Honey fungus

(Armillaria species)

Honey fungus is a devastating disease of trees, shrubs and other woody, and some herbaceous, plants. It attacks and kills the roots, then rots the dead wood.

1. Woody and some herbaceous plants start to die back, often suddenly during hot, dry weather
2. Leaves are smaller and paler than usual
3. Leaves produce autumn leaf colour and drop prematurely
4. Reduced growth and vigour
5. Reduced flowering – or very prolific flowering, followed by heavy fruiting, which may be followed by total die back
6. “Honey” coloured toadstools produced around dead or dying plants in autumn
7. Dead, dying and decaying roots
8. Sheets of white or creamy-white fungal growth (mycelium) between the bark and the wood, which smells strongly of “mushrooms”
9. Black “bootlace” or “shoelace” rhizomorphs spreading horizontally in the soil

Honey fungus is probably the most devastating disease of trees, shrubs and other woody and some herbaceous plants. No plant is completely immune, but some show good resistance.

It attacks and kills the roots of affected plants, then rots the dead wood, leading to the whole plant dying. Some or all of the above symptoms may be present on affected plants.

The clumps of “honey” coloured toadstools may be the first obvious signs of the presence of honey fungus. They don’t always appear, but once they do the disease will already be fairly widespread. Also, they are often confused with other, harmless, saprophytic fungi. Luckily, the spores produced by the toadstools play little part in the spread of the disease.

It is usually recommended to search through the soil for the black rhizomorph “bootlaces” to discover whether honey fungus disease is present. However, the only sure-fire way is to look for the paper-thin sheet of white or creamy-white mycelial fungal growth that forms just underneath the bark, and which characteristically smells strongly of “mushrooms”. This forms at or just above ground level, rarely spreading higher than 1m (3ft). However, this does mean removing the bark, which will then kill the plant.

• Woody trees, shrubs and climbers
• Some herbaceous perennials

Honey fungus is caused by several different species of Armillaria fungi. Some strains are more virulent than others, and affected plants may live through attacks of the least virulent ones.

It spreads underground through direct contact between the roots of infected plants to those of healthy plants. It also spreads from affected plants through the soil by its rhizomorphs – black, root-like structures, often referred to as “bootlaces” or “shoelaces”. These usually spread at a depth of 15-45cm (6-18in), at a rate of up to 1m (3ft) per year or more.

Honey fungus is more likely to cause problems with plants that are under stress for other reasons, especially a lack of moisture/drought, too much water/waterlogging and underfeeding. So try to provide ideal growing conditions and keep plants growing as strongly as possible to give them a chance of fending off attacks.

Infected roots and stumps should be removed to prevent the disease spreading to unaffected plants.

Avoid replanting in an infected site for a minimum of 12 months to starve the fungus.

It is often believed that honey fungus can be brought into the garden in bark mulches or that it will spread through mulches, but in reality this is highly unlikely.

There are no chemicals available for the control of honey fungus disease.

With honey fungus, prevention is certainly a far better option than trying to control or cure an outbreak.

If honey fungus is known to exist in your locality, you can attempt to stop it proliferating in your garden by growing those plants that are known to be less susceptible – or where attacks are not known to occur – and certainly don’t grow plants that are known to be very susceptible.

Since honey fungus outbreaks often start on tree stumps, or dead or dying trees or shrubs, always completely dig these out or otherwise remove them. A stump grinder is the best way to completely remove the stumps of large trees.

Wherever possible, practical and feasible you can prevent honey fungus entering your garden by inserting a vertical physical barrier of butyl rubber (good quality pond liner) or very heavy-duty plastic sheeting 45cm (18in) deep. This will stop the spread of the rhizomorphs. It must protrude 2.5-4cm (1-1½in) above soil level.

This can also be tried within the garden to isolate affected plants and stopping it spreading to unaffected areas.

Regular deep soil cultivation will break up the rhizomorphs and may help to limit their spread.

• Butyl pond liners