Bleeding canker of horse chestnut: new bacterium increases incidence

Pseudomonas bacteria cause increases in bleeding canker incidence

Bleeding canker is a disease that affects horse chestnut trees (Aesculus hippocastanum) and its incidence in the UK has dramatically increased over the last decade. In 2000 only four cases were reported, but this rose to more than 110 reports in 2006 to the Forest Research Tree Health Diagnostic and Advisory Service and survey results show that in 2007 49% of horse chestnut trees showed some degree of symptoms. So what is causing this dramatic increase in incidence?

The initial hypothesis was that mild winters and wet springs had favoured the spread of the Oomycete Phytophthora pathogens, which are known to occasionally cause bleeding cankers on horse chestnut trees.

But, while climate change may be playing a role, it is not the only factor that is driving the spread of the disease. It now seems that a different and newly arrived causal agent is responsible for most of the recent cases of stem bleeding and tree death.

Culturing from the margins of dying bark tissue of diseased trees has consistently revealed one species of gram-negative fluorescent bacterium. To find out if this bacterium is responsible for bleeding canker, Forest Research inoculated healthy young horse chestnut saplings with the bacterium. After several months many of the saplings developed symptoms of bleeding canker.

The bacterium has now been identified as a pathovar of the species known as Pseudomonas syringae; the specific name is Pseudomonas syringae pv aesculi. Find out more about the causal agent.

Find out more about the incidence of bleeding canker in the UK and across Europe.