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Remarks (public):For a complete description including images see 
Remarks (internal):This may be distinguished from A. montagnei Singer which has a pure white spore print, a non-gelatinized pileal surface, and an almost regular to indistinctly bilateral hymenophoral trama. Armillariella tabescens (Scop.: Fr.) Singer lacks a veil. A widespread and, at times, important root disease of forest trees and other perennial plantation crops. As its effects as a lethal pathogen are impressive its impact in this role is often over-estimated. On the other hand the damage that it does as a rot agency is often under-estimated. The severity of an attack of Armillaria root disease depends on the susceptibility and vigour of the host crop, the active mass of inoculum of the pathogen and the temperature and moisture status of the environment. When natural forest is cleared the relict stumps may become colonized by A. mellea and provide inoculum centres of an active mass sufficient to cause outbreaks of lethal disease in subsequent plantation crops. Leach (1937, 1939) showed that if trees with epiphytic root association of A. mellea are killed by careful ring barking, so that their starch reserves are exhausted, there is a reduced risk of their becoming infection centres. This has formed the basis of a method of control of the disease which is now widely adopted in the tropics, though recent work (47, 2564) has shown that the method may not have universal application. Other methods of control include careful species selection (in forestry) to ensure that highly susceptible species are not planted in areas of high infection risk, soil fumigation (in valuable orchard-crops), using carbon disulphide (Bliss, 1951) or methyl bromide (Larue et al., 1962) and removal of infective residues. The isolation of centres of inoculum by trenching is often recommended but is seldom effective.  
Description type:Non-original description 
Description:Armillariella mellea (Vahl: Fr.) P. Karst., Acta Soc. Fauna Flora fenn. 2: 4, 1881.
= Agaricus melleus Vahl: Fr., Syst. Mycol. 1: 30, 1821.
= Armillaria mellea (Vahl: Fr.) Kummer, 1871.
Carpophore caespitose, clitocyboid, polymorphic. Pileus 4-15 cm diam., fleshy, subglobose, expanding to convex, finally depressed at the centre or umbonate; surface varying from honey to tawny or deep brown, with small crowded, fibrillose squamules, margin entire, paler, striate. Lamellae adnate to subdecurrent, whitish to flesh pink, moderately distant but with lamellulae of four lengths, moderately thick, tough, up to 9 mm wide; edge entire, concolorous. Stipe 5-15 x 1-2,5 cm, central, equal or slightly thickened towards the base, fistulose, fibrous fleshy, surface ochraceous yellow when young, darkening to brown with age, white above the annulus; often arising from black rhizomorphs. Annulus persistent, rather thick, membranous, whitish, becoming more yellow in the outer portions, attached to the upper part of the stipe. Context white, woolly fleshy, consisting of thin-walled, hyaline hyphae, 2-8 µm diam., inflating up to 15 µm diam., without clamp-connexions. Smell and taste acrid. Spore print light cream. Basidiospores 7-12 x 5-7,5 (av. 10,5 x 5,5) µm, short ovoid to ellipsoid, with or without a suprahilar depression, hyaline, inamyloid, with a thin or slightly thickened wall. Basidia 35-47 x 5-9 µm, elongate clavate, bearing four sterigmata, up to 6 µm long. Lamella edge sterile, crowded with basidiole-like cheilocystidia 27-45 x 7-15 µm, fusoid, clavate to ventricose, hyaline, thin-walled. Pleurocystidia absent. Hymenophoral trama bilateral, with a broad mediostratum and diverging lateral strata, consisting of hyaline hyphae, 3-7 µm diam., inflating to 15 µm diam. Subhymenial layer interwoven, up to 12 µm thick. Pileal surface a repent cutis, up to 30 µm thick, consisting of loose, unbranched chains of cylindric elements, 15-45 x 6-8 µm, constricted at the septa, somewhat gelatinized. Development hemiangiocarpous.
Hosts: The fungus has been recorded as a pathogen on an extremely wide range of dicotyledonous and coniferous trees and shrubs (Raabe, 1962; Browne, 1968). It has also been found on potato (1: 357), narcissus (11: 376), strawberry (8: 727), bamboo (40: 436), geranium (12: 5227), sugarcane and banana (25: 355), but attack of monocotyledons and herbaceous plants is rare.
Disease: Armillaria root rot. Armillaria root disease. Honey fungus. Armillariella mellea has two roles as a plant pathogen. It may act as a lethal pathogen by invading and destroying the cambium of the root and lower stem, following which it may colonize and rot the woody tissues of the host after its death, or it may cause a butt rot without attacking the living tissues of the host. Both types of disease have a patchy distribution in the field, characteristically spreading from inoculum centres in the soil, so that the hosts which have been most recently attacked are found at the edge of the patch. Where lethal attack occurs the host dies rapidly and (in the case of conifers and certain dicotyledonous trees) the bark of the roots and stem base is cracked and covered with gum or resin exudate. Thick, creamy-white, fan-like sheets of mycelium are found under the bark and these are frequently accompanied by flattened, dark brown rhizomorphs (forma subcorticalis). These rhizomorphs may also be epiphytic on roots and grow out into the soil, where their form is more cylindrical (forma subterranea). Sporophores may be formed on the host in advanced stages of the disease.The butt rot is a typical soft white rot with pronounced black zone lines. It seldom advances more than 1-2 µm up the stem. The rotten wood is often luminous. Armillariella mellea also forms balanced mycorrhizal associations with the orchids Galeola septentrionalis and Gastrodia elata.
Geographical distribution: World wide, throughout forests of the temperate and tropic regions (CMI Map 143, ed. 3, 1969).
Physiological specialization: The fungus is extremely variable and there is still some doubt whether Armillaria fuscipes Petch and Armillariella tabescens (Clitocybe tabescens) are distinct. This variation extends to pathogenicity (Raabe, 1967) and may be expected to include host specialization also.
Transmission: By infection of woody debris and stumps by air-borne basidiospores, by the growth of rhizomorphs through the soil, by root contact with infected soil debris and transport of infected plant material.
Literature: Bliss, Phytopathology 41: 665-683, 1946; Browne, Pests and Diseases of Forest Plantations Trees, O.U.P., 1968; Garrett, Pathogenic Root-infecting Fungi, C.U.P., 1970; Leach, Proc. R. Soc. B 121: 561-573, 1937; Leach, Trans. Br. mycol. Soc. 23: 320-329, 1939; Larue et al., Calif. agric. 16: 8-9, 1962; Raabe, Hilgardia 33: 25-88, 1962; Raabe, Phytopathology 56: 1241-1244, 1966; 57: 73-75, 1967; Singer, Schweiz. Z. Pilzk. 48: 25-29, 65-69, 1970.

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