Section outline
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After becoming established in the roots, the vegetative mycelium of Armillaria develops a protective layer of thick-walled fungus cells, the pseudosclerotial envelope, about the body of the infected wood and the white mycelial fans in the cambium region. The rhizomorphs growing from the pseudosclerotium are also covered by this protective layer. The fungus may be acting as a secondary agent of damage in trees weakened by other means and can also grow saprophytically on trees killed by something else. When plants are stressed, for example, by drought or insect defoliation they become less resistant to the disease. As a result, infection of healthy roots more likely to occur but the pathogen may also be enabled to advance further in already infected roots. A major impediment in the chemical and biological control of Armillaria is the inability of the control agents to reach the site of inoculum inside wood in natural infections in sufficiently active state. The pathogen has also evolved highly sophisticated mechanisms of protection against outside deleterious effects. These include the production of antibiotics and the formation of pseudosclerotia. Armillaria spread typically occurs in the leaf-litter layer (duff) or underground via vegetative growth of rhizomorphs (dark, root-like mycelial structures) or mycelia (root-to-root contact). Rhizomorphs generally grow at a rate < 3 m per year, depending on the nutritional substrates, climate, and other environmental factors. In some situations, the radial growth of Armillaria results in an Armillaria root disease center or mortality center, but Armillaria often causes root disease that is more diffuse or scattered throughout the site.
- Short-range dissemination
Local spread is very important and is generally the dominant source of infection. The fungus can move by these means from an old root system, perhaps from a previous forest, to plants currently growing on the site. This leads to the appearance of disease in a previously uninfected population. The pathogen may survive for 50 years or more in stumps, it can wait until the new generation provides a large target for infection. The fungus can also move this way from a diseased tree to a neighboring healthy tree, leading to expanding areas of disease and mortality, usually called root disease centers. Root disease centers, as well as saprobic growth of the fungus, involve indeterminate growth through the forest. Resulting clones can cover many hectares, perhaps even miles, and be thousands of years old.
Mycelium can grow through direct root contacts and grafts with uninfected trees. Rhizomorphs can grow through soil to contact uninfected trees. Rhizomorphs are macroscopic, 1-5 mm diam., reddish brown to black, bundles of organized hyphae with an organized apically growing tip. Rhizomorphs grow through soil, produce branches, and look very much like roots (rhizomorph) or shoestrings (thus a common name for the disease, “shoestring root rot”). They use energy from a stump or killed tree to grow and infect a nearby tree. They can grow many meters through the soil.
- Long-range dissemination
Armillaria spp. is producing basidiospores, which are wind-dispersed to wreak death and destruction in new places, however, the spores don’t disperse very easily. There is indirect evidence that they do occasionally colonize stumps or wounds, especially in moist climates. Persistence in dead roots and stumps, the saprobic phase, may be dominant for some species that subsist by colonizing dead trees but rarely seem to kill them. Armillaria species may be abundant in the forest without a lot of obvious, damaging disease in some situations. Other species decay dead trees and stumps and build up energy to attack neighboring trees.