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Honey fungus: The tree killer

 

Honey fungus

The honey-coloured fruiting bodies of honey fungus (Armillaria spp) (image: Wikimedia Commons)

Honey fungi, Armillaria, are a group of parasitic fungi. They attack trees, shrubs and woody perennials, and are one of the most destructive fungal diseases in the UK.

They are also among some of the biggest living organisms in the world, their underground networks often covering many miles and living for up to a thousand years.

It is so successful because, unlike most parasites that rely on keeping their hosts alive in order to extract nutrients, it can kill its host and continue living on the decaying matter for many years. Watch our video on how to diagnose honey fungus

The fungi spread by long reddish brown root-like rhizomorphs that live close to the surface of the soil.

They attach themselves to the root collar of woody plants, killing off the root systems leaving the host unable to absorb nutrients and water. You can watch our video on how to diagnose honey fungus

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.

If it is identified in your soil, the only solution is to remove ALL infected material (or as much as you practically can), including stumps and roots. Then destroy all of this material by burning.

The fungus cannot survive on its own in the soil without its host. To ensure that you don't accidentally give spores a fresh host, it is vitally important to disinfect all tools after removal of infected material.

Preventing the spread of the fungus from an infected area is labour-intensive, but may well be your only course of action. You need to contain the fungus by burying a butyl rubber lining at least 45cm down, and which protrudes at least 2-3cm above soil level. This creates a barrier that the rhizomorphs cannot penetrate.

Once you have removed an infected plant or created a necessary barrier it is then possible to replant with a more resistant species.

 

Resistant plants

No woody plant is completely immune to attack, but if you think that your garden has honey fungus, then there are a number of trees that are judged by the RHS to have good resistance levels. These include: Juglans nigra, Carpinus betulus, Larix, Quercus ilex and Taxus.

And shrubs showing good resistance include: Berberis, Buxus sempervirens, Philadelphus, Sambucus, Lonicera nitida and Griselinia.

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

 

6 thoughts on “Honey fungus: The tree killer”

  • David Crossley

    Killed off mature Walnut in my gardenseveral years ago, followed by Eucalyptus gunnii and two silver birches, hawthorn then pyracantha. Armillatox, which is now prohibited for garden use, seemed to help a bit. Other eucalypts have not, so far, been affected. That's most of my 100' square back garden infested. Other trees alongside road at the front also affected, one died last year with incredible amount of fungi sprouting this year. Presumably it is pretty rife throughout our neighbourhood which was originally uncultivated parkland

    Reply
  • Adam

    I think the RHS list is a guide only. The fungus is attacking my elder tree and some bamboo, both of which are supposed to more resistant.

    I've put down some early purple orchid (orchis mascula) seeds because it controls the fungus. If only they would grow more easily ...

    Reply
  • Douglas Angwin

    Mature (30 years plus) winter flowering Cherry lost all of it's leaves about one year ago.Have now felled the tree. Did not take much notice until toadstools appeared at the base of stump. I have removed the stump and most roots. But in comparison to known symptoms there is no bootlaces or a strong smell and only half of the roots seem to be affected.

    Reply
  • Angus

    think this may be a reaction to the way you treat your garden. Have you been using chemicals or removed other fungus that is supposed to live naturally in your garden? The honey fungus is a natural part of the eco system, and it will appear in cultivated gardens that are not treated the way the need to be.

    It's probably there to balance out the harm you've done to your garden, which may take some years, so you could just leave it and let it do it's job.

    Reply
  • Roy

    I have a hawthorn hedge planted in a row of oak trees. Sections of the hawthorn 2 or three plants are totally dead an pull out of the ground fairly easily, there is no sign of fungus just litchen type growth on the branches but the main body is very soft internally when pulled up.
    Some branches on the oak have died but not sure how serious until the buds break into leaf.
    At a loss to know if this is honey fungus or something to do with a very large fairy ring nearby

    Reply
    • Ashridge Support
      Ashridge Support 1st July 2020 at 6:48 pm

      Too many variables I am afraid as it could also be related to the oak trees which may have one of the root rots - on which we are not qualified to advise. We have a honey fungus advice page at https://www.ashridgetrees.co.uk/gardening-advice/plant-diseases/honey-fungus-armillaria-mellea. Best thing to do is to call a local tree surgeon out to have a look. They will give you free advice and a free quote...

      Good luck

      Reply
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