Biology

In addition to its pathogenic activity, the fungus lives saprophytically on tree stumps and other buried wood. Armillaria exists largely as a mass of microscopically fine threads (hyphae) growing inside the roots and butts of live trees, old stumps, fallen trees, and other woody debris. The fungus derives its nutrition mainly from the woody tissues, resulting in their gradual decay. Beneath the bark of roots and lower parts of the stem the hyphae may be so abundant that they form a clearly visible creamy-white fungal mass (mycelium). In some species, the rhizomorphs are quite tough and elastic but in others they are fragile. They are usually branched, though the pattern varies between species. Old rhizomorphs are almost black but younger ones are reddish brown with a white core and growing tip. Infections occur when growing rhizomorph tips contact and then penetrate roots or root collars of susceptible live plants. Spread may also take place without rhizomorphs via grafts and contacts between infected and uninfected roots. As well as infecting living trees, the fungus can colonise stumps and trees killed by other agents. All of these food sources serve as foci for further spread and infection. Honey fungus reproduces by means of microscopic spores produced in the toadstools. These spores are of no importance in the local spread of disease but provide the means by which the fungus can colonise more distant areas. Rhizomorphs are of limited value in the diagnosis of the disease: they may be difficult to find and can be confused with small tree-roots, whereas in A. mellea, which is probably the most pathogenic, rhizomorphs are usually fragile and hard to find.