Honey Fungus - The Root of All Evil?

Some species of Honey Fungus are parasitic and can cause considerable damage to coniferous forestry plantations and gardens alike. However, most species are saprophytic and only degrade dead or dying woody material. It is essential to identify the species of Honey Fungus present before embarking on any course of remedial action.

 

Common Name

Honey Fungus or Bootlace Fungus

 

Botanical Name

Armillaria species (meli is ancient Greek meaning honey)

 

Diagnosis & Symptoms:

  • A tree has suddenly died or has died after a period of increasing ill health.
  • The crown of a tree shows a general and increasing deterioration over a period of time.
  • A tree has blown over to reveal a decayed root system.
  • Gum, resin or a watery liquid may exude from the tree stem.
  • Brown or honey-coloured toadstools (Figure 1) appear in clumps, in autumn, typically on or near the stem base.
  • The bark at the base of the stem is dead. Underneath the bark is a sheet of white or cream-coloured fungal tissue which later turns black (Figure 2).

 

  Honey Fungus Toadstools   Rhizomorphs or Bootlaces Honey Fungus 

 

Significance

Honey fungus kills the roots of the host plant. It then degrades the root system and the lower stem leaving the tree potentially unstable and liable to fail.

 

How it Spreads

The fungus spreads from infected plants by direct contact with the roots of neighbouring plants. It also spreads through the soil by means of the bootlace or rhizomorph structures.

 

Control – Cultural

Maintain a healthy plant stock – trees and shrubs under stress are more susceptible to infection.

Dig out and dispose of all infected plants with all of their root systems, if possible.

It may be possible in some situations to protect trees and shrubs with a sunken barrier.

Where A mellea or A ostoyae are present in the immediate area, avoid planting susceptible species of trees and shrubs (please see below).

 

Control – Chemical

The consensus of opinion (the RHS, The Forestry Commission and others) is that there is no effective chemical control although specific soil fumigants may kill the fungus in the stumps of small trees.

 

Geographical Distribution in the UK & Ability to Cause Disease

A saprophyte: an organism, especially a fungus or bacterium, that obtains food from dead or decaying organic matter. Encarta Dictionary

A parasite: a plant or animal that lives on or in another, usually larger, host organism in a way that harms or is of no advantage to the host. Encarta Dictionary

Pathogenicity: from pathogenic meaning causing disease, or able to cause disease. Encarta Dictionary

 

Species Distribution Habitat Pathogenicity
A. borealis Not reported in England, rare in Scotland Saprophytic only. Mainly recorded on birch in Scotland Weak parasite on birch and wild cherry
A. cepistipes  Rare in England, common in Scotland Saprophytic only  Relatively benign
A. gallica A common low altitude species Usually on soil near hardwoods Relatively benign. Weak or secondary parasite of hardwoods
A. mellea Common especially in SE England Usually with hardwoods, ornamental and orchard trees Aggressive parasite of deciduous trees
A. ostoyae Common throughout the UK Mostly restricted to conifers Serious parasite of conifers
A. tabescens Only reported in SE England Mostly saprophytic with oak stumps Reported to be aggressive towards Eucalyptus species

The above table is based on: Honey Fungus - Friend or Foe? By Paul F Hamlyn

 

Susceptible and More Resistant Tree & Shrub Species

 

Susceptible Trees and Shrubs include More Resistant Trees and Shrubs include
Azaleas Ash
Apples - all species Black walnut
Apricots Box
Birch Beech
Cedars Fothergilla
Cherries Holly
Cotoneaster Judas Tree
Forsythia Juniper
Hydrangea Larch - all species
Leyland cypress London Plane
Lilac Oak - all species
Peaches Pittosproum
Plums Plumbago
Privet Smoke Bush
Rhododdendrons Spruce
Viburnum Sumach
Willows Tamarisk
Wisteria Yew

Based on: Gardening Advice: Honey Fungus from the RHS and Diagnosis of Ill Health in Trees published by The Forestry Commission.

 

Summary

All too frequently Honey Fungus is found and identified as the primary cause of a tree or shrub failing in the garden. In many cases, however, Honey Fungus is a secondary cause colonizing an already stressed or weakened tree. In short, Honey Fungus can and does cause extensive damage in the garden but it is not 'the root of all evil'.

 

References

www.fungus.org.uk

www.rhs.org.uk

Diagnosis of ill-health in trees by RG Strouts and TG Winter published by The Forestry Commission

Don’t Worry (too much) Honey? TDA No.105 published by The Tree Advice Trust