Two species of the fungus-like (Oomycete) organism Phytophthora (P. cactorum and P. plurivora) have long been known to cause cankers (bark infections) in horse chestnut, though cases were relatively uncommon and confined mainly to southern England. But in recent years there has been a dramatic upsurge in cases of bleeding canker, in many parts of the UK, from which Phytophthora could not be detected. Work in the UK and the Netherlands has established that the bacterium Pseudomonas syringae pv. aesculi is the cause of these new cases.
The bleeding fluid is produced by the tree in response to the infection, which kills the inner bark, cambium and outer layers of wood, causing disruption to water and nutrient transport. If the canker girdles the stem, the stem dies.
Research on the bacterium is still in progress. It may require wounds to infect (which may include naturally occurring lenticels, or pores, in the bark) or might exist on plant surfaces and be spread by wind-blown rain. Phytophthora spreads in a similar way and also forms resting spores which can remain for long periods in the soil.