Honey Fungus: A Sticky Issue

Two words that are guaranteed to strike fear into a gardener’s heart: honey fungus. There is a widespread myth that this is a death sentence for your plants and you might as well give up now. Yes, it is a difficult fungus to deal with, but by adapting what and how you grow you can still have a beautiful garden.

“Honey fungus” is actually a name given to several species of one of the most widely distributed fungi in the world, Armillaria. It invades and kills the roots of susceptible woody and perennial plants and is known to be one of the world’s largest living organisms. A single specimen, growing in Oregon, USA, is estimated to be over 2,400 years old and covers some 8.4 sq. km.

You may see Armillaria as honey-coloured mushrooms in autumn around or on the bases of infected plants, but you will also see white fungus between the bark and the wood of the tree.

It spreads underground by black, bootlace-like root structures called rhizomorphs, although these are not always easy to spot. They can be as shallow as 15cm or as deep as 45cm and can spread as fast as 1m a year, so it is very efficient at finding new host plants. Favourite hosts are susceptible species or plants that are already under stress for another reason.

Is it honey fungus?

If you have a plant that dies quite quickly, then honey fungus is always a possibility, but so are other issues like an excess or lack of water. Symptoms to look for are:

• Branches or shoots dying, especially in summer when water supplies are lower
• Leaves that are smaller and paler than usual
• Heavy flowering and fruiting (as the plant tries to ensure continuation of the species)
• Early autumn colour
• Cracking of bark or “bleeding” of resin at the base of the stem
• Honey-coloured mushrooms around the base in autumn

Susceptible plants

No plants are completely immune to attack, but some are more prone than others. These include: Aesculus (horse chestnuts), birch, Buddleja, Ceanothus, cedar, Cercidiphyllum, Cotoneaster, leylandii, Forsythia, walnut, Laburnum, privet, Liquidambar, Photinia, oak, Rhododendrons, willows, Sorbs, lilac, Thuja, Viburnum and Wiegela.

What to do?

First things first: don’t panic! There’s no need.

If the fungus has a live host, the spread appears to slow down until it has been killed, then another is required. If you have an infected plant, do not be tempted to dig it up and burn it straight away. It is possible to dig down and place a barrier around it using pond liner or a strong plastic sheet. This should go down to 50cm below the soil surface and protrude at least 5cm above it.

If you see the mushrooms:

• Spray them with water and sprinkle sugar over them. This overloads them with carbohydrates and weakens them OR
• Sprinkle bicarbonate of soda (baking soda) over them as this is alkaline and the fungus prefers acidic conditions.

Tolerant plants

The Royal Horticultural Society maintains a comprehensive and very useful list of plants that show tolerance to honey fungus, although none can be described as being truly “resistant”. See the list here: https://www.rhs.org.uk/advice/pdfs/honey-fungus-host-list.pdf

When you add new plants

As you plant, add friendly mycorrhizal fungi around the rootball to help the new plant establish and give it the best chance of survival. A healthy plant will be better able to fight off attack by any pests and diseases. For this reason, one of the best things you can do for all your plants is feed them. See Fertilisers

Newer developments in biological control

Research is progressing into using antagonistic fungi such as Trichoderma and Dactylium species to control Armillaria mellea (one of the main species) through antibiotic production, competition for resources and also direct parasitism. Trichoderma occurs naturally in soils and may prove to be very useful in the fight to control the spread of honey fungus.