Emma Schaffert and Luke Hailey of Bartlett Tree Research discuss the cause, symptoms and treatment of Honey Fungus.
Honey fungus or Armillaria root rot is a soil-borne disease that attacks trees’ roots and trunk. It is caused by members of the genus Armillaria, most commonly in the UK by A. mellea and A. gallica.
A. mellea is considered the most aggressive species and infects trees in many places worldwide. Honey fungus infects more than 500 species of trees and shrubs, although some are more resistant to infection. It is a particular threat to ornamental and urban trees.
Armillaria fi rst attacks the roots of the tree, infecting and killing the cambium. It then moves into the base of the trunk (root collar). This causes whole tree decline and once inside the root collar it can girdle and kill the tree.
Burying a tree’s root collar when planting – a common mistake – can allow rapid infection of this vulnerable area. Trees with root infections are more susceptible to windthrow.
Honey fungus can live on dead wood as well as live trees, allowing it to continue to grow after killing the host. It can persist on remaining root systems for decades after the tree’s death and then infect new hosts on the same site. This makes it highly persistent as it is nearly impossible to remove all infected roots.
Symptoms include yellowing and dieback in the crown in response to root loss; loose or peeling bark with white mycelial fans underneath and a strong smell of mushrooms from infected areas. Black ‘bootlace’ rhizomorphs are sometimes also present under the bark and within the root system.
Clumps of brown to golden fungi may appear on the base of trees, stumps, logs and other deadwood in the autumn and the death of trees and shrubs in the surrounding area could also indicate the presence of honey fungus.
There are currently no chemical controls for honey fungus. Root collar excavation can be used to extend the life of infected trees, causing the fungus to recede. Where trees are heavily infested it is often too late to save a tree and removal is recommended to ensure safety.
The majority of control practices are based on removing infected wood such as stumps by methods such as pulling, excavation, grinding and breaking up any remaining rhizomorphs through air excavation techniques. The less infected material that is left in the soil, the lower the chance of further infections. Once broken up and suffi ciently exposed, honey fungus will often die through drying out and being attacked by natural enemies such as trichoderma fungi, although complete sanitation is unlikely. Root barriers can block the spread through the soil to new hosts, but this is only really a temporary measure.
Acer negundo, Juglans nigra, and Taxus baccata are thought to be the species most resistant to
honey fungus and are often used to replace removed infected trees.
Maintaining the general health of uninfected trees reduces the chance of them becoming infected and plant stress is strongly associated with the disease.
Check for buried root collars, excavate them and fill the hole with free-draining material if needed. Regular air excavation of the root zone and application of mycorrhizae (fungi that form a symbiosis with the tree) may also have some benefit. Damp sites can be especially predisposed to the spread of honey fungus and may require increased drainage.
Emma Schaffert is a Research Technician for the Bartlett Tree Research and Diagnostic Laboratory.
Luke Hailey is a research technician and plant ecologist He graduated from the University of Reading in 2012 with First Class Honours in Biological Sciences.