Ganoderma Butt Rot is caused by Ganoderma applanatum (formerly Fomes applanatus). This fungus is an important decomposer of logs and stumps but it also colonises wounds, and can cause decay of sapwood and heartwood in roots, butts and trunks of trees. It affects native tree species such as acacias, eucalypts, figs and beech, as well as many introduced species such as oaks, elms, ash and some conifers.
Ganoderma applanatum produces very distinctive, shelf-like fruiting structures or brackets. They are most commonly found on stumps or near the base of living trees, often at the site of an old wound. The brackets of Ganoderma applanatum are some of the largest produced by any fungus. They are hard and woody and grow perennially for 5 to 10 years, reaching as much as 50 cm or more across and 10 cm thick. The brackets do not have a stem, but are attached over most of their width to the host tree or stump. Their upper surfaces are dark reddish-brown to black, often irregular and knobbly, with a cream to white-coloured margin, and they often have a varnished appearance. The underside of the bracket is white to light yellow but turns brown when scratched or bruised.
A new, fresh, fertile layer of millions of tiny, white pores is produced on the undersurface each year. It is in these pores that the spores are formed. The spores are rusty red-brown in colour and produced in such numbers that they often colour the bark beneath the bracket. Spores are windborne and infection occurs through wounds.
Infected trees are slow in growth rate, and the leaves are often small and yellowed. Wood which is infected by the mycelium of the fungus has a light coloured, mottled appearance. In advanced stages of decay the wood readily fractures across the grain. It remains firm for a time but eventually becomes soft and spongy. Columns of decaying wood often extend above and below the brackets.
There is no control for Ganoderma butt rot and once a tree is infected it is only a matter of time until it dies. It may take several years to kill host trees but meanwhile they can pose a hazard because they may be very susceptible to windthrow. Often the only overt evidence of infection is the presence of brackets. Therefore any tree with brackets on it should be removed promptly if it is in a location where property damage may occur or where people could be struck by falling limbs.
Wounding of trees should be avoided to reduce the chance of infection and any live trees with broken tops or sizeable wounds should be removed.