is a serious disease of ash trees caused by a fungus called Chalara fraxinea (C. fraxinea). The disease causes stem lesions, leaf loss and crown dieback in affected trees and can lead to tree death.

C. fraxinea was confirmed for the first time in the UK in February 2012, when it was found in young trees imported from Europe to a Buckinghamshire nursery. Since then, it has been found at a number of sites that have received stocks of young ash plants from nurseries within the past five years.

In October 2012, scientists confirmed a small number of cases in East Anglia in mature ash trees not connected to the nursery stocks. An emergency survey of the countryside was undertaken, and infected mature trees were found in a number of counties. Forest scientists now believe that the disease has been spread by natural means such as spores being carried on the wind.

C. fraxinea is being treated as a quarantine pest under national emergency measures.

More can be found in the 2013 QJF

n the meantime, the RFS urges members to follow the advice below:

  • Frequently inspect any ash trees in your care, especially any that have been planted during the past five years.
  • Exercise good plant hygiene: biosecurity is paramount in preventing the spread of the disease. Clean and disinfect footwear, tools and vehicles when moving between sites.
  • Make yourself familiar with the symptoms of Chalara dieback using the pictorial guide and video guides on the FC’s Chalara website.
  • Report any suspected cases immediately to Forest Research.

For the most up to date details on the disease, how to identify it and report suspected cases, visit the Forest Research pages on Chalara.

See also:

  • Ash dieback advisory note (FC website).
  • Forestry Commission England grants (scroll to the end of this page to download details).

Sudden oak death

Sudden oak death has caused extensive damage to trees in parts of the USA and has occurred in parts of Europe and Britain. However, it appears that native British oaks are not as susceptible to the disease as American oaks. The non-native species Rhododendron is host to the fungus organism Phytophthora ramorum, which has been implicated as a causal agent in sudden oak death.

Phytophthora ramorum affected very few trees in the UK until 2009 when it was found to be infecting and killing large numbers of Japanese larch trees in South West England. In 2010, it was found on Japanese larches in Wales, Northern Ireland and the Republic of Ireland. This change in the pathogen’s behaviour was the first time in the world that it had infected large numbers of conifer tree species. To find out the latest developments, visit the Government’s Forest Research information pages.

Red band needle blight

Red band needle blight causes needle defoliation which, in severe cases, may kill trees. Over the past two decades, the incidence of this disease has increased dramatically in Britain. The increase could be due to a rise in rainfall during spring and summer and warmer spring temperatures which encourage spore dispersal and infection. Climate change may increase outbreaks if warming trends continue.

For more information, or if you are concerned about a tree disease, please visit the Government’s Forest Research website at