Larch is one of the few conifers that changes its clothes with the seasons, and that is part of its appeal. It is also why ramorum disease catches people out. By the time a larch looks wrong, infection may have been at work in the tree for weeks.

If you own a few larch, look after a shelterbelt, or simply walk through a plantation, it pays to know what you are looking at — and to know what not to do next.

What ramorum disease actually is

Phytophthora ramorum is not a fungus but a fungus-like organism, an oomycete, and it behaves much like one. It needs moisture, produces spores in mild, damp weather, and travels with water: wind-driven rain, splashing, run-off, contaminated soil and plant material, boots and tyres.

In Britain it has two faces. On shrubs such as rhododendron, viburnum, camellia, magnolia and pieris it causes foliar blight — dark lesions on leaves and shoots, and, crucially, enormous numbers of spores. On larch it is far more destructive, killing the inner bark and cambium and girdling branches and trunks. Japanese larch and hybrid larch are the worst affected; European larch can be infected too, though it often suffers less.

It is a regulated disease in the UK, and suspected cases must be reported to the plant health authorities. It does not harm people or animals, and the wood is still useable in many cases, but only under the conditions the authorities set.

Symptoms to look for on larch

The canopy and shoots

• From midsummer, individual branches or whole crowns turn greyish-brown, wilt and look scorched.
• Dieback usually starts at the top or on one side and works down and inward. Trees beside a wet flush, a pond or a thicket of rhododendron tend to go first.
• Young shoots may blacken, wither and curl over at the tip, with the needles clinging on long after they should have dropped.

Trunks, branches and bark

• Dark, sunken cankers on branches and trunks, sometimes with resin oozing and running down the bark.
• Under the bark at the edge of a suspect canker, the tissue is stained dark brown to black, with a fairly clear boundary between affected and healthy wood.
• Sometimes the first clue is simply that a group of trees has thinned from the top and died within a season or two.

Not every sick larch has ramorum

Larch has plenty of ordinary troubles. Needle blight turns foliage brown and causes heavy needle fall, often earlier in the season and worse in sheltered, humid stands. Larch aphids bring yellowing, sticky honeydew and sooty mould. Larch canker, a different disease entirely, produces sunken resinous cankers and is common on European larch. Drought, waterlogging, frost and root damage all cause dieback too.

What sets ramorum apart is speed and pattern: several trees going over within weeks, often in a cluster, usually in the wettest part of a site. Even so, do not trust your own diagnosis. Field identification is genuinely difficult, and only laboratory testing gives an answer.

Reporting a suspected case

1. Stop. Do not fell, prune or move anything off site. Needles, bark and soil can all carry the organism, and a tidy-up can spread it further.
2. Record what you see: date, location (a grid reference if you have one), how many trees are affected, and the pattern. Photographs from a distance plus close-ups of foliage, cankers and resin are genuinely useful.
3. Report it. In Great Britain, Forest Research's TreeAlert is the standard route for tree pest and disease reports. For garden plants and nursery stock, contact your nation's plant health authority — Defra and APHA in England, the Welsh Government in Wales, the Scottish Government's plant health team in Scotland, and DAERA in Northern Ireland.
4. Wait for advice. If a case is confirmed, you will be told what has to happen — typically felling of infected larch and nearby trees, with restrictions on moving timber, needles and soil. A felling licence may also be needed, so check with your nation's forestry authority.
5. Keep notes of who you spoke to and when. It makes conversations with neighbours and contractors much easier later.

Biosecurity you can actually keep up with

Biosecurity sounds bureaucratic. In practice it is a handful of habits, and most take seconds.

• Clean boots and tools before you leave a site, not when you get home. Brush off mud and soil first — disinfectant does not work well through a layer of mud — then use a plant-safe disinfectant at the label rate, made up fresh.
• Stay on tracks where you can, and keep vehicles out of larch crops. Wash wheels and arches before driving to another wood.
• Do not move plant material, needles, leaf litter or soil between sites. That includes the boot-load of prunings you were going to take to the tip.
• Buy from reputable nurseries and check plant passports for larch and known host shrubs. If a plant arrives looking poorly, tell the nursery rather than planting it out.
• Keep larch away from rhododendron, viburnum, camellia, magnolia and pieris. Those shrubs can produce spores in huge numbers, and larch growing beside them is at greater risk.
• Inspect your larch from midsummer onwards. Look at the canopy against the sky from a distance before you go in close to the bark.

A simple routine worth keeping

Pick one week in July or August and walk your larch. Look at the tops of the trees first, then the trunks, then the ground for resin and fallen shoots. Photograph anything you are unsure about and note the date. If something looks like ramorum, report it that week rather than at the end of the season — the sooner it is looked at, the more options everyone has.

Remember that most poorly larches are simply dry, waterlogged or crawling with aphids. If your trees are part of a managed woodland, or you are thinking about felling, speak to a forestry consultant or your plant health authority first: there are rules about felling and about moving timber, and dead or dying larch drops branches without warning. Felling is a job for trained people with the right kit.

Photo: wal_ 172619 / Pexels