What is honey fungus?
Honey fungus, or Armillaria mellea, is a parasitic fungus that damages and kills the roots of many trees and shrubs causing the plant to die. It is the single most destructive plant disease in the UK and has been top of the RHS annual disease and pest ranking for over 20 years. Armillaria particularly thrives during warm, dry summers when plants are weakened by higher stress levels.
How to spot honey fungus and what does it look like?
The most obvious sign of honey fungus is discovering papery whitish strings of mycelia beneath the bark at the base and roots. Sometimes this is visible but if unsure peel away a little bark to check.
In autumn fruiting bodies appear above ground. These can be an orangey honey colour (hence the name) to various shades of brown. Not finding these toadstools does not mean you don’t have the fungus. Likewise removing the toadstools will not get rid of the fungus.
Other signs (see What damage does it cause? below) can be harder to detect as they may affect the host plant for several years before causing its death and can often be mistaken as symptoms of other problems, such as drought.
What trees does honey fungus affect?
Honey fungus is spread to a large range of trees, including:
- Alder (Alnus)
- Ceanothus (California lilac)
- Horse chestnut (Aesculus)
- Katsura tree (Cercidiphyllum)
- Privet (Ligustrum)
- Sorbus (Rowan)
- Willow (Salix)
How does honey fungus spread?
Honey fungus spreads via dark reddish brown/black bootlace strings (rhizomorphs) that travel through the soil. These can be found an inch to eight inches below ground, sometimes even deeper and can travel over three feet a year which is why it is vital to treat outbreaks thoroughly.
How can you treat honey fungus?
There are no sprays or treatments available to tackle honey fungus so the bad news is that once you are certain you have it in your garden there is no alternative but to dig out the affected plant and destroy it completely by burning or taking it to landfill. Make sure to properly remove all the roots. Once the host plant is removed the bootlace rhizomorphs spreading out from it can no longer survive.