Q What is horse chestnut bleeding canker?

A It is a potentially fatal disease of horse chestnuts that has become much more prevalent in the last few years.

Caption: Bleeding canker is most obvious in spring and autumn

Q How do I recognise horse chestnut bleeding canker?

A Areas of weeping or bleeding occur on the trunk at various heights and sometimes on the main branches. This starts in spring, when the liquid is dark and gummy but clear. Bleeding increases as the weather warms up, the liquid turning cloudy and amber-coloured. It tends to dry out to a hard crust in summer, then more liquid appears in autumn. The bark in affected areas darkens and the wood below dies, creating a canker. This dead tissue may then be invaded by wood-rotting fungi and, in due course, these can produce fruiting bodies in the form of brackets or nodules. If the cankers join up to girdle the trunk, or a branch, then the area above the girdle will die. Well-established infections can produce other symptoms including late flushing of the leaves in spring, premature leaf fall in autumn and dieback. However, these symptoms can have other causes so, on their own, do not indicate bleeding canker.

Q What causes horse chestnut bleeding canker?

A Horse chestnut bleeding canker was first noticed in the 1970s, but was uncommon and confined to the south of England. The cause was two species of fungus, Phytophthora cactorum and P. citricola. These are from the genus responsible for many plant diseases, including sudden oak death (P. ramorum) and potato blight (P. infestans). At the turn of the century, a new form of the disease appeared, caused by a bacterium (Pseudomonas syringae pv aesculi). This is responsible for the recent epidemic.

Q Where does horse chestnut bleeding canker occur?

A The disease was first noticed in 2000. Early reports were widely scattered in England but mostly in the south. It has since spread rapidly through most of Britain. A survey by the Forestry Commission in 2007 found 49% of the horse chestnuts assessed were affected to some extent, though this was worse in the south-east with up to three-quarters of trees affected. The Forestry Commission estimates that 35,000 to 50,000 trees are affected countrywide. The disease is now also widespread in Belgium, France, Germany and the Netherlands.

Q What plants does horse chestnut bleeding canker attack?

A The bacterial form of the disease only attacks horse chestnuts, and has been found on trees of all ages. The less-serious fungal form also attacks birch, maple (including sycamore), oak and willow.

Q What damage does horse chestnut bleeding canker do?

A By girdling the bark the cankers can kill affected trees and, because they are smaller, young trees succumb more quickly. Older trees may be weakened but can survive for many years and sometime their own defence mechanisms can halt the spread of the disease, though damaged areas will not recover.

Q Could I mistake horse chestnut bleeding canker for any thing else?

A The substantial bleeding typical of this disease does not occur with any other horse chestnut problems. 

Q How serious is horse chestnut bleeding canker?

A Horse chestnuts are not native to Britain, but were introduced in the 16th century — they originate in the Balkans. They rarely occur in woodland in any numbers, and the timber is weak so of little value. However they are of significant ornamental value in parks and gardens and as street trees. Here, and in landscape features, such as the double avenue of horse chestnuts leading up to the prehistoric stone circle at Avebury, their loss would be significant.

Q Why has horse chestnut bleeding canker become such a problem?

A Research continues, but it appears that this is a newly evolved bacterium, which appears to be very infectious and which trees have little resistance to.

Q When do horse chestnut bleeding canker attacks occur?

A Bleeding is most obvious in spring and autumn, indicating that the bacterium is most active in mild, moist weather. However it is likely that infection can spread at any time.

Q How do I treat trees affected by horse chestnut bleeding canker?

A There is no treatment that can cure or halt the spread of the disease. Dealing with affected trees must depend on the extent of the damage, the likelihood that that the tree may become a hazard, and the number of horse chestnut trees in the vicinity. Generally the less interference with the tree the better. There is no point trying to cut out the cankers, as you might with fruit trees, as this will not eliminate them and could further damage the tree as well as releasing more bacteria to affect other trees. However, if cankers girdle and kill a branch it should be removed as horse-chestnut branches can dry out rapidly and fall without warning. The dead wood should be burned. If an affected tree is likely to present a hazard by falling prematurely or dropping branches, then professional advice should be sought to ascertain the level of risk. If a few trees among many become infected there may be an argument for removing them to protect the unaffected trees. However as so little is known about how the disease spreads, there is no certainty that this would be effective.

Q How do I find professional help for horse chestnut bleeding canker?

A The Tree Advice Trust has a helpline, which can be a useful first port of call. It costs £1.50 per minute (09065 161147). If you need someone to visit, try Which? Trusted Traders. Otherwise contact the Arboricultural Association (trees.org. uk, 01242 522152), which produces lists of approved consultants to give advice and contractors to do practical work.

Q Are there any varieties resistant to horse chestnut bleeding canker?

A So far no resistant species or varieties have been identified. Common horse chestnut (Aesculus hippocastanum), red horse chestnut (A. x carnea) and several other species, including A. flava and A. indica, are known to be susceptible, while the double-flowered variety A. hippocastanum 'Baumanii' appears to be particularly vulnerable.