Armillaria root rot

Armillaria root rot
Armillaria luteobubalina, widespread in Australia, is a primary cause of Eucalyptus tree death and forest dieback resulting from Armillaria root rot.
Causal agents	Several species of the genus Armillaria
Hosts	Several tree species

Armillaria root rot is a fungal root rot caused by several different members of the genus Armillaria. The symptoms are variable depending on the host infected, ranging from stunted leaves to chlorotic needles and dieback of twigs and branches. However, all infected hosts display symptoms characteristic of being infected by a white rotting fungus. The most effective ways of management focus on limiting the spread of the fungus, planting resistant species, and removing infected material. This disease poses a threat to the lumber industry as well as affecting recreational areas.

Because this disease is caused by multiple species within the genus Armillaria, it has an extremely broad host range. Hundreds of trees and shrubs are susceptible to root rot to varying degrees. In fact, the only two genera of tree known to be resistant to Armillaria root rot are larch and birch. Further investigation is being conducted for additional species, but at this time there is no further evidence leading to the belief that more exist.

As a result of the multitude of possible hosts, symptoms also range a great deal from one infection to another. All infected hosts do however display symptoms common to white rotting fungi. These include light or bleached wood as a result of the degradation of essential cell wall compounds such as lignin and hemicellulose. In addition to these, the host will show above-ground symptoms due to fungal infection of the vasculature. These include chlorosis in the needles as well as dieback of twigs and branches. The extent of these symptoms varies with the degree of infection as well as the susceptibility of the host. In addition to these symptoms, the trunks of conifers will also exude excess resin in a process known as resinopsis which results in a layer of resin, debris, and fungal tissue forming around infected roots. Deciduous trees occasionally develop sunken cankers but most often fail to exhibit these symptoms on the trunk, and will instead simply display the other symptoms.