Honey fungus

Confirm it before you believe it

More garden plants are declared dead of honey fungus than die of it. Drought, waterlogging, root damage from building work and Phytophthora all kill shrubs in ways that look identical from above, and the diagnosis usually gets made from a clump of toadstools in the lawn.

There are three signs, and they are not equally reliable.

The definitive one is a sheet of creamy white fungal growth between the bark and the wood at the base of the trunk, extending down into the main roots. Cut a flap of bark away below soil level with a knife and look. It is white to cream, sheet or fan-shaped, and it smells powerfully of mushroom. Nothing else does that.

The second is the rhizomorphs. Black, flattened, tough strands one to three millimetres across, like bootlaces, running through the soil around the roots and under the bark. Snap one and it is white inside. These are how the fungus travels between plants, and it moves through soil at up to about a metre a year.

The third is the toadstools, and they are the weakest evidence. Honey-coloured caps with a ring on the stem, in dense clumps at the base of a stump or over a root run, in autumn. They appear in some years and not others, and several harmless fungi are similar enough to fool anyone.

Not every Armillaria is the same

Britain has several species and they differ considerably in what they do. Armillaria mellea is the one that kills garden shrubs. Armillaria gallica lives mostly on dead wood and takes only plants already failing for another reason. Armillaria ostoyae is largely a conifer problem.

That is worth knowing because a clump of toadstools on an old stump at the end of the garden, with everything around it in good health, is a fungus doing the job fungi do. Toadstools plus a dying hedge is a different conversation.

Why there is no cure

No fungicide is approved for honey fungus in Great Britain, at any scale, for amateur or professional use. Products that were once recommended for it, including Armillatox and Jeyes Fluid, are not approved as pesticides and cannot legally be used as one. They are sold as cleaning products.

The reason is structural rather than a gap in the market. By the time a plant shows symptoms, the fungus is inside the root system and behind the bark, and it is also in the soil for metres around as rhizomorphs and in every piece of buried woody debris. There is nothing to spray and nowhere to spray it.

Saying so plainly is more use than the hedged non-advice that fills this search. What follows is management, and management works.

What to do about it

Dig out the dead plant with as much of the root system as you can get. This is the one action that changes the outcome. The fungus lives on that dead root mass and uses it as a base to reach whatever you plant next, so a stump ground level with the soil and its roots left in place has removed the eyesore and none of the problem.

Then go looking for the other reservoirs. Old stumps from trees felled years ago, buried fence posts, lengths of timber left in a trench, a heap of prunings in a corner. Every one is a food source.

Where you want to protect something specific, sink a barrier. A vertical sheet of heavy butyl or thick plastic to 45 centimetres, with a couple of centimetres proud of the surface, stops rhizomorphs at the line.

Keep everything else growing strongly. This fungus is far more damaging to stressed plants, so watering through a drought, mulching and keeping mowers away from trunks all raise the odds for the plants you still have.

What to plant afterwards

Two routes are open, and the second is better than most people expect.

Choose woody plants with some resistance and accept that resistance is relative. Yew, box, hornbeam and beech come through infected ground far better than privet, rhododendron, rose, apple, birch, wisteria, viburnum and hydrangea, which are among the readiest to die. Check a current resistance list rather than working from memory, because these lists get revised.

Or stop planting woody things there. Honey fungus decays woody tissue, and herbaceous perennials, bulbs, ferns, grasses and annuals are broadly unaffected. A boundary that has swallowed three shrubs in five years can become an excellent perennial border, and that is a real answer rather than a consolation.

What it gets mistaken for

Phytophthora root rot also kills shrubs from the roots up and is common on wet ground. It leaves no white sheet under the bark and no bootlaces, and the affected roots go dark and brittle. Rhododendron, yew and lawson cypress are common victims of that one.

Waterlogging kills roots outright and produces the same crown die-back and sudden collapse, usually after a wet winter on heavy ground, with several plants affected across a low-lying area rather than spreading from one point.

Fairy rings and the general run of autumn toadstools in a lawn are fungi feeding on organic matter in the turf. They are not attacking anything, and they need no action beyond a mower.

Bracket fungi on a standing trunk are decay fungi of the wood rather than root killers, and they indicate a different problem, usually one for an arboriculturist to look at on safety grounds.