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.
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 fruiting body of the fungus is visible, by which time it may be too late to act. Honey fungus (Armillaria spp.) is an example of a white rot fungus which 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, whereby the fungi obtain energy from the tree sugars made during photosynthesis and 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, along with 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 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 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.
Chalara 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 which 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
In the meantime, the RFS urges members to follow the advice below:
- Frequently inspect any ash trees in your care, and especially any which 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.
- Ash dieback advisory note (FC website)
- Forestry Commission England grants (scroll to 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 in 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 www.forestresearch.gov.uk