Phytophthora ramorum in Larch: Symptoms, Spread and What to Do

By late August, a larch plantation should still be a wall of soft green. If instead you are looking at patches of brown, thin crowns and bare shoots on trees that were healthy in spring, it is worth stopping and taking a closer look. Phytophthora ramorum does not move slowly once it takes hold in larch, and in the UK it is a notifiable disease. Knowing the signs, understanding how it travels, and reporting it promptly can change what happens to the woodland around you.

What Phytophthora ramorum actually is

It is a water mould rather than a true fungus, which is why wet weather suits it so well. The organism produces spores in damp conditions and can infect a wide range of trees and shrubs, including rhododendron, bilberry, sweet chestnut and beech. On most of those hosts it causes localised damage. On larch it behaves differently: bark lesions girdle the trunk quickly, and infected trees shed spores in enormous numbers. That combination makes larch the engine of an outbreak rather than just another casualty.

One reassurance worth stating plainly: this is a plant disease. It poses no risk to people, pets or livestock, and there is no need to avoid walking through an affected wood on health grounds.

Symptoms: what sudden larch dieback looks like

The name is apt. A tree can look reasonably well in June and be visibly failing by September. Because larch is deciduous and drops its needles in winter, the best window for surveying is roughly June to October, when healthy trees should be fully leafed.

Early signs

• Needles wilting and turning a dull grey-brown or black, often starting on lower or inner branches, or on one side of the crown.
• Bare shoots where needles have dropped, giving the tree a thin, ragged look against its neighbours.
• Dark, sunken lesions in the bark, sometimes with black or reddish resin bleeding from the trunk.
• Individual trees affected in patches rather than evenly across a stand.

Later signs

• Crowns dying back branch by branch, or from the top down.
• Extensive resin bleed and staining on the trunk, with dark discolouration in the wood beneath the bark.
• Whole trees dead within a season or two, standing among still-green companions.

Larch canker, frost damage and drought stress can all produce confusingly similar symptoms, which is exactly why a suspected case needs confirming by the plant health authority rather than by eye alone.

How the disease spreads

Spores travel in moist air and wind, in rain splash, and in water running through a site. That accounts for spread over moderate distances, particularly in wet, mild conditions. The faster and more damaging routes are the ones people create.

• Mud and soil on boots, bicycle tyres, vehicle wheels, machinery and tools.
• Movement of infected plants, especially rhododendron and other nursery stock.
• Brash, needles and fallen timber carried between sites, sometimes as firewood.
• Water running from an infected area into a neighbouring catchment.

Larch amplifies all of this. An infected tree releases spores prolifically, which then settle on the rhododendron, bilberry and other understory plants nearby, keeping the disease circulating in the landscape long after the larch has gone.

Why larch is treated so seriously

Japanese larch and hybrid larch are highly susceptible; European larch generally less so, though it is not immune. In UK forestry, larch is often the main sporulation host, meaning an outbreak in a larch crop puts every other susceptible plant within reach at risk. That is the reasoning behind the control approach: removing infected larch, rather than leaving it standing, is intended to cut off the spore supply at source.

Reporting a suspected case

Do not take samples away from the site, and do not fell anything on suspicion alone. Take clear photographs of the crown, the trunk and any resin bleeding, note the grid reference and the tree species, and contact the relevant authority for the nation you are in:

• England — report through TreeAlert or contact the Forestry Commission / APHA.
• Scotland — report through TreeAlert or contact Scottish Forestry.
• Wales — report through TreeAlert or contact the Welsh Government's plant health team.
• Northern Ireland — use TreeCheck or contact DAERA.

If you are managing the woodland on someone else's behalf, tell the owner as well, and let your neighbours know if they have larch or rhododendron nearby. Early reports are far more useful than tidy ones.

Biosecurity: the part you control

Good hygiene will not stop wind-borne spores, but it reliably prevents the spread that people cause. The principle is simple: arrive clean, leave clean, and assume every site could be infected.

1. Brush mud and plant debris off boots, tools and tyres before you leave.
2. Clean and disinfect footwear and equipment between sites, using a product approved for plant health use and following the label.
3. Stay on tracks and paths where you can, particularly in wet weather.
4. Keep vehicles and machinery on hard standing, and wash them down before moving to another wood.
5. Never move brash, needles, soil or plant material off a site you suspect is affected.

What happens after a report

An inspector will visit, and if the disease is confirmed, a Statutory Plant Health Notice is usually served. This sets out what must be felled and by when, and it is a legal requirement rather than a recommendation. In practice, infected larch is often harvested as timber where it is safe and accessible, with brash and low-value material chipped, burned or otherwise dealt with on site to reduce spore load.

Restocking comes next, and it is worth planning early. Replanting with larch in a known outbreak area is generally unwise; a mix of species not known to be susceptible will give the site a better long-term footing. If you are facing a notice, speak to a forestry agent or woodland adviser, and check whether any restocking support is available for your situation before committing to a plan.

What to do this season

Walk your larch between June and October, ideally in dry weather, and look at the whole crown rather than just the nearest trees. Note anything that looks wrong with a grid reference and a photograph. Clean your boots before you get back in the car. If something seems off, report it rather than waiting to see how it develops — with this disease, a fortnight's delay can mean several more trees in the frame. For legal or grant matters arising from a notice, take advice from the relevant plant health authority or a qualified forestry professional.

Photo: Ian Probets / Pexels