Larch Canker: A UK Checklist for Spotting and Managing Infection

Larch canker rarely announces itself with a dying tree. More often it starts as a small patch of dead bark, a bead of resin on a branch, or a hollow where a twig once grew. Find it at that stage and you have options. Leave it a few seasons and you may be looking at a girdled trunk, a stag-headed crown and a decision you would rather not have to make.

The fungus behind it, Lachnellula willkommii, is widespread in the UK. It is a wound parasite rather than an aggressive killer: it exploits dead branch stubs, pruning cuts, frost cracks and squirrel damage, then spreads slowly through the bark. This checklist is about finding it early and knowing where your own efforts stop and a professional's start.

What a larch canker looks like

Classic cankers are lens- or diamond-shaped patches of sunken, dead bark. On a young tree with smooth bark, the lesion often looks like a dark, slightly flattened area with a raised rim of callus where the tree has tried to wall it off. Older infections can show concentric rings of callus, like a target drawn on the trunk.

Resin is the giveaway. A canker bleeds without being cut — clear to amber, sometimes hardening into a white-crusted tear running down the bark. Trace the resin upwards; the wound usually sits above where the run accumulates. Look particularly on the undersides of branches and the sheltered side of the trunk, where moisture lingers longest.

Then look for fruiting bodies. In damp weather, mostly in spring and again in autumn, the fungus pushes out small cup-shaped structures a few millimetres across, pale and hairy on the outside with an orange-yellow disc inside. They appear on dead bark around the edge of an old canker and are the most reliable confirmation you will get in the field.

A field checklist for a suspect tree

1. Follow the resin. Work upwards until you find the source rather than treating the stain as the problem.
2. Map the lesion. Is it a branch canker or a trunk canker? Roughly what proportion of the circumference is affected? Your hand span is a good enough measure for a first judgement.
3. Look underneath. Check the lower surfaces of branches and any side of the trunk that stays shaded and damp.
4. Hunt for cups. Return after rain in spring or autumn and inspect the edges of dead bark. Photograph what you find, with something for scale.
5. Read the crown. Thinning foliage, short shoots, dead tips and tufts of brown needles held on the tree all point to a problem below.
6. Check the base and root flare. Resin at the butt can come from root damage, drought or waterlogging, not canker.
7. Widen your view. One affected tree in a mixed stand is a different situation from a cluster of them along a sheltered edge.
8. Record it. Date, location, photographs and a note on what you did. A baseline makes the next visit far more useful.

Telling it apart from the lookalikes

Several things cause resin bleeds and dieback on larch, and they call for different responses. A vertical split with resin along the line, often on the sunniest side of the trunk, is more likely a frost crack. Strips of bark removed higher up, with tooth marks, mean squirrels. Frayed stems on young trees point to deer. Old mechanical wounds, mower strikes and strimmer damage all produce resin and callus.

The important one to rule out is Phytophthora ramorum, which has affected larch — particularly Japanese larch — in parts of Britain. It tends to cause dark, resinous bleeding lesions with rapid crown decline, and it can move through a stand far faster than canker. Because it is a regulated plant disease, suspected cases should be reported to the plant health authority for your part of the UK rather than quietly pruned out. If in doubt, report it and let someone else make the call.

Pruning: what it can and cannot do

Canker is a wound disease, which makes pruning both a treatment and a risk. It works well on branches and on small trees. It cannot save a trunk that is already badly girdled.

What to remove

Take off the whole affected branch at the branch collar where you can. If that is not possible, cut back into clean wood, ideally at least 30cm beyond the visible lesion. Inspect the cut surface: if brown staining runs into the wood, cut further back. Sanitise your tools between cuts and especially between trees, using methylated spirits or a garden disinfectant.

When to prune

Choose a dry spell. Spores are dispersed in wet weather, so avoid working through the wettest weeks of spring and autumn, and avoid cutting in frost. Late winter is often a sensible window, but a dry, settled day matters more than the month.

What to do with the arisings

Burn or remove infected material rather than chipping it and leaving it in the stand. Do not drag brash through trees you are trying to protect, and clean boots and kit if you are moving between woodlands.

Slowing things down: site and stand management

• Canker thrives in still, humid air. Thinning dense stands helps, but time operations for dry weather and avoid wounding stems.
• European larch is generally the most susceptible; Japanese and hybrid larch tend to fare better. On mild, sheltered, heavy-soil sites, larch may simply be the wrong tree.
• Keep mowers, strimmers and machinery clear of trunks. Most cankers start at a wound you could have avoided.
• Manage squirrels and deer where damage is persistent, and keep vegetation from resting against stems.
• Plant stock from a reputable nursery, and do not move larch material between sites without checking the current plant health guidance.

When to bring in a professional

Call someone in if the canker is on the main stem, particularly if it wraps a significant share of the circumference — no amount of careful pruning will fix that, and the tree may need felling or monitoring. It becomes a safety matter if the tree stands near a road, footpath, building or boundary.

Get help too if you cannot confidently rule out Phytophthora ramorum, if several trees across a stand are affected, or if the trees are valuable specimens. Protected trees need consent before work begins: check with your local authority about Tree Preservation Orders and conservation areas, and with the relevant forestry authority about felling licences, which vary across the UK. Anything requiring climbing or chainsaw work above head height should go to a qualified arborist working to BS 3998. If a tree could damage property or injure someone, take advice on your position before acting.

A routine worth keeping

Walk your larches twice a year — once in early spring before the needles are out, once in late summer — following the same route each time so that changes stand out. Ten minutes per stand is usually enough. Carry secateurs, a phone and something to mark a tree with.

If you find a suspect lesion, photograph it, note the date and check again in a month. No change gives you a baseline. Visible growth gives you a reason to act, or to pick up the phone. Canker is a manageable nuisance in many gardens and a real constraint in some woodlands, and the difference between the two is usually how early someone looked.

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