- 12th January 2021 -

Identifying and dealing with Ramorum disease

Identifying and dealing with Ramorum disease

Imported on infected plants and driven across the country by windswept rain, Ramorum disease (Phytophthora ramorum) is a serious threat to a variety of trees and bushes, especially the larch.

What is Ramorum disease?

Ramorum disease – or blight – is a species of the Phytophthora genus, a group of water molds which were responsible for the infamous potato blight that devastated 1800s Ireland. The disease’s latin name, Phytophthora ramorum, means the ‘plant-destroyer’. 

Ramorum disease was first detected in the UK in 2002, in imported viburnum plants. Since 2009, it has been devastating the UK’s population of larch trees, earning it another nickname: sudden larch death.  

Borne on moist winds,  Ramorum disease is particularly common in the wetter western parts of Scotland, Northern Ireland and the south west although it is found throughout the UK.

Once the spores have landed on leaves, they break down the cell wall. The micro-organism then spreads within the plant, blocking off its water transport system leading to branch dieback. Larch trees are particularly vulnerable and can die very quickly once infected.

One of the factors that make the disease so deadly is its ability to spread quickly while evading detection.

Another worrying aspect of the disease is its adaptability to different species of tree. In the UK, it has been found in ash, beech, birch, cherry, English oak, horse chestnut, sessile oak, sweet chestnut and sycamore trees.

It can also jump from larch trees to neighbouring firs (e.g. Douglas fir, noble fir, grand fir, western hemlock, etc.)

Ramorum disease has been found in bushes and shrubs including bilberry. This is worrying news for the blueberry industry as this plant is closely related.

Other native species, for example heather, have so far been unaffected in the wild but have shown susceptibility under lab conditions.

Non-native plants under threat include the Sitka spruce, rhododendron, pieris and camellia.

Outside of the UK, the disease’s devastating ability to switch hosts has been seen in the United States where it has attacked native oak and tanoak species.

Devon’s sweet chestnuts under threat

In 2015, Ramorum disease was found to have taken hold of sweet chestnut trees in several sites across Devon and Cornwall. While there are fewer sweet chestnuts in the UK than there are larches, there are a greater proportion of veteran trees. It is therefore vital that landowners with sweet chestnuts remain vigilant to help prevent a devastating cultural loss.

Symptoms of Ramorum disease

How would you spot Ramorum disease in your trees?

When the mold first breaks down the leaves’ cell walls it leaves blackened patches at the base of the petioles and along the midrib.

Later, once the organism has penetrated deep into the tree and started blocking nutrient transport, it causes leaves to blacken and wilt and branches to die back. 

Areas of black coloured ‘bleeding’ on the trunk are another giveaway.

What can you do about Ramorum disease in your trees?

First, it is important to note that Ramorum disease is a notifiable disease so you must report any suspected cases. We recommend you contact Arborcure for a tree survey so we can confirm the presence of the disease and suggest a way forward.

Where felling is needed to prevent damage to the wider ecosystem, Arborcure can provide a professional service.


Tree diseases such as Ramorum disease need to be reported and dealt with quickly. Do you need a trusted firm to inspect your trees and carry out the necessary work to minimise the damage? Get in touch with Arborcure today.

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