honey fungus (Armillaria root rot)

Symptoms

Plants decline, leaf out poorly, then whole branches or the whole shrub/tree dies — often suddenly in summer; under the bark at the base and on the roots there is a sheet of white fungal growth that smells strongly of mushroom; flat black "bootlace" rhizomorphs run through the soil; honey-coloured toadstools may appear in clusters at the base in autumn.

Plants affected

• apple
• sweet cherry
• arborvitae (thuja)
• silver birch
• forsythia / border forsythia
• black mulberry
• black poplar
• Cyprus golden oak
• sessile oak
• white willow
• European hornbeam
• European ash

Easily confused with

• bacterial canker of cherry

  How to tell them apart: Pseudomonas syringae pv. morsprunorum (bacterial canker) shows sunken, dark cankers on branches and trunk that often ooze amber-to-clear gum (gummosis); affected limbs leaf out then suddenly wilt and die back from the canker outward, and leaves develop angular brown 'shot-hole' spots. Damage starts above ground on specific branches and the bark at the canker is discoloured and may smell sour when cut. Armillaria mellea (honey fungus) attacks the roots and crown at soil level: peel back the bark near the base and you find a white, fan-shaped mat of mycelium smelling strongly of mushrooms, plus black bootlace-like rhizomorphs in the soil. With Armillaria the whole tree declines uniformly from the bottom up, and honey-coloured toadstools may cluster at the trunk base in autumn.

• red-necked longhorn beetle

  How to tell them apart: Aromia bungii (red-necked longhorn beetle) reveals itself by piles of fibrous, sawdust-like frass and coarse wood shavings pushed out of bark cracks low on the trunk and main scaffold limbs, with oval-to-round exit holes (about 6-10 mm) and adults that are glossy black with a bright orange-red 'collar' (pronotum). Damage is borer tunnelling under the bark, so wilting branches dry out while the wood beneath shows galleries packed with frass. Armillaria mellea (honey fungus) instead shows creamy-white, fan-shaped mycelial sheets between bark and wood at the root collar, black bootlace-like rhizomorphs in the soil and on roots, and honey-coloured toadstools clustered at the tree base in autumn. With Armillaria the bark at the collar is sunken and rotten and there are no insect exit holes or sawdust frass. NOTE: Aromia bungii is an EU-quarantine priority pest — if you see the orange-collared beetle, oval exit holes or wood-shaving frass, report it to your National Plant Protection Organisation (NPPO) immediately.

• bronze birch borer

  How to tell them apart: Agrilus anxius (bronze birch borer) attacks the upper trunk and main branches: look for raised, rusty-brown lumpy ridges over the bark, tiny D-shaped (flat-topped) exit holes about 3-5 mm wide, and serpentine, sawdust-packed larval galleries winding just beneath the bark when you peel it back. Dieback starts at the very top of the crown and works downward, with sparse yellow foliage on stressed, often sun-exposed birches. Armillaria mellea (honey fungus) attacks the base and roots: peel the bark at the collar to find flat, white, fan-shaped sheets of mycelium that smell of mushrooms, and dig at the soil line for black, bootlace-like rhizomorphs. In autumn, clumps of honey-coloured toadstools with a ring on the stem appear at the trunk base or over the roots.

• Phytophthora root + crown rot

  How to tell them apart: Armillaria mellea (honey fungus): peel back the bark at the base of the trunk and roots — you find flat, fan-shaped sheets of creamy-white to cream mycelium with a fresh mushroom smell, plus black, bootlace-like rhizomorphs running through the soil and over the roots. In autumn, clumps of honey-coloured toadstools with a ring on the stem appear at the collar. Decay is moist and stringy. Phytophthora cinnamomi (root and collar rot): the bark at the collar shows dark, sunken lesions and reddish-brown to black staining of the inner bark, often with a dark, sometimes resin- or sap-bleeding margin where dead tissue meets healthy wood. There are no white fans, no rhizomorphs and no toadstools; fine roots are blackened and rotted, and decline is worst on wet, poorly drained or waterlogged soils.

• lavender root rot

  How to tell them apart: On a declining Thuja occidentalis, Armillaria mellea and Phytophthora nicotianae produce the same dull yellowing and thinning canopy, so confirm by examining the root collar. For Armillaria, peel the bark near the base to find creamy-white, fan-shaped mycelial sheets that smell of mushroom, plus black bootlace-like rhizomorphs in the soil and honey-coloured mushroom clusters at the stem base in autumn. For Phytophthora nicotianae there are no fans or rhizomorphs; instead you see a dark, water-soaked or resin-stained lesion at the crown with reddish-brown discolouration under the bark, almost always on heavy, wet or waterlogged soil.

• cypress canker

  How to tell them apart: On Thuja occidentalis both Seiridium cardinale (cypress canker) and Armillaria mellea (honey fungus) cause whole-plant browning and scattered dead branches, but the tell-tale signs are in different places. Seiridium shows discrete sunken cankers on twigs, branches and the upper stem, often cracked with reddish-brown bark and a characteristic bleeding of clear-to-amber resin around the lesion; foliage beyond a girdled branch turns rusty-brown while the rest of the plant may stay green, giving a patchy 'flagging' look. Armillaria attacks from below: peel back the bark at the root collar and base of the trunk and you find a creamy-white fan of mycelium smelling of mushrooms, often with black bootlace rhizomorphs in the surrounding soil and honey-coloured toadstools clustered at the base in autumn; decline here is more uniform and the whole plant fades together rather than branch by branch.

• emerald ash borer

  How to tell them apart: On Fraxinus excelsior both Agrilus planipennis (emerald ash borer) and Armillaria mellea (honey fungus) produce a top-down thinning, dying crown with epicormic shoots, but the decisive signs differ in location and form. Agrilus planipennis is confirmed on the TRUNK and branches: tiny 3-4 mm D-shaped (flat-topped) exit holes, vertical bark splits, and S-shaped serpentine larval galleries packed with pale sawdust-like frass just under the bark, often with jagged woodpecker flecking. Armillaria mellea is confirmed at the ROOT COLLAR and base: creamy-white, fan-shaped mycelial sheets that smell of mushroom beneath the bark, flat black bootlace-like rhizomorphs on the roots and in the soil, and clumps of honey-coloured toadstools at the stem base in autumn. Round holes or no holes plus white fans and bootlaces = fungus; D-shaped holes plus snaking galleries = beetle.

• ash dieback (Chalara)

  How to tell them apart: Armillaria mellea (honey fungus) and Hymenoscyphus fraxineus (ash dieback) both thin the crown and kill ash branches, so check the stem base and the dead twigs. For Armillaria, peel the bark at the root collar: a creamy-white, fan-shaped sheet of mycelium that smells of mushrooms lies between bark and wood, black shoelace-like rhizomorphs run through the soil and roots, and in autumn dense tufts of honey-brown toadstools with a ring on the stalk erupt at the base. For ash dieback, look up the shoots instead: elongated diamond-shaped lesions with a dark brown sunken centre girdle young stems where a side-shoot died, leaf stalks and the midrib turn blackish-brown before the leaf wilts, dead side branches trigger bushy epicormic shoots on the trunk, and in summer tiny white stalked cups appear on last year's blackened leaf-stalk debris on the ground.

• vine weevil

  How to tell them apart: Both Armillaria mellea (honey fungus) and Otiorhynchus sulcatus (vine weevil) attack roots and the root collar of privet, so above ground both can look identical: yellowing leaves, wilting, branch dieback and sudden collapse of an otherwise healthy shrub. To separate them you must dig: Armillaria shows flat, fan-shaped sheets of creamy-white mycelium beneath the bark at the base of the trunk, plus tough black bootlace-like strands (rhizomorphs) in the soil and honey-coloured mushrooms in clusters at the stem base in autumn. The vine weevil instead reveals fat, C-shaped, legless white grubs with a tan-brown head feeding in the rootball, and stripped or stubby roots — never mycelial fans or black laces. A useful above-ground clue for the weevil is irregular notching cut into the leaf margins by the night-feeding adult beetles, which Armillaria never produces.

Common questions

How do I recognise honey fungus (Armillaria root rot) on apple?

Plants decline, leaf out poorly, then whole branches or the whole shrub/tree dies — often suddenly in summer; under the bark at the base and on the roots there is a sheet of white fungal growth that smells strongly of mushroom; flat black "bootlace" rhizomorphs run through the soil; honey-coloured toadstools may appear in clusters at the base in autumn.

Do I have to report honey fungus (Armillaria root rot)?

No. honey fungus (Armillaria root rot) is not a notifiable organism, so there is nothing to report to an authority. Treat it as a normal garden problem.

How is honey fungus (Armillaria root rot) treated?

Mainly with biological and cultural measures — removing affected material, changing the conditions that favour it, and treating with agents approved in your country. The exact steps and their timing depend on the plant, the season and where you live: Dr. Phyto works them out in a free diagnosis.