Added on 02/01/2012Pest/Disease

Bleeding Canker of Horse Chestnut


Possibly Pseudomonas syringae pv. aesculi



This recently identified and serious disease affects species of horse chestnut (Aesculus spp.).
Phytopthora Bleeding Canker was first reported on horse chestnut in the UK in the 1970s. It is caused by Phytopthora spp. - serious but generally uncommon fungal pathogens that infect deciduous tree species - particularly horse and sweet chestnut, alder and lime. Infections start at bark wounds and cause a black tarry ooze from the bark. Deep red, dead areas of inner bark are seen when outer bark is cut away. P. cambivora and P. citricola are found in horse chestnut and others. P. alni (common and serious) is found only on alder. All cause bark death, crown thinning and dieback.
Over the last 5 years, the incidence of bleeding cankers on stem and branches of Aesculus hippocastanum has risen markedly in the UK, the Netherlands, France and Germany.
Laboratory analysis has shown that fungal Phytopthora is present in only 5-10% of cases, and so is no longer the primary causal agent. Evidence is increasingly implicating a bacterial pathogen, possibly Pseudomonas syringae pv. aesculi, for the increase in these symptoms on horse chestnut. This may act either singly, or in combination with fungi that have also been found in the cankers.



Rusty-red to yellow-brown and black patches appear on the trunk. In the spring a dark and transparent sticky liquid oozes from the patches, changing to rusty-coloured and cloudy during the season. The inner bark (phloem) is usually dead or dying, with an orange-brown mottled or striated colour. As the disease advances, cankers can girdle the trunk, causing crown thinning and dieback.
Infection is thought to start at existing wounds in the bark, but little is currently known about the exact transmission or infection process.
Large trees can be killed where the disease is severe and areas of killed bark are extensive. Younger trees (10-30 years old) can be killed in only 3-5 years as their trunks can be girdled more quickly, and so are at greater risk.
In the UK the disease is now very widespread and the Forestry Commission estimate that up to 50,000 trees are affected, with probably a few thousand already felled.
The disease affects almost all ages of tree, from young trees with a DBH of only 10cm to mature specimen trees, including many highly visible trees in parks, gardens and avenues. The disease has struck the prominent double avenue of horse chestnuts leading to the prehistoric Avebury stone circle.
Research has shown about 50% of the horse chestnuts studied in Hampshire to be affected with bleeding canker (Straw and Green, unpublished data), with a higher proportion in amenity situations than in woodlands. Slightly more red horse chestnuts (A. x carnea) were affected than the white A. hippocastanum.



There is no chemical treatment available to control or stop development of the canker.
Where a significant number of horse chestnuts are affected, a site survey to assess the extent of the problem is the first course of action. See Forest Research guidance.
Trees will sometimes recover from the disease, so trees with minor symptoms should initially be monitored.
Horse chestnuts are prone to sudden limb drop when under stress or as the wood dries out, so remove major dying branches and those with significant cankers.
If the entire trunk is girdled by the cankers the tree will die, and should be removed. The best option is to burn or bury infected waste on site, to minimise spread to other trees or sites. Chipping of brash is not recommended, as this may further spread the pathogen through the air. Logs harvested from the tree fro firewood should be stored away from other trees. Tools and footware should also be sterilised after use.
Do not replant the site with Aesculus spp. as these are likely to also be infected within a few years.


Text by Ian Monger November 2007. Photos by Chris Skellern.