Tree Pests and Diseases

How “natural” are our woodlands?

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. When the ecosystem is in balance, insects or animals feeding on trees isn’t a problem.

The problem is, woodlands in the UK today are no longer completely “natural”. The grey squirrel, for example, shouldn’t live in our woodlands. It was imported from North America over 100 years ago and is responsible for serious damage to hundreds of thousands of trees every year. The introduction of animals, plants or disease from abroad can wipe out entire species of trees or areas of woodland if we don’t take action.

Oak Processionary Moth caterpillars

What’s causing the problem?


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.


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.


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.

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’.

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.

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  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. 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.

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.

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