DISEASE
 
   NAME
     Armillaria Root Rot, Honey Fungi
     Armillaria luteobubalina
  
Description
This naturally occurring fungus grows between the bark and wood of trees producing distinctive cream sheets of hyphae. The toadstool-like fruiting bodies are yellowish brown and appear from the soil or sprout out of the base of the host plant.

Image by Dr Brett Summerell

Symptoms
This is a vigorous fungus that attacks the roots of trees and is not normally noticed until dieback starts to occur. Affected plants generally appear declined, with some dieback and by this stage; the disease may be well established. Leaves can turn yellow, shrivel and fall from the plant and the branches die back. Citrus trees may produce a heavy crop of fruit just before death. Splits often occur on the trunk of affected trees and the bark may lift revealing white sheets of mycelium or hyphae under the bark. These hyphae will have a distinctive “mushroom” smell. Affected roots become spongy, powdery or jelly-like and when dissected reveal similar white sheaths under the bark. Infected plants may survive for many years before final death of the tree.

Image by Dr Brett Summerell

Source and Dispersal
This fungus is a native species that is naturally found in Eucalyptus forests and woodlands throughout eastern Australia and south-western Australia. It is commonly found on old tree trunks and decaying wood where honey coloured toadstools appear from the base of the host or surrounding soil in May-June. The toadstools produce white spores that are dispersed by wind, but generally the fungus spreads underground through contact between infected and uninfected roots. The fungus can grow about 1-1.5 metres per year along a root. This species, unlike those in the northern hemisphere does not produce rhizomorphs, thick strands that are flat resembling shoe-strings.

  Mycelium on wood  
Image by Dr Brett Summerell

The fungus is not spread by contaminated soil as it only grows in root material. Spread to new sites is either through spore movement (very rare), movement of infected plants or through movement of contaminated wood chip used for mulch. Dead or dying trees should be inspected for the presence of this pathogen prior to use as mulch.
This fungus can survive for many years in infested root and stem material depending on the size of the material and speed of decomposition of the wood. 

Favoured Conditions
This fungus prefers sandy soil types and is more frequently found in more freely drained soils. Moisture is required for growth of the fungus along the root system. Generally the damage caused to plants is greater on plants that are already under stress or weakened. The retention of infested root systems and stumps in the soil has contributed to an increase in the occurrence of this disease. 

Affected Plants
The host range for this fungus is extremely wide and includes many ornamentals and Australian native plants. Fruit trees and perennials are also commonly attacked. The most susceptible species include oaks, camellias, azaleas, roses and eucalypts. 
Amaryllis, Narcissus and Hippeastrum species are also infected.

   Cedrus species trunk     Image by Dr Brett Summerell

Cedrus species are also affected by Armillaria root rot causing the roots to rot and the trunk to swell and spilt open. There is no control for these infections.

Non-chemical Control
Control of this disease is totally dependent on removal of the inoculum of the fungus from the soil. To be effective this will require removal of the infested roots and stem, a process that may be difficult in garden beds. Infected plants should be removed and disposed of, but it is not necessary to remove the surrounding soil as the fungus only occurs in the plant.  When clearing affected land for cultivation, remove all stumps and roots and allow 2 to 3 years prior to replanting. 

Trees that are in the early stages of attack may be saved by removal of the affected roots and leaving remaining roots exposed for several years. Top soil around the trunk should also be removed for a distance of up to 1m and affected trees should be fertilised and watered to encourage vigour.

Chemical Control
There is no practical or effective chemical control. Fumigation has been carried out to eradicate the fungus but success is dependent on the removal of large sources of inoculum. However many of the chemicals used for this purpose are highly toxic and have restricted usage now.
Note
Always read the label for registration details and direction of use prior to application of any chemicals.

Amendments by
Dr Brett Summerell
Director Science and Public Programs
Royal Botanic Gardens Trust, Sydney