The Forestry Commission today published the results of a survey that indicates the extent of symptoms of bleeding canker of horse chestnut trees throughout Great Britain.

The condition has been affecting increasing numbers of horse chestnut trees in recent years. Bleeding cankers are lesions that ooze or "bleed" liquid from the trunks or branches of trees. In the case of horse chestnut, some trees eventually die, some recover, and others have to be felled for safety reasons because the condition weakens the trunks or branches until they are in danger of falling.

For several decades small numbers of horse chestnut trees have been known to suffer from bleeding cankers caused by one or the other of the fungus-like organisms Phytophthora citricola or Phytophthora cactorum. Infection rates stayed at a low level and therefore did not cause concern.

However, from about four or five years ago the Forestry Commission detected a steep increase in the number of reports of bleeding canker. In the vast majority of cases analysed since then, scientists at the Commission's Forest Research agency have not found Phytophthora organisms to be the causal agent. Instead, it gradually became clear that in many samples taken from the affected trees another organism, eventually identified as the bacterium Pseudomonas syringae pathovar aesculi, was causing the canker symptoms.

Responding to the concerns of the public and tree owners, and those of woodland managers worried that the infection might spread to other species, the Commission conducted two research projects during 2007.

The first gathered information from land owners, land managers, local authorities and tree-related organisations to ascertain the approximate extent and location of horse chestnut trees across Great Britain. Then, informed by this data, the Commission inspected sample groups of horse chestnut trees in all regions of Britain to estimate the incidence and distribution of symptoms of bleeding canker.

For this survey, the researchers divided Britain into 11 regions (Scotland, Wales and the nine RDA government regions of England) and inspected samples of horse chestnut trees in all 11 regions. From this survey it was learned that:

• trees with symptoms of bleeding canker were found in all 11 regions;
• an average of 49 per cent of the trees inspected displayed symptoms that indicate or might indicate bleeding canker. Symptoms found included cankers themselves, dieback of entire branches, and other features, such as cracks in trunks and branches which can appear after the bark has been killed by the organism that causes bleeding canker; and
• there were regional variations in the proportion of trees inspected that displayed symptoms. The south east of England had the highest levels of symptomatic trees (more than 70% of those inspected), but no region had less than 30%. In Scotland the proportion of inspected trees affected was 42%, and in Wales it was 36%.

Announcing the survey report's publication, Roddie Burgess, head of the Forestry Commission's plant health service, said,

"Horse chestnut trees are a much-loved feature of the British landscape, and we want to be able to offer management advice to tree owners to help them minimise the impact of this condition. We also need to learn as much as we can about the extent of the risk to horse chestnut and, if any, to other tree species.

"The information that we've obtained from these surveys takes us a step further towards meeting these objectives. It gives us a scientifically sound picture of the extent of the symptoms, and a basis upon which we can make decisions about what further research is needed and what steps might be needed to manage it.

"However, we do not yet have the complete picture. For example, we don't yet know the total number of horse chestnut trees in Britain affected by bleeding canker; or how many of the symptoms seen in the survey will eventually prove to have been caused by bleeding canker, which can only be learned by observation over time. Nor do we yet know exactly what proportion of the cankers is caused by each of the three known canker-causing pathogens: Phytophthora cactorum, Phytophthora citricola and Pseudomonas syringae pathovar aesculi. However, we suspect the majority are likely to be caused by Pseudomonas .

"We will therefore carefully consider the results of the survey and their implications before deciding what further steps we can take."

However, Mr Burgess added:

"There is a limit to what can be done to combat diseases of trees. The key things are to encourage tree owners and woodland managers to practise good woodland management and tree care to ensure maximum tree health. The healthier a tree is, the better it is able to withstand attacks by pests and diseases.

"Therefore the main thrust of our work is likely to be to gain as much understanding of the condition as possible so that we can provide appropriate advice to owners on how to manage affected trees and how to control the spread of the organisms that are causing the problem."

Mr Burgess said the Commission had received anecdotal evidence, which it had not yet been able to assess scientifically, that some uninfected horse chestnut trees have been found very close to others that have been heavily infected for some time. He added,

"This suggests the possibility that some of our horse chestnut trees have a degree of genetically inherited resistance to the pathogens that cause bleeding canker, and therefore their seed could form the basis of resistant stock for future generations of the species."

The full report and a Question and Answer factsheet are available for downloading from the Forestry Commission's website at www.forestry.gov.uk/planthealth. Further scientific information about bleeding canker of horse chestnut is available on the Forest Research website, www.forestresearch.gov.uk.

NOTES TO EDITORS:
1. Horse chestnut (Aesculus hippocastanum) is a native of the Balkans region of south-eastern Europe. It is believed to have been introduced to Great Britain in the 1500s. Exactly how many there are in Great Britain is unknown - there are approximately half a million in woodland situations, plus an unknown number in other situations such as streets, parks and gardens.
2. It is estimated that about 3000 horse chestnut trees with bleeding canker have had to be felled for public safety or similar reasons in recent years because the condition has weakened trunks or branches to a point where they have been in danger of falling.
3. The Forestry Commission is the government department for forestry in Great Britain, with responsibility for protecting the health of forest trees and timber. For further information, visit www.forestry.gov.uk.
4. Forest Research is the agency of the Forestry Commission that conducts world-class scientific research and technical development relevant to forestry for internal and external clients. For further information, visit www.forestresearch.gov.uk.

MEDIA CONTACTS:

The news media are invited to consult the Question & Answer factsheet on bleeding canker of horse chestnut on our website at www.forestry.gov.uk/planthealth, which should answer most questions. Answers from the website may be quoted by the news media and attributed to "a Forestry Commission spokesman". Otherwise, the contacts are:

• for media circulating or broadcasting throughout Great Britain, contact Charlton Clark, 0131 314 6500;
• for media circulating or broadcasting solely in England - Stuart Burgess 01223 346027 / 07785 748351 or Jo Fowler, 01223 346034 / 07833 672903;
• for media circulating or broadcasting solely in Wales - Clive Davies, 0845 604 0845;
• for media circulating or broadcasting solely in Scotland - Paul Munro, 0131 314 6507.