by Robert D. Raabe
Department of Environmental Science and Management
University of California, Berkeley
Armillaria mellea is a common disease producing fungus found in much of California . It commonly occurs naturally in roots of oaks but does not damage them unless they are weakened by other factors. When oaks are cut down, the fungus moves through the dead wood more rapidly than through living wood and can exist in old roots for many years. It also does this in roots of other infected trees. Infection takes place by roots of susceptible plants coming in contact with roots in which the fungus is active. Some plants are naturally susceptible to being invaded by the fungus. Many plants are resistant to the fungus and though the fungus may infect them, little damage occurs. Such plants, however, if they are weakened in any way may become susceptible and the fungus may kill them.
The plants listed here are divided into three groups. Those listed as resistant are rarely damaged by the fungus. Those listed as moderately resistant frequently become infected but rarely are killed by the fungus. Those listed as susceptible are severely infected and usually are killed by the fungus.
The fungus is variable in its ability to infect plants and to damage them. Thus in some areas where the fungus occurs, more plant species may be killed than in areas where other strains of the fungus occur.