Bleeding canker is an infection of the bark of horse chestnut by the bacterium Pseudomonas syringae pv. aesculi or, more rarely, by two species of the fungus-like (Oomycete) organism Phytophthora. It causes the affected bark to bleed a dark sticky fluid.

It is specific to horse chestnuts, and both the white-flowered Aesculus hippocastanum and the red A. × carnea are affected.

Cankers can be seen at any time of year.

You may see the following symptoms:

• Dark or reddish-brown sticky liquid oozing from cracks in the bark where the infections occur. In dry weather, this dries out to form a rusty-brown or black deposit
• Cutting away the outer bark over infected areas will reveal a brown or purple discoloured area of inner bark, with a diffuse edge if the infection is still spreading and a sharply defined edge if it is stable. Healthy inner bark is a white or pinkish colour
• On older cankers, the dead bark may fall away to expose the wood

The effect on the tree is variable. Some infections last for years, more or less stable, and with little effect on the crown. Others spread rapidly and cause crown deterioration, die-back and even death of part or all of the tree.

Non-chemical control

• Some infections stabilise, so do not immediately resort to removal but keep affected trees under observation
• If the tree’s condition continues to deteriorate, consider removing it because it may be a source of infection for other horse chestnuts
• Burn the affected material if possible, rather than composting or chipping
• Disinfect tools with a product such as Jeyes Fluid after use
• Consider raising horse chestnuts in situ from seed, to avoid the risk of importing the disease on infected plants. Neither pathogen is likely to be seed borne

Chemical control

There is no chemical control currently available.

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 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.