Bleeding Canker in Horse Chestnuts

There has been much written the press recently about the threat to our Horse Chestnut trees from bleeding canker. The following short article provides an overview of the disease and the limited control methods applicable.

Bleeding canker of Horse chestnut (Aesculus hippocastanum) has been occurring in the UK since the 1970s, and has become increasingly common in the past 4 or 5 years.

Relatively little is known about this disease, other than it is caused by two distinct species of Phytophthora – P. cactorum and P. citricola. The process of infection is unclear.

Horse Chestnut trees, which are infected, will display cankers. These may appear as a cracks in the bark or on the stem or possibly on the larger branches. The cankers ooze a dark almost black gummy liquid that dries as a crust. Over several years the fungus can extend through the phloem and cambium layer of the tree and in some cases can encircle the branch or stem of the plant.

Seriously infected trees will show extensive thinning of the crown and multiple lesions will be apparent. This will invariably lead to crown death, or ‘stag heading’.

The method by which this fungus spreads is not clear. Wet, warm weather seem to be providing the optimum conditions, as Phytophthora spores are known to be dispersed in rain splash and via mist.
Diagnosis of horse chestnut bleeding canker can be problematic. Bleeding canker is distinct from Phytophthora root disease. In these cases, tree stems are infected by fungus growing up from infected roots and/or root collars. Stem or branch lesions giving rise to bleeding cankers are unconnected to root infections, suggesting that the Phytophthora species involved causes direct infection of bark.

Confusingly, the symptoms that manifest on Horse Chestnuts can appear very similar to stem bleeding due to infection by Phytophthora ramorum. This is the cause of Sudden Oak Death in the USA, and more recently found in Britain. Although similar, bleeding canker of Horse Chestnut is caused by different species of Phytophthora. It is unlikely that Horse Chestnuts with bleeding canker have been infected P. ramorum unless an already infected species is nearby.

The reasons for the rise in reports of Horse Chestnut bleeding canker over the last few years cannot be determined. Milder winters and wet springs may have increased the likelihood of infection. Until recently, the disease was considered to be uncommon and had only been reported from the south of England. Recent reports from 2003 onwards would indicate it has spread north into Scotland. Trees of all ages are affected, but it is on large, mature trees the effects are most obvious. 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.

There are no fully effective control measures, although 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 paint based on a fungicide approved for use against Phytophthora. The recommended method is to use a sterilised cutting blade to remove a strip of bark at least 5cm wide from around the periphery of the canker. All the excised bark should then be collected and burned.