Phytophthora ramorum is a fungal-like organism that causes the death of a wide range of trees and shrubs. The greatest impact so far has been on larch plantations, leading to thousands of hectares of felling around the UK.
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
Symptoms include:
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.
Credit: Ben Jones / Forestry Commission
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.
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).
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.
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.
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