Bleeding canker is an infection of the bark of several trees by a number of different species of the fungus-like micro-organism Phytophthora, causing the affected bark to bleed a dark sticky fluid. Cankers may be present at any time of year.
Trees commonly affected in the UK include Aesculus (horse chestnut), Tilia (lime) and Alnus (alder).
Horse chestnuts continue to be affected by Phytophthora bleeding canker at a relatively low level, but have recently been affected by a huge upsurge in cases of bleeding canker caused by a bacterium.
Holly is affected by Phytophthora ilicis, which is more commonly the cause of leaf blight but can also infect bark, causing cankers.
Alder is affected by the newly described P. alni, which has been shown to be a new species formed by the hybridisation of two other species, neither of which affects alder. This is a worrying development and spread of the disease is being monitored by Forest Research scientists.