Acute oak decline

A new disease that affects native oak trees in Britain and can cause their death.

Disease overview

Close up of fresh lesion beneath the bark of Oak with symptoms of Acute Oak Decline. Send, Woking, Surrey, EnglandAcute Oak Decline (AOD) is a new disease mainly affecting native oak trees in Britain and considered to have first made a presence in Britain 30‑35 years ago. It affects mostly English or pedunculate oak (Quercus robur) and sessile oak (Quercus petraea).

Affected trees have vertical, weeping fissures that seep black fluid down the trunk (stem bleeds). In the live tissue beneath the bleeds a lesion is formed. This is a sign of tissue decay. Some trees die four to six years after onset of symptoms.

The larval galleries of the native buprestid beetle Agrilus biguttatus  are usually found in association with the lesions and various species of bacteria have been isolated from the lesions. The high co-occurrence of the beetle and the bacteria suggest that these agents play a role in AOD. Forest Research is involved in studies to investigate these relationships in more detail.

Close up of bark removed from tree suffering with Acute Oak Decline symptoms, showing lesion and cavities.  Send, Woking, Surrey, EnglandThis resource will help you to identify affected trees and differentiate AOD from other oak disorders, namely chronic oak dieback . Step-by-step instructions on taking samples for verification of AOD are provided and recommendations on how to manage AOD are given.

Disease details

  • Affects  mostly mature trees aged over 50 years but has recently been found on younger trees with diameter 10 – 12 cm
  • Affects  oak species. The most commonly affected species are English or pedunculate oak (Quercus robur) and sessile oak (Quercus petraea) but turkey oak (Q. cerris) and bali oak (Q. Fabri) are also affected in Britian. In Spain Q. ilex (holm oak) and Q. pyrenaica (Pyrenean oak) are affected by the bacteria.
  • Significant spread across England, and recently has been found in Wales in the Welsh borders  – see distribution maps
  • Thousands of oak trees are affected
  • Most prevalent in the Midlands and South East England
  • Tree death is likely to be caused by multiple agents
  • For infection to occur, it is likely the trees need to be weakened (predisposed) by certain factors
  • Thinning of canopy as tree nears death
  • Symptoms can develop rapidly over 18 months
  • Some trees die within four or six years of onset of symptoms


  • Extensive stem bleeding - dark weeping patches on the stem
  • Dark fluid seeps through vertical cracks between bark plates and runs down the tree trunk
  • Fluid may dry and cake on tree stems at certain times of the year
  • In the early stages trees may have one or just a few bleeding points which may increase in number over time
  • Patches of decay (lesions) form in the live tissue beneath bleed points
  • ‘D-shaped’ exit holes of the beetle Agrilus biguttatus may be present in bark plates of affected trees. (One third of cases show this). The ‘D-shaped’ exit holes are approximately 4mm wide and 3mm high.
  • A network of larval galleries of Agrilus biguttatus is present close to dying tissue


Weeping patches or stem bleeds are a general symptom or host response to tissue attack from a range of pests and pathogens, for example Phytophthora species. A stem bleed alone does not indicate AOD.

Our AOD symptoms page provides more information and photographs to help you diagnose this condition and differentiate it from Chronic Oak Decline.

You can report oak trees with symptoms of extensive bleeding to our Tree Health Diagnostic and Advisory Service.

Disease management

Forest Research is involved in numerous studies to understand the exact cause and spread of this condition. Until evidence based information is available we provide guidelines on managing the disease.

These guidelines are based on general principles to reduce the risk of transferring pathogenic bacteria found in bleeds to healthy trees.

Research is underway to determine the possible role the native buprestid beetle plays in this complex, and whether there are ways of managing it sensitively.

Our research

Research scientist working on Acute Oak Decline in tree health laboratory inserting newly emerged two spotted oak buprestid (Agrilus biguttatus) larvae into slits in the bark of English oak (Quercus robur). Location: Alice Holt research station, Hampshire, England.As part of a programme funded by Defra, our scientists are collaborating with scientists from universities and research organisations as well as stakeholders to investigate the causes, distribution and scale of AOD in the UK . This research will help to develop effective AOD management and prevention strategies, map the distribution of the disease in Britain and predict the risk of spread of the disease.

Our work includes:

  • Studies of Agrilus biguttatus to investigate its life cycle and behaviour, and to determine whether this species plays a role in the disease, perhaps by spreading pathogenic bacteria
  • Formal identification and characterisation of unnamed and unknown bacteria isolated by Forest Research from symptomatic oak
  • Investigations on the role of bacteria in causing tissue death in affected oaks
  • Development of rapid diagnostic tools to detect bacterial species
  • Systematic survey of a number of statistically selected sites in England and Wales
  • Mapping symptomatic and unaffected trees in selected AOD sites
  • Intensive monitoring of symptom development in individual trees
  • Spatial analysis of mapping and monitoring data to determine distribution and spread of this condition, levels of tree mortality and/or recovery and changes in the severity of the condition within sites.

Find out more about Agrilus biguttatus our latest findings on bacteria implicated in AOD.

Related resources

More about different forms of oak decline and dieback

If you suspect a tree is affected by AOD you can report it using the Forestry Commission’s Tree Alert form.

We can also help you identify diseased trees and provide guidance on prevention and management. Please contact our Tree Health and Disease Diagnostic Advisory Service.


Dr Sandra Denman

Alternatively, enquires may be addressed to our Tree Health Diagnostic & Advisory Service at

What's of interest

FC AOD advisory pages

Managing acute oak decline
Forestry Commission Practice Note 15.

Current tree health issues (PDF-1375K)

Acute oak decline, Phytophthora diseases and Dothistroma needle blight.

Presentation to Royal Institution of Chartered Surveyors in March 2011.

Related pages

Related documents