A new disease of oak with symptoms of stem bleeding
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.
The 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.
The 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.
The 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.
Underneath the outer bark at the bleeding point, the inner bark a wet and slimy lesion forms and the tissue breaks down developing into a fluid filled cavity. The larval galleries of Agrilus bigattutus, a native bark boring beetle, are almost always visible in and around the lesions.
A 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.
In 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.
The Agrilus larva can even sometimes be seen when a panel is opened in the tree stem.
Beetle coming out
In 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.