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Armillaria root rot - fact sheet
Armillaria luteobubalina is a soilborne fungus that causes root rot of a wide variety of plants including many native and introduced ornamental plants. The fungus is native to Australia and causes losses in natural ecosystems, in forest plantations and in fruit crops and ornamental plants. The host range of the fungus is very large and poorly defined (at least 50 families and over 200 species) with little information on the presence of resistant or tolerant species.
Symptoms of the disease
Early symptoms of the disease can often be difficult to detect but may include dieback of the limbs and branches, yellowing of foliage, splits in the trunk of the infected tree, poor vigour, exudates from the trunk (kino production), scars may form on the trunk and darkening of the larger roots. Removal of the bark may reveal the presence of mycelial fans - these are large sheets of fungal growth, usually white in colour, which will have a characteristic mushroom odour. The surface of the affected timber is often pitted in appearance. The fungus produces mushrooms in May-June. These are olive brown to yellow in colour, can be up to 12 cm in diameter with a stipe (stalk) of up to 15 cm high, although usually less. The stipe will have an annulus (the ring of tissue around the stipe) that should be quite obvious. The spores of the fungus are white and these often coat the surface of the mushrooms.
Infection occurs via the roots usually as a result of infected roots coming into contact with uninfected roots and the fungus growing across. The fungus does not appear to readily produce rhizomorphs (specialised fungal threads that can grow through the soil) and it is less likely that the fungus can spread through the soil by its own devices. The fungus is able to infect new areas by several means. Very rarely the spores of the fungus can fly through the air and land on dead wood surfaces and initiate infection. More commonly the fungus will be introduced into an area by the transportation of infected material such as the transplantation of infected plants, contaminated roots, or contaminated mulches. Hygiene is obviously important in minimising the spread of this fungus.
Soil conditions that favour the development of the disease are poorly defined. It is thought that the fungus prefers lighter soils or clays with reasonable drainage but this is not always the case. It is claimed that the disease is more severe on nutrient poor soils or with some characteristic that is not optimal for plant growth. There is however very little clear evidence for this, particularly in Australia. Drought is often associated with severe symptoms. It appears that the stress involved predisposes the tree to infection and also allows the fungus to more rapidly colonise the root system of the plant. Similarly stresses resulting from flooding can also predispose trees to severe infection. It is fair to say that any factor that stresses trees is likely to result in a weakened defence system and an increased likelihood of the disease developing.
The fungus can survive in soil for extremely long periods of time and there are estimates of up to survival for 50 years, although 20 years would be more likely.
At present there is no one simple method for controlling Armillaria luteobubalina. A combination of sanitation measures, good horticultural management and the addition of organic matter to soils can be expected to retard the activity of Armillaria.
Prevention of the disease: hygiene is essential for ensuring the disease is not spread from infested to uninfested sites.
Removal of inoculum: removal of infected material from infested sites will reduce the impact of the disease in subsequent plantings. It is difficult to ensure that all sources of inoculum are removed from the site and this may be difficult as the fungus can survive in relatively small pieces of root. In these instances we recommend leaving the area unplanted to allow the small pieces of infected material decay. Isolation of infected areas by trenching and root barriers can be very effective in some situations where the area of infection is well known and defined. Trenching needs to be at least 0.6 m deep.
Clearing, aerating and drying the root collar can be very useful on high value trees that are infected. This involves excavating around trees so that infected areas are exposed to the air. This halts the activity of the fungus because the surface wood and bark dries out. This technique has been used in a variety of situations with a number of species of Armillaria with some good results.
Chemical control: there are no effective chemicals to control the disease in trees.
Decay organisms: one method of reducing the inoculum of the fungus is by introducing a decay organism (eg. Phanerochaete filamentosa) into infested dead material in the soil, eg. stumps. This is a long term control method as it will take some time for the decay organism to be effective and displace the Armillaria from the stump.
Biological Control: there are some promising developments in biological control of Armillaria. Many centre on the use of the soil fungus Trichoderma. This fungus is a ubiquitous soil inhabitant that is active on the root and there are research trials on its effectiveness on several Armillaria species in different parts of the world.
It should not be expected that total control of Armillaria root rot will be achieved. However, all of the above factors will need to be part of an integrated program of disease management.
This map show the extent of Armillaria root rot in the Royal Botanic Garden Sydney and the Domain in 2012.
Click on map to enlarge