Honey fungus can wreak havoc in gardens. Named after its honey-coloured mushrooms, which sometimes appear in late summer and autumn, it comprises several species in the Armillaria genus, which spread underground and attack and kill the roots of woody and perennial plants.
Native fungi, honey fungus usually causes few problems in the wild, where other fungi are able to compete with them and keep their growth in check. However, in managed situations like gardens, where fungi is not usually tolerated, honey fungus can have an advantage over other species and can therefore take hold, to devastating effect.
Most woody and herbaceous perennials are at risk from honey fungus, including birch, hydrangea, privet, apple, pear, magnolia and beech. Roses, flowering currant, willow and viburnum seem particularly vulnerable.
How to identify honey fungus
Honey fungus symptoms to look out for include: die-back, pale foliage, an absence of flowers, bleeding and cracking bark, and eventual death.
Honey fungus can be hard to identify as it spreads underground and doesn’t always bear fruiting bodies (mushrooms or toadstools) above ground. Indeed, it can take years for the fungus to kill the shrub or tree, and some symptoms, such as dieback, may be mistaken for symptoms of other problems such as a lack of water or planting in the wrong place.
If you suspect your shrub or tree is affected with honey fungus, it’s best to peel back some of the bark at ground level. Here, strings of white ‘mycelium’ growing between the bark and the wood will signify infection. Other symptoms include bootlace-like fungal growths known as rhizomorphs, which appear red or purple at first, and then mature to black. Finally, the honey-coloured mushrooms themselves may appear in autumn.
Honey fungus mushrooms grow in clumps and can be different sizes. A honey fungus mushroom has a white ring of tissue, known as a annulus, growing around the stalk just below the mushroom cap. However these are often hard to find and may be barely noticeable. Although the mushrooms usually appear on infected tree stumps, they can also pop up in soil, growing from rhizomorphs below ground. These rhizomorphs can extend up to 30m from the original source of infection.
How honey fungus causes problems
There are different species of honey fungus, which vary in the extent to which they cause plants harm. The more aggressive species can kill otherwise healthy plants but some will simply restrict growth or kill only already weakened plants. Therefore it may not be necessary to try to eradicate the fungus, but focus on keeping your plants healthy and living with it.
Honey fungus and the law
There are no laws to consider when tackling honey fungus. However it’s neighbourly to not allow the spread of the fungus into neighbouring gardens.
How to remove honey fungus
There are no chemical controls for honey fungus at present. The only option is to dig out the affected plant, complete with all its roots, and burn it or take it to landfill. This should be enough to kill the bootlace rhizomorphs that spread out from the host plant, but if the fungus has already taken hold on other plants, you may need to remove them as well.
Other tips to control the spread of honey fungus in the garden include:
- Keeping your plants healthy (well-watered, well-pruned and not stressed)
- Removing a layer of soil around the base of plant stems – trees and shrubs planted too deeply are more likely to be affected
- Removing as much of the infected plant, including the roots and soil, as possible. Don’t leave stumps
- Removing plants growing around the affected plant, particularly in a hedge
- Digging the soil to root out and break up infected roots or rhizomorphs
- Avoiding planting anything back into the soil for six months to a year, to kill off the fungus
- Replanting with less susceptible woody plants
- Recording where you’ve found honey fungus, to map which areas of soil are affected
