The honey-coloured fruiting bodies of honey fungus (Armillaria spp) (image: Wikimedia Commons)
Honey fungi, Armillaria, are a group of parasitic fungi. They attack trees, shrubs and woody perennials, and are one of the most destructive fungal diseases in the UK.
They are also among some of the biggest living organisms in the world, their underground networks often covering many miles and living for up to a thousand years.
It is so successful because, unlike most parasites that rely on keeping their hosts alive in order to extract nutrients, it can kill its host and continue living on the decaying matter for many years. Watch our video on how to diagnose honey fungus
The fungi spread by long reddish brown root-like rhizomorphs that live close to the surface of the soil.
They attach themselves to the root collar of woody plants, killing off the root systems leaving the host unable to absorb nutrients and water. You can watch our video on how to diagnose honey fungus
Honey fungus symptoms
- The die back of upper parts of the plant. This can happen quickly particularly in hot dry weather, or may take years to gradually kill off branches.
- The leaves may fail to develop in spring, or be smaller and paler than usual.
- The plant may fail to flower, or in some cases suddenly flower and fruit profusely before dying.
- There can be signs of cracking and bleeding at the base of the trunk or stem. The red-brown rhizomorphs can sometimes be seen between the bark and the wood of trees.
- The fruiting bodies of the fungi are honey coloured toadstools. They may be seen in autumn growing on infected wood, at the base of a tree or on a nearby stump.
- Below the ground, the tree roots steadily rot from the ends inwards.
- The base of the trunk is often covered in white fungal strands (smelling strongly of that damp mushroomy odour) that can spread up between the bark and the wood, in severe cases for up to a metre.
Treating honey fungus infections
At present there is no chemical fungicidal control for honey fungus.
If it is identified in your soil, the only solution is to remove ALL infected material (or as much as you practically can), including stumps and roots. Then destroy all of this material by burning.
The fungus cannot survive on its own in the soil without its host. To ensure that you don't accidentally give spores a fresh host, it is vitally important to disinfect all tools after removal of infected material.
Preventing the spread of the fungus from an infected area is labour-intensive, but may well be your only course of action. You need to contain the fungus by burying a butyl rubber lining at least 45cm down, and which protrudes at least 2-3cm above soil level. This creates a barrier that the rhizomorphs cannot penetrate.
Once you have removed an infected plant or created a necessary barrier it is then possible to replant with a more resistant species.
No woody plant is completely immune to attack, but if you think that your garden has honey fungus, then there are a number of trees that are judged by the RHS to have good resistance levels. These include: Juglans nigra, Carpinus betulus, Larix, Quercus ilex and Taxus.
Have you had any first-hand experience of dealing with honey fungus? Please do let us know in the comments box below.