Honey fungus: symptoms and treatments

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.

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What damage does honey fungus cause?

• Tips of branches dying away, particularly in hot weather
• Poor or pale foliage
• No flowers
• Bleeding and cracking bark especially at the base of stems
• Death of plant

What plants does honey fungus attack?

Honey fungus can attack most woody and herbaceous perennials. Following comparisons of over 5,000 recorded cases by the RHS and a study into susceptibility by the University of California the RHS has released a list of trees, shrubs and hedges most at risk of honey fungus and those with a moderate and low risk of infection, so if your plot is riddled with honey fungus all is not lost.

Most vulnerable to honey fungus: lilac, buddleia, cotoneaster, forsythia, willow flowering currant, viburnum, privet, birch, leyland cypress, viburnum, hawthorn, eleagnus, walnut

Moderately vulnerable: apple, maple, prunus, magnolia, roses, hornbeam, roses, holly

Low vulnerability: olive, yew, laurel, honeysuckle, box, lavender, pear, fig, mulberry, rosemary, camellia, heather

Download the full PDF from the RHS here

What trees does honey fungus affect?

Honey fungus is spread to a large range of trees, including:

•  Alder (Alnus)
• Amelanchier
• Apple
• Beech
• Birch
• Ceanothus (California lilac)
• Cedar
• Cotoneaster
• Fir
• Hawthorn
• Holly
• Horse chestnut (Aesculus)
• Katsura tree (Cercidiphyllum)
• Magnolia
• Pear
• Privet (Ligustrum)
• Rhododendron
• Sorbus (Rowan)
• Lilac
• 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.

Is honey fungus dangerous?

When eaten some people have allergic reactions and honey fungus can be poisonous when not cooked properly. Some varieties are also known to cause sickness when eaten within a day of drinking alcohol, and people have also reported dogs becoming sick after consuming honey fungus.

Honey fungus facts

• The largest known organism in the world by area is a honey fungus (Armillaria ostoyae, or dark honey fungus) that spans over 8.9 km2 (2,200 acres) in Malheaur National Forest, Oregon. It is estimated to be 2,400 years old and weigh 605 tons.
• Honey fungus is edible but not recommended because of the risk of allergic reaction.
• Armillaria mellea is bioluminescent.

Visit our section on Pests and Diseases for more solutions to garden problems, including vine weevils, box tree moths and powdery mildew

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