Honey Fungus is a serious disease that affects apples, plums and damsons.
What does it look like?
Clumps of honey-coloured mushrooms sometimes appear briefly on infected stumps from late summer into autumn, but their absence doesn't mean the fungus isn't there. Peeling back the bark at ground level can reveal sheets of white fungus between bark and wood that smell strongly of mushrooms, the most characteristic symptom to confirm the diagnosis.
What causes it
The rhizomorphs develop mostly 2.5 to 20cm below the soil surface in moist soil, but may be found deeper in dry soils. They can grow at up to 1m a year, and the fungus can attack plants up to 30m from the source, so an old infected stump or a badly affected neighbour is a risk to healthy plants nearby.
Is this disease active now?
The fungus itself is active in the soil year-round, but the honey-coloured mushrooms that give the clearest visible sign only appear from late summer into autumn.
🌱 Plants at risk
Symptoms
Above ground, the upper part of the plant can die back, sometimes suddenly in hot, dry weather, with paler, smaller leaves, poor flowering or an unusually heavy flowering just before death, early autumn colour, and bleeding or cracked bark at the base. Below ground, black or brown root-like 'bootlaces' called rhizomorphs spread out through the soil from the infection.
Life cycle
The fungus persists as mycelium and rhizomorphs in the soil and in infected wood year-round, with mushrooms fruiting above ground only in late summer and autumn. Spread happens through direct root-to-root contact and through rhizomorphs travelling independently through soil; newer research also points to spores playing a bigger role in starting a fresh infection than previously thought.
Why this matters in Scotland
The RHS calls honey fungus the most destructive fungal disease in UK gardens. No Scottish incidence figures were found.
Rhizomorphs develop mostly in moist soil, which a damp Scottish garden may supply, but no source measures how that changes the risk here.
Controls: what works
Excavate and destroy all infected root and stump material, by burning or via landfill
Remove the infected plant with all its roots and the stump, and burn it or take it to landfill. Replant with less susceptible woody plants, and avoid planting anything back into the soil for six months to a year (Gardeners' World).
Install a 45cm-deep vertical barrier of butyl rubber or heavy-duty plastic to protect healthy plants nearby
A strip of butyl rubber or heavy-duty plastic sunk 45cm deep, protruding 2 to 3cm above the soil, blocks the rhizomorphs and protects a healthy plant from an infection next door.
Deep cultivation to break up rhizomorphs in the soil
Physically disrupting the rhizomorphs interrupts the route the fungus uses to travel between plants, useful alongside removal and barriers rather than as a stand-alone fix.
Avoid using infested compost around woody plants
Armillaria gallica produces large, easily visible rhizomorphs that are quite often found in compost heaps, so avoid using infested compost around woody plants.
Myths about honey fungus
“All trees and shrubs are equally at risk from honey fungus, so plant choice makes no difference.”
Susceptibility varies a lot by species. Birch, privet, and rhododendron sit among the plants particularly susceptible, with privet specifically flagged as the most susceptible known host, alongside others including Aesculus, Buddleja, Ceanothus, Cedrus, Cotoneaster, leylandii, Forsythia, Juglans, Laburnum, Liquidambar, Photinia, Quercus, Salix, Sorbus, Syringa, Thuja, Viburnum, and Weigela. By contrast, bamboo, box, and ginkgo are among plants rarely recorded as being affected, alongside others including Callicarpa, Catalpa, Chaenomeles, Chimonanthus, Cordyline, Erica, Garrya, Hypericum, Jasminum, Pittosporum, Rhamnus, Sarcococca, Tamarix, and Vaccinium.
Our read This is a real and useful difference for planning what to plant near an old stump or a known infection site, even though it isn't a guarantee.
What to do: When replanting near ground with a history of honey fungus, favour species from the rarely-affected list over the particularly susceptible one, but don't treat any woody plant as risk-free since none are described as fully immune.
In Scotland: Some plants on the more resistant list, such as Erica and Vaccinium, are already common choices in Scottish gardens for acidic, damp soil, which makes this a practical substitution rather than a compromise on what would grow well here anyway.
1 source
- RHSLists specific genera as particularly susceptible to honey fungus and separate genera as rarely recorded as being affected, while stating no plants are completely immune.
“Cutting an infected stump down to ground level and treating it is enough to stop honey fungus spreading.”
Surface-level removal leaves the part of the plant the fungus depends on. The only effective remedy described is to excavate and destroy, by burning or landfill, all of the infected root and stump material, because that root and stump wood is the food base the rhizomorphs feed on and spread from; leaving it in the ground leaves the source intact regardless of what happens above ground.
Our read There's no chemical shortcut to fall back on either, since no fungicide treatment is available for this disease, which is what makes full excavation the only real lever rather than one option among several.
What to do: Dig out and remove the full root system and stump of an infected plant rather than just felling it, and dispose of the material by burning or taking it to landfill rather than composting or leaving it buried.
1 source
- RHSStates the only effective remedy is to excavate and destroy all of the infected root and stump material by burning or landfill, and that there are no chemical controls available.
“If you don't see honey-coloured mushrooms at the base of a tree, honey fungus isn't present, and if you do, the plant is already doomed.”
Both halves of this claim oversimplify. The absence of mushrooms is not proof the fungus isn't active, since many plants are killed before any mushrooms appear at all, and the mushrooms themselves only fruit in a narrow late summer to autumn window even when the underlying infection runs year-round. Mushrooms are a confirming sign when present, not a required one, and their appearance reflects an infection already established rather than automatically meaning the whole plant is beyond any response.
Our read Framed as a strict either/or, this doesn't hold up, but the underlying caution that this disease is usually diagnosed late is fair: RHS's own overall guidance is that a plant already showing symptoms is usually already lost, so mushrooms are more often a late confirming sign than an early warning.
What to do: Don't rely on mushrooms as the only check. Look for the other signs too, bootlace rhizomorphs in the soil and white fungal growth under bark at the base, at any time of year, rather than waiting for a late-summer mushroom flush to confirm anything.
1 source
- RHSStates the absence of mushrooms is no indication the fungus is not active in the soil, and that many plants may be killed before mushrooms appear.
“Black shoestring-like growths in the soil near a dying tree confirm honey fungus.”
The black or brown root-like cords, called rhizomorphs, are a diagnostic feature of honey fungus and typically run 2.5-20cm below the soil surface in moist conditions, deeper if the soil has dried out. Penn State Extension independently describes the same structure in Armillaria root rot as dark, shoestring-like rhizomes beneath bark, on roots, and in the soil, which lines up with the RHS description rather than being a one-source quirk.
Our read Rhizomorphs are one of several confirming signs rather than the only one; peeling bark at the base to check for a sheet of white fungal growth (often with a mushroom smell) is the other main check, since not every infected plant shows rhizomorphs at a diggable depth.
What to do: Dig gently near the base of a suspect stump or dying woody plant and check for these cords, and also peel back a patch of bark at ground level to look for white fungal growth underneath. Either sign on its own is a reasonable basis to treat the plant as infected.
2 sources
- RHSDescribes black or brown root-like rhizomorphs developing mainly 2.5-20cm below the soil surface, plus white fungal growth under bark, as the two below-ground diagnostic signs.
- Penn State ExtensionDescribes dark, shoestring-like rhizomorphs beneath bark, on roots, and in the soil as a key diagnostic feature of Armillaria root rot.
“Digging a barrier trench around an infected area stops honey fungus spreading to nearby plants.”
A vertical physical barrier does block the specific route this fungus uses to travel, since rhizomorphs spread through the soil itself rather than through the air. The trench has to be deep enough to matter: a 45cm-deep strip of butyl rubber pond liner or heavy-duty plastic sheet buried in the soil is specifically what blocks rhizomorphs from crossing.
Our read This only checks out for the exact spec, not for a shallow decorative trench. Depth is the whole mechanism, since rhizomorphs can sit deeper than 20cm in dry soil, so a barrier shallower than 45cm leaves an easy route underneath it.
What to do: If protecting a healthy tree next to a known infection, sink a continuous strip of butyl rubber or heavy-duty plastic sheet at least 45cm deep between the two, with no gaps along its length.
1 source
- RHSStates a 45cm-deep vertical strip of butyl rubber pond lining or heavy-duty plastic sheet buried in the soil will block the rhizomorphs.
Frequently asked questions
What should I do first if I find honey fungus in my garden?
The first thing to try: Excavate and destroy all infected root and stump material, by burning or via landfill. Then: Install a 45cm-deep vertical barrier of butyl rubber or heavy-duty plastic to protect healthy plants nearby.
Is there a spray that works?
No. There are no amateur chemical treatments available for honey fungus. The controls that work are cultural: the measures listed above.
When should I watch for honey fungus?
The fungus itself is active in the soil year-round, but the honey-coloured mushrooms that give the clearest visible sign only appear from late summer into autumn.
Sources
Show the 6 sources
- RHS1: identification, symptoms above and below ground, the moisture-and-proximity trigger, life cycle and spread mechanism, removing infected material, vertical barriers, deep cultivation, absence of a chemical control, the 'most destructive' severity framing
- RHS1: honey fungus named among the problems on the apple growing guide
- RHS1: honey fungus named among the problems on the plum growing guide
- RHS1: honey fungus named among the problems on the damson growing guide
- RHS1: honey fungus named among the problems on the cherry growing guide
- Gardener's World12: apple and pear among the host plants, dieback, bleeding and cracking bark, digging out the plant with all its roots, no chemical controls, replanting with less susceptible plants and leaving the soil unplanted for six months to a year