Roots of trees infected with Armillaria mellea show rotting in the form of white to yellowish, fan-shaped fungal mats between the bark and the wood. Dark brown to black fungal threads (called rhizomorphs) can sometimes be seen on the root surface and honey-colored mushrooms grow at the base of the tree. Affected trees often show a general decline in vigor, noted by yellowing foliage, dieback of twigs and branches, and reduced leaf size and number. The decay and collapse of the wood may happen suddenly or over several years. Initially the diseased trees are scattered but due to the concentric spread of the fungus from its primary infection point, circular areas of diseased trees appear. In the absence of mushrooms, the symptoms can be easily confused with Phytophthora root rot or any other root problem caused by a fungus or nematodes.
The disease is caused by multiple fungi within the genus Armillaria, among other A. mellea, and has a wide range of hosts. It shows varying symptoms depending on both the species of fungus and the host in question. The fungus survives on dead roots in the soil or the lower severed stumps for several years in the absence of host. They do so by forming resistant bodies called rhizomorphs. When conditions are favorable, it grows and produces thread-like extensions that spread in the soil in a concentric way in search of healthy roots. This explains why trees usually die in a circular area, that expands each year as the fungus grows along roots of infected trees over to roots of adjacent healthy trees. In general, they infect and kill trees that have been already weakened by other fungi or insects. They can be a problems in orchards that have been planted in recently cleared forestlands.
Trees infected with Armillaria mellea usually cannot be saved. Trees, stumps and any large remaining roots need to be removed and burned. The hole should be left open for about a year (solarization). At early stages of the diseases, excavating the soil around the base of the tree down to the first layer of lateral roots may delay the progress of the disease because it prevents the fungus from gaining access to the crown of the tree.
Always consider an integrated approach with preventive measures together with biological treatments if available. Soil fumigation with carbon disulphide may ensure absence of Armillaria mellea in an orchard prior to planting the young trees. The application of the products can also be carried out by irrigation, but never by spraying. Irrigate the feet early in the attack when the vegetation is yellowing with products containing phosetyl aluminum, metalaxyl or thiophanate-methyl fungicides. To prevent the spread of the disease, agricultural tools should be disinfected with bleach or after use.