Oak decline is a term that describes the deterioration of oak trees, in some cases eventually leading to death of the tree. A slow-acting phenomenon known as chronic oak decline or chronic oak die-back, thought to be due to an interaction between many different factors (insect damage, disease and environmental conditions) has been known since the early part of the twentieth century.

In recent years another problem called acute oak decline has come to the fore. Thought to be caused by bacterial pathogens, this can lead to a rapid deterioration of affected trees.

Oak decline is unrelated to the disease known as sudden oak death which, despite its name, does not notably affect English oak trees.

You may see the following symptoms:

Chronic oak decline

• Mainly affects pedunculate oak (Quercus robur)
• Progressive deterioration of the crown, taking many years or even decades
• Deterioration begins with leaves becoming paler and smaller
• Twigs start to die back, followed by small branches
• The dieback may progress to large branches, and in the most severe cases the tree dies
• Others may recover partially or stabilise, although the problem may begin again at a later date
• Recovering trees often have a healthy-looking lower crown, but with large dead branches in the upper crown projecting above the green lower canopy like antlers (known as a ‘staghead’ effect)

Acute oak decline

• Mainly affects mature (greater than 50 years old) trees of pedunculate oak, sessile oak (Quercus petraea) and hybrids between the two, although some younger trees have been affected and occasionally also other Quercus species
• A dark fluid weeps down the trunk from cracks (about 5-10cm long) in the bark. This may stop and dry out at certain times of year, and can be washed off by heavy rain
• Multiple bleeding patches may be present, from close to ground level to high in the canopy
• Some affected trees have died within four to five years
• Unlike chronic oak decline, symptoms of deterioration in the crown may not appear until just before the tree dies

Non-chemical control

• Aim to achieve healthy, vigorous growth by matching the tree to its site (soil conditions, climate, etc.)
• Evidence suggests that trees derived from those of local provenance are best adapted to the local conditions and more likely to resist insect and disease attack
• The presence of large numbers of oak trees with extensive bleeding should be reported to the Tree Health Diagnostic Advisory Service of Forest Research (part of the Forestry Commission) (see the link below) – this will help in their investigation of the problem. Acute oak decline is not a notifiable disease, however, and there is no legal requirement to report cases
• Current advice is to leave affected trees in situ (unless there is an immediate concern about safety) and to monitor them
• However, if only a few trees are affected, amongst a large number of healthy trees of the same species, it may be prudent to fell them
• Pruning of affected trees is not recommended, unless dead branches pose an immediate health and safety risk
• Do not compost any part of an infected tree, nor use any resulting wood chips as a mulch or soil conditioner

Chemical control

There are no chemicals available for the control of acute oak decline. However, it is recommended that any equipment used for the felling or pruning of affected trees is disinfected after use.

Further information

The above is a summary of the current recommendations given by the Forestry Commission (the full recommendations can be found on this link).

Chronic oak decline has been known throughout Britain since the early twentieth century. It is thought to be caused by the interaction of a range of contributory factors, which are also likely to vary between sites. Factors that have been implicated include:

• Honey fungus
• Phytophthora root rot
• Other root decay fungi
• Powdery mildew
• Drought and high winds
• Wood-boring beetles
• Invertebrates causing defoliation (mostly caterpillars of moths)

Acute oak decline has so far been found most commonly in south-east England, the Midlands, East Anglia, the Welsh borders and south-east Wales, and is still the subject of extensive investigation and research. A previous case of acute decline in the 1920s was thought to have been triggered by heavy infestations of the oak leaf-roller moth (Tortrix viridana). However, in the recent outbreaks 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.