A new disorder of oak with symptoms of stem bleeding
What is acute oak decline (AOD)?
AOD is a relatively new condition of oak trees in Britain, thought to have started 20 - 30 years ago but seemingly now on the increase.
Mature trees (>50 years old) of both native oak species (Quercus robur, known as pedunculate or English oak, and Quercus petraea - sessile oak) are affected. Key symptoms are black weeping patches on stems (called stem bleeds); lesions and necrotic tissue underlying the bleed points, and larval galleries are usually present weaving a sinuous path along the cambial zone.
The cause of the problem is likely to be complex involving multiple agents. At this early stage of the investigation we know that the buprestid beetle Agrilus biguttatus is often found in co-occurrence with various species of bacteria on trees with symptoms of AOD. Studies are underway to elucidate the roles these organisms play in this particular decline condition of oak. Compare with oak decline and chronic oak decline.
- Affected trees are characterised by symptoms of extensive stem bleeding evident as dark weeping patches on the stem surface between bark plates.
- Dark fluid seeps through small longitudinal fissures in the bark of stems and runs down the tree trunk. In advanced stages of AOD many bleeding points may be present on the stem but in the early stages only a single or few bleeds are apparent.
Note: Weeping patches or stem bleeds are a general symptom or host response that may be formed when underlying tissues are attacked. A range of organisms can elicit this type of response. For this reason a stem bleed alone is not diagnostic for AOD.
- The fluid may dry and cake on tree stems at certain times of the year.
- Lesions and necrotic patches form in the stem tissues underlying the weeping points; the galleries of Agrilus biguttatus are in close proximity to necrotic patches.
Left: Panel from a tree with typical symptoms of AOD showing two weeping points on the stem.
Right: Bark of panel split longitudinally and open outwards revealing lesions and galleries in the innerbark (phloem) onto the sapwood – these are typical symptoms of AOD in the internal tissue of affected trees.
In contrast to chronic oak decline, some of the trees affected with AOD die within 4 to 5 years of the onset of symptoms. In the early stages of this condition no changes in canopy health are noticeable but as trees approach death canopies may be visibly thinner.
Research into acute oak decline
- Studies to elucidate the role of Agrilus biguttatus in AOD and investigate links between the buprestid beetle, and its larvae and bacteria are in progress.
- Many of the bacteria isolated from symptomatic oak were previously not named and some were even unknown to science. The identity of these bacteria is being formalised and the role that bacteria play in causing tissue death in oaks affected with AOD is being investigated.
- Rapid diagnostic tools to detect the bacterial species concerned are being developed.
- Mapping of selected AOD sites has recorded the position of symptomatic as well as unaffected trees. Intensive monitoring of symptom development in individual trees on the mapped sites is underway. Data obtained from the monitoring and mapping exercise will be analysed spatially. Results will give insights into the expanding distribution of this condition, levels of tree mortality and/or recovery, as well as changes in the severity of the condition within sites.
European reports of similar conditions
Reports of Agrilus biguttatus attack on oaks with similar symptoms of stem bleeding or ‘slime flux’ on stems have also been made in Belgium (Flanders) (Van Steenkiste et al., 2004), Germany (Hartmann and Blank, 1992), The Netherlands and Poland (Moraal and Hilszczanski, 2000).
A similar condition caused by bacteria on Mediterranean oak species (Quercus ilex or holm oak and Quercus pyrenacia the Pyrenean oak) has been reported in Spain, although no association with Agrilus was documented (Biosca et al., 2003; Poza-Carrion et al., 2008).
Distribution of acute oak decline
Over the past three to four years there have been a growing number of reports on oak trees with symptoms of stem bleeding. The incidence of AOD in Britain is unquantified at this stage but estimates put the figure at a few thousand affected trees. The condition appears to be most prevalent in the Midlands and the south east.
Reporting affected trees
Oak trees with symptoms of extensive bleeding may be reported to our Tree Health Diagnostic and Advisory service, using their standard report form.
Management of acute oak decline
Until the exact cause of AOD is known we can only offer management guidelines:
- Specific management advice
- Example of information notice to the public regarding stem bleeding of oak
Managing acute oak decline (PDF-1150K)
Forestry Commission Practice Note 15.
More detailed advice and recommendations for managing symptomatic trees, as well as more general information about the condition, is currently being produced as a Practice Note.