Quick facts

Common name: acute oak decline

Scientific name: none

What does it affect?: English oak and sessile oak

Areas affected so far: southern England and Wales

Origin: none

What does acute oak decline look like?

Oak declines are a complex phenomenon that have occurred a number of times across Europe over the last 200+ years. The current decline in the UK has been observed over the last 20 or so years. Trees over 50 seem to be the worst affected and mature trees can die within five years.

Symptoms include:

  • General thinning of the crown. This can be quite sudden, occurring over a two-year period.
  • Extensive stem bleeding - this looks like dark weeping patches on the stem.
  • Dark fluid that seeps through vertical cracks between pieces of bark and runs down the tree trunk.
  • Stem bleeds that can stop and heal as the tree recovers from a stressed state.
  • As the tree becomes stressed, secondary pests and diseases begin to take advantage of the weakened tree.

What is acute oak decline?

Acute oak decline is a combination of factors which cause oak trees to become stressed. Environmental stresses like soil conditions, drought, waterlogging and pollution can all impact the tree. Insects, fungi and bacteria then move in on the vulnerable tree and push it into decline.

Trunk of an oak going through decline

Credit: Martin Fowler / Alamy Stock Photo

What happens to the tree?

Symptoms of acute oak decline are essentially normal tree responses to stress. Thinning of leaves on the crown is caused by a tree struggling to take up water and wet patches on the bark known as bleeds can be a response to drought.

If the stress is particularly severe or prolonged, the tree can reach a tipping point where it runs out of energy to get through the winter or fight off pests. These trees can die within a few years.

Where has acute oak decline impacted?

Oak declines have been observed around the world for over 250 years. Oaks in Germany, for example, have been going through a period of decline since the 1990s.

In the UK, oak decline has been observed across the southern parts of England and Wales. It’s not a disease, so it doesn’t spread as such, but as environmental conditions become more unpredictable, cases of oak decline might increase. Declines in other species might also occur.

How did acute oak decline get here and what impact will it have?

Oak declines don’t ‘arrive’ as such, but changes in environmental conditions have caused them to come about. The evidence suggests that oak declines might become more frequent and severe as the climate changes. More frequent environmental changes like prolonged periods of drought or waterlogging will make the problem much worse.

What are we doing about it?

To combat acute oak decline, we are planting oaks as much as possible. In addition, we plant in areas where oak can naturally regenerate to make sure the next generation of trees is better adapted to our changing climate.

As well as this work, we have:

  • Developed a UK and Ireland sourced and grown assurance scheme to make sure all the trees we plant and sell are produced in the UK and Ireland. This reduces the risk of importing new pests and diseases that could add stress to our native oaks.
  • Funded research on the causes of oak decline in the UK.
  • Lobbied the government to improve biosecurity at border points to stop new pests and diseases entering the UK.
  • Partnered with Observatree, a tree health citizen science project which trains volunteers to spot pests and diseases, thereby helping tree health authorities identify and manage outbreaks early.
Horse chestnut tree discoloured to yellow after a leaf miner infestation

Trees woods and wildlife

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