The reasons for the increase in number of affected trees is being investigated by scientists at Forest Research. The initial hypothesis was that the recent spate of mild winters and wet springs had favoured spread and infection by the Phytophthoras previously found to cause bleeding cankers on horse chestnut. However, although climate may indeed be playing a part in the incidence of this disorder, it now seems that a causal agent other than Phytophthora is responsible for the vast majority of the cases of stem bleeding and tree death over the past 4-5 years.
Culturing from the margins of the dying bark tissue of affected trees in the south of England usually yields several types of micro-organism. However, one turns up consistently – a species of gram-negative fluorescent bacterium. At Forest Research, healthy young horse chestnut saplings were inoculated with the bacterium and after several months many of the saplings developed typical signs of the bleeding canker disease. This bacterium has now been identified as a pathovar of the species known as Pseudomonas syringae. The specific name is Pseudomonas syringae pv aesculi.
In The Netherlands, where recent surveys have revealed that one in three of all horse chestnuts are now affected to a greater or lesser extent by the same disorder, research has also been underway to discover the cause. Their findings also indicate that the same species of bacterium appears to the causal agent. The research group undertaking work on this disorder in The Netherlands is known as Aesculaap: see www.kastanjeziekte.wur.nl/uk/index_uk.htm (this the link for the English version of the Aesculaap website).