Armillaria

Armillaria sp. infects the roots and root collars of some trees and woody plants. It can kill the roots, subsequently killing the infected tree or plant, or it can decay the roots causing a living tree to fall over.

It can infect a wide range of species at any age. The older and healthier an infected tree or plant is, the greater resistance it has to death. Plants that are stressed and weakened become less resistant and allow the fungi to infect healthy roots and advance further in roots that are already infected.

Armillaria luteobubalina is considered a primary pathogen that infects the roots of healthy trees while other species are considered a secondary pathogen which attacks already weakened trees.

Symptoms
Stunted leaves, wilted foliage, reduced growth, branch dieback, tree death.

Signs of disease
Fungi fruiting bodies of Armillaria sp. appear in autumn and are usually found in clumps on infected plants. They are usually found at the base of trees and plants and rarely extends more than 500mm up the trunk. They can also be found in the soil above infected roots.

Fruiting bodies are usually between 10-15cm tall with caps 6-12cm wide at maturity. The caps (pileus) and stems (stipe) are a yellow to honey-brown colour. Gills are cream and can have a pinkish/yellowish tinge. The stem has a collar (annulus) below the cap (fig 1 and 2).

Armillaria sp. mainly exists as a large mass of microscopic threads (hyphae) that grow inside roots and butts of living trees, old stumps and woody debris. Mycelium might be seen as a creamy-white fungal mass under the bark of infected areas, and some species of Armillaria produce rhizomorphs that can appear like ‘bootlaces’ and grow into the soil or beneath dead bark. Old rhizomorphs are black and younger ones are red-brown with a white tip.

Spread
The fungus primarily spreads by physical contact. This occurs when rhizomorphs from an infected root spreads to other plants and penetrates the roots or buttress’ of susceptible plants. It reproduces with microscopic spores from the fruiting bodies.

Control
Completely removing Armillaria from a site is difficult and cannot always be guaranteed. The best option is to remove all woody material from an infected plant, including the roots. This option, however, can involve removing a large section of soil around the tree which might be detrimental to the beneficial mycorrhizae, and this risk must be weighed against the benefit. Any infected debris that is removed should be disposed of, and woodchips should not be used as mulch.

If all the infected parts cannot be removed, an option is to install root barriers around the known infected area at least .5m deep (up to 1m may be necessary in some sites) to try and stop the fungus spreading to non-infected areas. This can only be effective if it is certain that the fungus has not spread beyond the intended root barrier.

Once the infected material and soil is removed, the soil can be replaced with suitable clean soil. It is recommended that the site not be replanted in straight away but instead left for up to 12 months. During this time the site should have compost and organic matter added to encourage saprophytic fungi to decompose the remaining pieces of root that may be left behind. The site can be replanted with a species that is resistant to Armillaria.

Some resistant species:
Fraxinus excelsior
Geijera parvivlora
Liquidamber
Platanus x hispanica
Robinia pseudoacacia
Tilia sp.

References
Greig, B,J,W, Gregory, S,C, Strouts, R.G, 1991, Honey Fungus, Forestry Commission Leaflet 6
Shaw, C,G III, Kile, G,A, 1991, Armillaria Root Disease, United States Department of Agriculture, Forest Service, Agriculture Handbook No. 691
Smith, I, W, Smith, D, I, 2003, Armillaria root rot: A disease of native and introduced trees, Forest Science Centre, Department of Sustainability and Environment, State of Victoria.

Images curtesy of the Mushroom Observer, a source for mycological images.