Pseudomonas syringae (Bleeding Canker of the Horse Chestnut)

CuPC33 Lab Test: Positive

A bacterial infection which exists in various strains; P. syringae pv. Aesculi attacks the horse chestnut; a variant, pv. Aceris attacks maples (Acer). Approximately 70% of the UK’s 2 million horse chestnuts are infected and most will die in the next 8-10 years unless they are treated.


Forest Research scientists estimate some 35,000 to 50,000 trees are affected and probably a few thousands have already been felled as a result of the disease.

Horse Chestnuts ranging from 10-15 year old vigorous saplings to large mature amenity trees can all be affected by the disorder.  Many are highly visible amenity trees in parks and public gardens, others form important features in avenues, historic gardens and landmarks such as the double avenue of horse chestnuts leading up to the prehistoric Avebury stone circle.

In a detailed survey of around 230 Horse Chestnuts in Hampshire, about half were found to be suffering from bleeding canker (Straw and Green, unpublished data). A higher proportion of trees in towns and rural areas displayed symptoms compared with woodland trees, while slightly more red horse chestnuts (A. x  carnea) were affected compared with white flowered trees (A. hippocastanum).

Other strains attack a variety of commercial crops including wheat, barley, beets, crab apple, peas, tomatoes, rice and soyabeans. Treatments developed for these cash crops have proved effective under controlled conditions but are not suitable for treating trees.