Bleeding Canker of Horse Chestnut is a common canker of horse chestnut trees (Aesculus hippocastanum, also known as conker trees) that is known to be caused by infection with several different pathogens.
Infections by the gram-negative fluorescent bacterium Pseudomonas syringae pathovar aesculi are a new phenomenon, and have caused most of the bleeding cankers on horse chestnut that are now frequently seen in Britain.[1]
Contents |
In the past few years, the bacterial pathogen Pseudomonas syringae pv. aesculi has a emerged as a new and virulent agent for this disease in Western Europe. Specific to horse chestnut trees, this pathogen infects the bark (cambium) around the trunk and main branches. As it spreads, it cuts off the water supply to the crown; and when it completely encircles the trunk, the tree will die.[1]
This particular infective agent emerged in the past few years, and has now spread rapidly to infect many trees in Western Europe.[2]
Initially the outbreak was attributed to Phytophthora, until DNA tests suggested that a pathovar of Pseudomonas syringae was responsible; and this hypothesis was confirmed in 2007 with tests satisfying Koch's postulates.[1] [3]
The disease has risen markedly in the UK since 2003,[3] and now approximately one half of all horse chestnuts in Great Britain are affected and showing symptoms to some degree.[2] The disease is spreading at an alarming rate in the Netherlands,[4] where one third of all horse chestnuts are affected to a greater or lesser extent.[5] A similar upsurge is reported in Belgium and France.[3]
Some bleeding cankers on horse chestnuts are caused by fungal pathogens including principally the fungus Phytophthora.[6]
Phytophthora was responsible for mass die-off of horse chestnut trees in the 1940s worldwide, and is still the principal agent of bleeding canker on horse chestnuts in North America. It was considered uncommon in the United Kingdom and only recorded in southern England,[3] but now accounts for 5-10% of bleeding canker on horse chestnuts in Britain.[1]
The first symptom is a sticky liquid oozing from blemishes on the bark of infected trunks. Later, the bark peels away, exposing a characteristically brown-orange stained inner bark below, and the whole tree shows a yellowing of foliage and premature leaf drop. Eventually the crown dies.[1]
Progression is slow, but younger trees can succumb to the disease in just a few years as the smaller diameter of their trunks means that they can be girdled more quickly.[7]
If the outer bark is cut away at the site of a bleeding canker, the discoloured and dying inner bark (which may be purple or brown or orange in colour) often appears mottled or zoned. A well-defined edge between the discoloured inner bark and the white or pinkish healthy areas suggests an infection that has stabilised; a diffuse boundary indicates that the infection is spreading.[8]
There is as yet no tried and tested treatment, and cutting out the infected branches may even be counter-productive. In some cases the disease will stop spreading and the tree will recover—and it is for this reason that experts recommend that infected trees should not be felled at first signs of infection.[9],[10]
In many cases the disease will progress and will eventually kill the tree, or weaken it to the point that it becomes dangerous and has to be felled.
An injection treatment using the garlic extract allicin is currently being trialed.[11]