Bleeding canker is an infection of the bark of several trees by a number of different species of the fungus-like (Oomycete) micro-organism Phytophthora, causing the affected bark to bleed a dark sticky fluid. Cankers may be present at any time of year.
Trees 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 Phytophthora alni, which has been shown to be a new species formed by the hybridisation of two other species, neither of which affects alder. P. alni is now widespread in Britain; further information can be found in the Forest Research disease profile. Another species, Phytophthora siskiyouensis, has also been found in recent years causing bleeding canker on alder.
Please note that two of the species capable of causing bleeding cankers on a range of trees (including beech and larch), Phytophthora ramorum and P. kernoviae, are notifiable pathogens. If they are suspected of being the cause of the symptom then this should be reported to the relevant plant health authority, whose contact details can be found on the UK Plant Health Information Portal.