Honey Fungus: The Tree Killer

What is Honey Fungus?

Honey fungus is a spreading, parasitic fungus that live yeah s on trees, woody shrubs, and occasionally herbaceous perennials.

It plays a crucial role in the regeneration of forests: at the destruction end!
It is capable of killing complete woodlands, which is wonderful for the churning gyre of biodiversity and evolution, but no fun for the human owners of those trees.

It’s one of the biggest living organisms in the world. The largest recorded honey fungus is nearly 4 miles square and several thousand years old.

Characteristics

• Honey fungus spreads through bootlace-like structures called rhizomorphs, which grow under the soil at a rate of about 1 metre a year.
• Rhizomorphs are red-brown or black and usually hard to find.
• Mushrooms appear in winter between November and January on wood, growing in dense clusters with sticky, yellow-brown caps.

How Does it Kill?

• Honey fungus kills its hosts by growing through the tree and encircling the cambium layer at ground level, cutting off the supply of sap and nutrients.
• Infected plants will suddenly start to die back or fail to produce leaves in the spring.
• Resin can seep from the trunks of conifers
• A “swan song” may occur where plants under attack flower and fruit before dying.
• Affected plants are covered in mycelium under the bark, a white fungal sheet, which has a strong mushroom smell.
• Unlike most parasites, which need a living host, honey fungus can kill its host and continue living on the decaying matter for many years.

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.

To reduce the impact of honey fungus in your garden:

1. Remove infected plants: Take out infected plants and as much of their root system as possible to prevent the fungus from spreading. If a tree is too big to pull, have the stump ground out until it is at least 8 inches (20cm) below soil level.
2. Prevent future infections: Get rid of trees that have died for other reasons and disinfect all tools after removal of infected material.
3. Improve soil quality: Mulch regularly with good organic matter to keep plants healthy and less vulnerable to honey fungus attacks.
4. Destroy infected material: Burn all infected material, including stumps and roots, to prevent the fungus from spreading.
5. Create a barrier: Bury a butyl rubber lining at least 45cm down and 2-3cm above soil level to create a barrier that prevents rhizomorphs from penetrating.
6. Choose a resistant species: Once you have removed an infected plant or created a necessary barrier, replant with a more resistant species (see lists below).

Honey Fungus Resistant Trees and Shrubs

No woody plant is completely resistant to honey fungus.
Only herbaceous perennials (and annual bedding plants, but they only live for a year anyway) are generally not at risk.

The RHS has a complete list of honey fungus resistant and susceptible plants, separated into four risk categories:

1. Top honey fungus host
2. Frequently affected
3. Sometimes affected
4. Rarely affected

Listed below are the “rarely affected” and “sometimes affected” trees, shrubs, and climbers that we grow here at Ashridge:

Most Resistant, Rarely Affected Trees

• Acacia, Mimosa
• Arbutus, Strawberry Tree
• Ficus, Fig Trees
• Ginkgo biloba, Maidenhair Tree
• Larix species, Larch
• Laurus, Bay Leaf Laurel. Good evergreen hedging.
• Morus, Mulberry
• Pyrus, pear trees: more the ornamentals like Chanticleer or Pendula than edible pears, which are mostly grafted onto quince rootstocks; quince is “sometimes affected”, see below
• Taxus baccata, Yew. The King of formal, native evergreen hedges.

Most Resistant, Rarely Affected Shrubs

• Buxus sempervirens, Boxwood. Slow growing evergreen hedging, very shade-tolerant.
• Callistemon, Bottle Brush Tree
• Convolvulus
• Griselinia. Good evergreen hedging.
• Hydrangea
• Hypericum
• Lavandula, Lavender
• Lonicera nitida, shrub honeysuckle. Good evergreen hedging.
• Philadelphus, Mock Orange. Decent informal hedging.
• Pittosporum
• Rhamnus species, Buckthorn. Good country hedging.
• Rosemary
• Rubus species, which includes Blackberry and Raspberry. .
• Salvia species
• Sarcococca, Sweet Box
• Symphoricarpos, Snowberry
• Tamarix. Good coastal hedging.
• Vaccinium, which includes blueberries and cranberries.

Moderately Resistant, Sometimes Affected Trees

• Acers, Maple and Sycamore. Field maple is good country hedging.
• Carpinus betulus, Hornbeam. Good formal hedging.
• Corylus, hazel. Good native hedging.
• Cydonia, quince
• Eucalyptus
• Euonymus
• Fagus, beech. The Queen of formal hedging.
• Ilex, holly. Good native evergreen hedging.
• Malus, Apples and Crab Apples
• Picea species, Spruces like Blue Spruce
• Pinus species, Pines like Scot’s Pine
• Populus species
• Prunus species, which include fruit trees like Cherry and Plum, and evergreen hedging like Cherry Laurel.
• Sambucus, Elderflower, can be used as country hedging, ideally as a single species.
• Tilia species, Lime trees
• Ulmus species, Elm trees

Moderately Resistant, Sometimes Affected Shrubs

• Acer negundo
• Actinida
• Berberis species, Barberry. Good thorny hedging.
• Cornus, dogwood. All are good hedging; Cornus sanguinea is the native species.
• Hebe
• Lavatera
• Perovskia
• Potentilla
• Roses
• Senecio

Most Resistant, Rarely Affected Climbing Plants

• Akebia, Chocolate Vine
• Hedera, Ivy
• Hydrangea
• Jasminum species, Jasmine (not Trachelospermum species, which are sometimes affected)
• Lonicera species, Honeysuckle
• Passiflora, Passion Flower
• Wisteria

Should gardeners live in fear of Armillaria?

In my opinion, having lost more than a couple of trees to it down the years (a lovely opportunity to grow something else), not really.

Honey fungus is found in practically every location that has groups of mature trees and rotting wood on the ground. In the wild, it is “in balance” with all sorts of other organisms, playing an essential role in the ecological turnover of forests.

It is mainly orchard owners and forestry managers who have a real cause for concern. Firstly, it’s their livelihood, and secondly man-made monocultures create the conditions where any pest or parasite can go berserk on the banquet before them, with no biological bouncers to bring them back into ‘balance’.

Have you had any first-hand experience of dealing with honey fungus? Please do let us know in the comments box below.

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