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Forest Research home > Research > Protecting trees > Oak decline / dieback > Acute oak decline

Typical symptoms of acute oak decline

A new disease of oak with symptoms of stem bleeding

Trees affected

Oak trees with symptoms of Acute Oak Decline (AOD) are being reported with increasing frequency in Britain. 

The disease is characterised by extensive bleeding or oozing of a dark fluid from small lesions or splits in the bark of tree stems.  Mostly mature oaks of more than 50 years old are affected.   Both pedunculate oak (Quercus robur) and sessile oak (Q. petraea) are attacked.

Typical symptoms

  • Bleeds on stemsThe splits are longitudinal forming in the cracks between the bark plates and are typically between 5 and 10cm long. They can be close to one another (10-20cm) or spaced further apart.
  • Dark fluid beginning to ooze from a split in the barkThe bleeding patches usually become visible between one and two metres above the ground and can extend high into the canopy. Beginning in spring the fluid runs down the stem, staining the bark.
  • Bleed drying up at certain times of yearThe bleeding often stops at certain times of the year, leaving dry, black streaks on the stems. The dried fluid can cake or form a crust around the split.
  • Lesion on inner bark below the bleeding point in the outer barkUnderneath the outer bark at the bleeding point, the inner bark breaks down creating a lesion, which develops into a fluid-filled cavity.
  • Necrotic tissue in the inner barkA cross-section taken through a bleeding point reveals a cavity between the outer bark and the heartwood and necrotic tissue breaching the cambium and progressing into the sapwood.
  • Inner bark showing signs of callusing in an attempt to heal the cavityIn longitudinal section black flecking may be seen in the inner bark running up the length of the cut section. In some cases the inner bark can show signs of callusing in an attempt to heal the cavity.
  • The tree canopy may show signs of deterioration, but in many cases this does not occur until the tree is near death.
  • D-shaped exit hole of Agrilus biguttatus close to a bleeding patch on the stem of an oak treeIn some cases there is evidence of insect attack associated with the stem bleeding, particularly by the buprestid beetle, Agrilus biguttatus. ‘D'-shaped exit holes of Agrilus might occur in close proximity to some stem bleeds. Agrilus beetles are not considered to be a cause of acute oak decline, but they are thought to be 'opportunistic' on the weakened trees, that is, they take advantage of the tree's weakened state and infest it which further weakens trees and may even hasten their death.
  • The establishment of AOD on a site is typically characterised by low numbers of trees being affected initially, increasing to the point where more than half the oaks on the site have extensive stem bleeding.

See also