Armillaria Root Rot – The Root of All Evil

Toolbox Talk

1st October 2019 | Info


Armillaria Root Rot is one of the most common root diseases that affects garden trees and shrubs and is caused by Honey fungus (Armillaria x). There are many species of Honey fungus, several of which cause the deadly root rot due to having parasitic qualities. Some of the species include Armillaria mellea and Armillaria ostoyae.


When checking if your tree has been infected with Honey Fungus you should peel away the bark at its base. If there is a creamy to white layer of mycelium (fungal tissue), then the tree has been infected. Rhizomorphs also form on the roots of the tree. Mycelium and rhizomorphs are present all-year-round, whereas toadstools (which also may identify that your tree is infected) appear in the late summer to autumn and are a honey colour. You will also be able to detect infection above-ground as branches may begin to die back, small and paler leaves may form, there may be premature autumnal colour or bark may experience cracking and bleeding at its base.

Susceptible Trees

Some plants have a stronger resistance to the disease, including the Box Elder (Acer negundo). Whereas others are more susceptible; e.g. Beech, Birch and Apple. 

Click here for our downloadable list of susceptible vs non-susceptible species. Tolerant and Less Tolerant Species of Armillaria Root Rot


Predominantly, Honey fungus forms on roots, trunks and stumps. Rhizomorphs thrive in moist soil, just below the surface, but can also develop deeper in dry soils. Typically, wooded areas and gardens are the most affected.

Spread and Reproduction:

Infected trees develop rhizomorphs which protrude from its roots. These then spread underground and attacks further roots of trees in turn, killing them and decaying its wood.

Treatment and Disposal:

Currently, there are no chemicals available for the control of honey fungus.  The only remedy is to remove and destroy all infected roots and stumps as the rhizomorphs will not be able to feed on the source, and they cannot grow when separated from the infected source. Once removed these should be burnt or sent to landfill.

If an area does become infected, it is important to prevent it from spreading to any unaffected areas. It is suggested that you create physical barriers within the soil to block the rhizomorphs from spread – e.g. using pond lining or heavy-duty plastic sheets. It is also encouraged that you actively carry out deep cultivation to break up the existing rhizomorphs to limit the spread too.

Sources: Forest Research – Pathology Advice Note 1 – Honey Fungus, RHS –

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