Trees which have been affected for some years may show crown symptoms (Photo 1).
However, the early symptoms tend to be limited to bleeding lesions; scattered drops of rusty-red, yellow-brown or almost black, gummy liquid ooze from small or large patches of dying bark on the stems or branches of infected horse chestnuts (Photos 2 and 3).
Bleeding patches may be associated with the base of the tree at the soil surface or may start higher up the trunk at about one metre, and then extend upwards. Early in the year (spring) the exudate from bleeding patches is a dark colour but transparent. However, as the weather becomes warmer, bleeding from infected tissues becomes more copious and runs some way down the tree. At this time it is often a conspicuous rusty-colour and no longer transparent but cloudy or opaque. Under dry conditions during the summer, this exudate dries to leave a dark, brittle crust near the point of exit in the bark (Photo 4).
Renewed bleeding can may be seen later in the year, often in autumn. This suggests that pathogen activity is greatest under moist, mild conditions of spring and autumn.
After some months the centre of the bleeding bark patch may become cracked. In time, fruit bodies of wood-rotting fungi often appear on the surface of the dead bark, protruding out of the bark cracks (Photo 5).
Over several years, and particularly if a tree has multiple bleeding cankers, the areas of dead phloem and cambium underneath the bleeding areas may coalesce and extend until they encircle the entire trunk or branch. When this happens crown symptoms become visible, typically consisting of yellowing of foliage, premature leaf drop and eventually, crown death (Photos 6 and 7).
Sometimes, part of the crown will fail to flush, and later in the year the remaining foliage withers and dies. In trees with chronic dieback caused by the disease, the leaves may also be smaller, and seem thinner and more flaccid than the foliage of healthy trees.
The inner bark (phloem) under the bleeding patches is usually necrotic or dead, with an orange-brown colour which is often clearly mottled or zoned (Photos 8 and 9).
Underneath the wood may be stained blue-black (Photo 10).
Sometimes white fungal mycelium (Photo 11) can be seen under the dying bark but this is usually indicative of Armillaria (honey fungus) or other decay fungi which invade the moribund tissue, and not the original cause of bark death. In these instances it is rarely possible to isolate the agent that originally caused the bleeding canker.
Sometimes Armillaria does attack healthy trees and causes stem bleeding, as it invades via the tree root system. However, in such cases the stem bleeding is likely to be confined to the root collar and lower stem.
Prior to the current upsurge in the incidence of bleeding canker, affected trees were invariably found to be suffering from attack by Phytophthora. However, although this continues to be the case for a few of the trees that are examined (5-10%), the majority of the horse chestnuts currently suffering from bleeding canker in the UK are not infected by Phytophthora. DNA analysis of the diseased tissue taken from a small number of tree fails has also failed to detect any Phytophthora. Instead, the most frequently isolated agent is the gram-negative fluorescent bacterium which we have now identified as Pseudomonas syringae pathovar aesculi.
Tests have confirmed that this pathovar of Pseudomonas syringae is responsible for causing bleeding cankers on horse chestnut that are now frequently seen in Britain. Confirming that a particular organism is responsible for causing a disease by inoculating it into healthy plants to confirm that it causes the disease and its associated symptoms, and then re-isolating it the same organism from the infected plants is known as satisfying Koch’s Postulates.