Tree Damage Alert No 92
Horse chestnut bleeding canker
Bleeding canker of Horse chestnut (Aesculus hippocastanum) was first recorded in the UK in the 1970s (Brasier and Strouts, 1976) and has become increasingly common over the past 4 or 5 years. We know relatively little about this disease, other than it is caused by two species of Phytophthora – P. cactorum and P. citricola. The process of infection is also unknown.
Symptoms
Horse chestnut trees have cankers, which initially may be apparent as a crack in the bark on the stem and occasionally on large branches. These ooze rusty-red, yellow-brown or almost black gummy liquid which dries forming a dark coloured, brittle crust. Over several years the fungus can extend through the phloem and cambium of the tree and may even girdle the entire stem or branch. Large or numerous lesions are often associated with extensive yellowing and thinning of the crown, and eventually crown death. In some cases, parts of the crown often associated with individual branches, will fail to flush, and later in the year the remaining foliage withers and dies
Infection and Diagnosis
Infection and disease development are very unclear. Warm and wet conditions may optimise the risk of infection as Phytophthora spores tend to be dispersed in rain splash and via mist.
Confirming the diagnosis of horse chestnut bleeding canker can be difficult. Bleeding canker is distinct from Phytophthora root disease; here tree stems are infected by the fungus growing up from infected roots or root collars. In contrast, stem or branch lesions giving rise to bleeding cankers are not connected to root infections, suggesting that the Phytophthora species involved can cause direct infection of bark, in the absence of any wound or injury.
Confusion with other diseases - The symptoms on Horse chestnuts can look very similar to stem bleeding due to infection by Phytophthora ramorum. This causes Sudden Oak Death in the USA. Despite the similarities, bleeding canker of Horse chestnut is caused by different species of Phytophthora. Unless a horse chestnut with bleeding canker is near to a source of P. ramorum disease such as infected rhododendrons, the chance of it being caused by P. ramorum is remote.
Confusingly, mycelium and fruit bodies of other fungi (e.g. Armillaria species) may also be present. These result from secondary infection.
Why the bleeding increase? - We don’t know why there has been such a rise in the reported incidence of Horse chestnut bleeding canker in the last few years. The trend towards milder winters and wet springs might increase the likelihood of infection. Until recently, the disease was considered to be uncommon and had only been reported from the south of England (Strouts and Winter, 2000). Reports from 2003 indicate it has spread as far north as Glasgow. Trees of all ages have been found with the disease, but the impact is most striking on large, mature trees. Apart from Horse chestnut, bleeding cankers associated with P. citricola and P. cactorum have also been recorded on Limes (Tilia species) although, interestingly, there has not been a corresponding increase in the number of reported cases.
Can we control the disease? - There are no fully effective control measures although Strouts and Winter (2000) report that if the cankers are small, it may be possible to excise the infection by cutting out all the dead and necrotic bark, and then treating the wound with a paint based on a fungicide approved for use against Phytophthora. They recommend sterilising the cutting blade and removing a strip of bark at least 5cm wide from around the periphery of the canker. All excised bark should then be collected and burned
Authors: - Joan Webber, and Kath Thorpe, Forest Research, Alice Holt Lodge, Wrecclesham, Farnham GU10 4LH
References
Brasier, C. M. and Strouts, R.G. (1976). New records of Phytophthora on trees in Britain. Phytophthora root rot and bleeding canker of horse chestnut (Aesculus hippocastanum L.). European Journal of Forest Pathology 6, 129-136.
Brasier, C.M. et al (in press). First report of Ramorum bleeding canker on Quercus falcata caused by Phytophthora ramorum. Plant Pathology
Strouts, R. G. and Winter, T. G. (2000). Diagnosis of ill-health in trees. The Stationery Office, Norwich.
.
This is one in an occasional series of Tree Damage Alerts produced for the benefit of the arboricultural profession and issued by the Arboricultural Advisory and Information Service.