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Bleeding canker of horse chestnut: Causal Agent

Phytophthora and Pseudomonas cause bleeding canker

Bleeding canker is a disease that affects horse chestnut trees (Aesculus hippocastanum). It was first reported in Britain in the 1970s at that time considered a relatively uncommon tree disorder. Today the incidence is dramatically increased due to the arrival of a ‘new’ pathogen.

Before this current upsurge in incidence, affected trees were invariably found to be infected with one of two fungal-like pathogens: Phytophthora cactorum or Phytophthora citricola. However, recent DNA analysis of tissue taken from diseased horse chestnut trees often failed to detect any Phytophthora and analysis shows that Phytophthora is only responsible for bleeding canker in 5-10% of affected horse chestnut trees today.

An entirely different pathogen is now causing widespread disease in trees across Europe. Find out how Forest Research scientists identified this new pathogen as Pseudomonas syringae pathovar aesculi.

Researchers are now using molecular technology to characterise the pathogen’s biology.

Both Pseudomonas and Pseudomonas pathogens can cause similar symptoms: cankers on the stem, bleeding on the trunk and branches, occasional scaffold and crown die back. Find out more about the symptoms of horse chestnut bleeding canker.

Another pathogen that can cause bleeding cankers is Phytophthora ramorum. However, this pathogen has only ever been found in one specimen of horse chestnut in England. Unless a horse chestnut with bleeding cankers is located near to a source of P. ramorum – such as infected rhododendrons or larch – then the likelihood of infection by P. ramorum is negligible.

Despite this, P. ramorum must be taken seriously because it is a quarantine organism. If you suspect that a tree is suffering from P. ramorum infection, you should contact the Forestry Commission:

Plant Health Branch
Forestry Commission
Tel: 0300 067 5155

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