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Honey Fungus: (Armillaria species)

This is a particularly virulent and in all cases fatal root and root collar killing/root and butt rotting species of fungus.

Symptoms & Diagnosis

A tree has suddenly died or has died after a short or lengthy period of increasing ill health, or shows a general deterioration in crown condition indicative of a root or root collar problem. Infected trees may also blow over to reveal rotted roots.

 

Additional Indicators

  • Other trees or shrubs in the immediate vicinity have died over a number of years. The recent death is another in a series of deaths of adjacent trees in a hedge or closely planted row.
  • Resin, gum or a watery liquid exudes from the lower stem of the affected tree.
  • Clumps of Armillaria toadstools appear at or near the tree base or on nearby stumps.

Confirmation

  • Bark at the root collar and/or of roots is dead and when cut away reveals thin, white or creamy white, paper thin cohesive sheets of fungal tissue (mycelium), sometimes with fan-like striations, sandwiched between it and the underlying wood.
  • The mycelium may also marble the thickness of the dead bark.
  • Infected bark usually has a strong ‘mushroomy’ or fungoid smell
  • Dead roots may be decayed and the rot is stringy and distinctly wet.

Honey fungus toadstools

  • These grow mostly in clumps and only in autumn.
  • They are brown, or honey-yellow brown, with roughly the stature and texture of tall cultivated mushrooms. The stalks always have a whitish collar near the cap; the gills are clearly joined to the stalk; the spore powder is white or cream and often conspicuous on surfaces beneath the caps. An excellent test method to ascertain 100% is to place a cap gills down on white paper for a few hours. If the spore print is not white or cream, it is not Honey fungus.
  • Texture is soft, but not tough.
  • The flesh is white with an astringent taste.
  • The gills are whitish but later develop into a brown colour.
  • Decay characteristics include wet, brown or yellowish with black lines.

Status

Honey fungus is very common and widespread on wooded or previously wooded sites. The disease is very uncommon in urban, roadside trees.

Significance

With the exception of Dutch elm disease, Honey fungus probably kills and debilitates more amenity trees in this country than any other single living agent.

Host Trees

An extremely wide range of woody plants are attacked though susceptibility varies greatly.

Resistant Trees

Acer negundo, Juglans hindsii, Juglans nigra and Taxus baccata are highly resistant if not immune.
Trees with a useful degree of resistance include Abies species(Pine), Crataegus species (Hawthorn), Holly (ilex aquifolium), Common Ash (Fraxinus excelsior), Quercus species (Oak), Hornbeam, Tilia species (Lime) and Fagus species (Beech).

Infection & Development

  • The fungus resides in buried wood (stumps, posts etc.) and in dead roots on living trees.
  • It spreads locally by growing throuhg the soil and leaf litter as strands (rhizomorphs) whose tips can infect undamaged, live bark of roots and the root collar, or dead wood.
  • The fungus grows through and kills the phloem (food cells) and cambium (inner bark layer that enables incremental growth) and later invades and decays the wood.
  • The tree dies once it is girdled at the root collar or as the result of extensive root killing.
  • Some trees blow over while still alive as the result of extensive root decay.
  • On rare occasions, airborne spores also give rise to new infections.

Control: Therapy

None available

Prevention

  • To prevent continuing spread of the disease ideally dig out all infected material then replant.
  • In hedges and screens, also remove the apparently healthy plant next to the one showing symptoms, in both directions.
  • If this is not practicably possible then dig out as much infected material as possible and replant with resistant species.
  • Obstruct rhizomorph spread by inserting durable plastic sheeting barriers in the ground between the source of the infection and the plants to be protected.
  • Where none of the control measures are successful/practicable plant with immune species, or accept the likely further occasional losses, removing plants as they die.
  • The tree growing area could also be relocated and the infected area used for another purpose.
  • The toadstools are of no practical significance in the spread of the disease, and are useful only as a visible above ground indicator.
  • The value of available chemical control measures is dubious, although recent research has shown certain soil fumigants to be useful in killing the fungus in tree stumps.

Remarks

  • Plants 30m or more from infected trees or stumps are not at risk.