Honey fungus can be a very serious disease that affects woody and herbaceous plants alike. The name honey fungus refers to a range of species in the genus Armillaria that target the roots of perennial and woody plants. There are 45 different species of the fungus and in fact, the largest organism in the world is a specimen of Armillaria solidipes, which is over 3.4 square miles in size. It will often kill plants, especially young ones. In older trees, the fungus will often just weaken them to the point that they are killed by other minor diseases or through wind damage. It can be very hard to identify honey fungus as the stump that has the initial infection could be very far away from the newly infected plants and the mushrooms are only present from mid-summer to autumn.
Honey Fungus causes the dieback of branches and leaves in woody and herbaceous plants and can also stop leaves emerging in the spring. The dieback is often quite rapid. There are several diseases that can cause these symptoms and so you will need to check for the additional signs of the fungus. In a Honey Fungus outbreak, there will be an old tree stump that is rotting nearby. This may have white mould growing underneath its bark or have small honey coloured (though can be whitish or brown) mushrooms sprouting on it. The white mould will have a strong mushroomy smell to it and can also be found under the bark of the infected tree near the base. Further evidence of a Honey Fungus infection is the presence of dark bootlace-like structures called rhizomorphs in the soil around the infected tree. The shape and thickness of these structures vary depending on the species of Honey fungus but they will be present.
Rotting tree stump nearby.
Dieback of branches
Dark staining or white mould underneath bark at base of the tree with strong mushroom scent
Rotting tree stump nearby
Clumps of honey coloured small mushrooms near plants
Bootlace-like structure in the soil around infected plants.
There are no biological treatments for this disease.
There are no chemical treatments for this disease. Excavation and destruction is needed to completely eradicate, all infected stump and root material should be burnt or sent to landfill.
The rhizomorphs burrow through the soil and infect the root systems of plants. The fungus can kill the plant especially if it is young. In older trees, it may only weaken it and leave it susceptible to other attacks. The mushrooms also create spores that can infect dead wood and in new areas.
The planting of resistant and immune species as hedges. Not having many susceptible species planted together. When removing a tree, remove the whole tree so that there is no stump left to get infected. Physical barriers can be useful to stop the spread to unaffected areas, something like a pond liner (butyl rubber) or heavy plastic sheeting can be placed underground to limit honey fungus spread, this should also extend 2-3cm out above the soil. Regular mechanical action to break up root-like structures known as rhizomorphs is also helpful to limit the spread.