Royal Horticultural Society

What is phytophthora bleeding canker?

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.

Links

Horse chestnut bleeding canker
Holly leaf blight

Symptoms

You may see the following symptoms:

• Reddish-brown fluid oozing from cracks to the bark, drying to a dark, tarry material.
• Above the infected area, the foliage may be pale and sparse and the branches may start to die back. Eventually the branch may die.
• Cutting away the outer bark over infected areas will reveal a reddish-brown discoloured area of inner bark, with a diffused 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.

Control

Non-chemical control

Infected smaller branches are best removed and destroyed to prevent the spread of infection.

Scrape back cankers on trunks and larger branches to remove the outer bark and reveal the extent of the infection. Exposure of the canker to dry conditions may cause it to dry out and slough off. If the cankers are small it may also be possible to remove the infections by cutting away the dead bark. However, as noted above, if infection has reached the cambium, healing is only possible from the edges of the infection. This will leave the wood exposed for many years and is likely to lead to decay.

Chemical control

There is no chemical control available to gardeners for control of bleeding canker.

Biology

The genus Phytophthora contains a large number of species which cause root rots, foliage blights, fruit rots and bark cankers on a wide variety of plants. These fungus-like organisms are among the most damaging plant pathogens. They spread by releasing swimming spores (zoospores) into water and may be dispersed in water or in wind-blown rain. They also produce resting spores that can be spread in soil.

The species that cause bleeding canker on trees infect through wounds and also probably through naturally-occurring cracks and lenticels (breathing pores) in the outer bark. Infections spread in the living inner bark, killing it and provoking the tree to produce a dark liquid which bleeds out onto the outer bark.

Growth of the lesion (infected area) is related to bark moisture content and in dry conditions cankers may stabilise and be sloughed off. However, if the infection reaches the cambium and kills it, the infected area can only heal by regrowth from the edges. Cankers may persist for several years, stabilising in dry conditions and then breaking out again when rainfall is high.