Quick facts

Common name: ramorum blight, sudden larch death

Scientific name: Phytophthora ramorum

What does it affect?: English oak, sessile oak, sweet chestnut, larch, beech, sycamore, horse chestnut, cherry, ash, birch

Areas affected so far: throughout the UK but most severe in the west

Origin: unknown

What does Phytophthora ramorum look like?

Symptoms include:

  • Blackened base of the leaf near the petiole and along the midrib of the leaf.
  • Withered and blackened leaves or needles leading to dieback of the outer branches.
  • Areas of black “bleeding” on the trunk.
  • In larch, the disease progresses very quickly so whole trees will be dead within a short period of time.

What is Phytophthora ramorum?

Phytophthora species are microscopic fungal-like organisms closely related to algae. Their name literally means ’the plant-destroyer’ and they have been responsible for some of the worst plant disease epidemics in history, including potato blight which led to the disastrous Irish potato famine in the 1800s.

Resin bleeds dried Into white crust, Phytophthora Ramorum

Credit: Ben Jones / Forestry Commission

What happens to the tree?

Phytophthora ramorum spores spread via wind-driven rain. When they land on a leaf they grow into the tree by breaking down the cell walls in the leaf. This leads to the initial blackening symptoms. It then grows within the tree and blocks its water transport system leading to dieback and eventual death. While this process is happening the pathogen will also be spreading onto other nearby trees and shrubs.

Where has Phytophthora ramorum impacted?

Trees in the wetter parts of the UK are particularly susceptible because the pathogen requires water to spread and infect its host. Major outbreaks on larch have occurred in south-west Scotland, Wales, Northern Ireland and Cornwall.

Phytophthora ramorum moves around within plant networks very easily because it can’t be easily detected and is therefore hard to stop any spread. As a result, it is now widespread worldwide. It has had a particularly severe impact in western parts of the US where its host is the native tanoak (Notholithocarpus densiflorus).

How did Phytophthora ramorum get here and what impact will it have?

Phytophthora ramorum was first discovered in the UK in 2002 on an imported Viburnum plant. It can spread naturally in wind-blown rain (up to 4 miles) but it is the plant trade that spread this disease across the globe.

This disease is here to stay. Before its discovery there were approximately 154,000 hectares of larch planted in Great Britain, 5% of the total woodland area. Most of this has been lost and larch can no longer be used as a timber species. There is also a risk that this pathogen can become aggressive on other hosts; this would be devastating to the natural environment. For example, in 2015 infected sweet chestnut trees were found at a small number of sites in South-West England, mostly in Devon and Cornwall.

What are we doing about it?

If forestry authorities identify P. ramorum in an area the land owner is issued with an official notice to fell all larch. This slows the spread by taking away the pathogens host and preventing spores from spreading.

As part of our work to stop the spread of pests and diseases we have:

  • Developed a UK and Ireland Sourced and Grown assurance scheme to make sure that all the trees we plant and sell are produced 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

What we are doing about tree pests and diseases

We are fighting back against pests and diseases. Find out what we're doing to prevent the spread and protect the UK’s trees. 

Learn more

What to do if you spot it

If you think you have spotted signs of this disease, report it to the plant health authorities via TreeAlert if you are in Britain or TreeCheck in Northern Ireland.

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