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Armillaria root rot

Scientific Name
Armillaria ostoyae
Scientific Author
(Romagn.) Herink
Taxonomy
Marasmiaceae, Agaricales, Agaricomycetidae, Basidiomycetes

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Pathogen

Diagnostic Notes

Symptoms: Armillaria ostoyae colonizes the root system of woody plants and causes non-specific symptoms such as reduction of shoot growth, changes in foliar characteristics, crown dieback, stress-induced reproduction, basal stem indicators (exudates, cankers, cracks, or flutes at or just above the root collar), and death. Signs of the fungus (mycelial fans, rhizomorphs and basidiocarps) can occur on the root collar and lower bole of the tree. A detailed description of these symptoms and signs is provided by Morrison et al. (1991). Typical symptoms on pines are: The whole crown becomes brown and dies simultaneously, strong resin exudation at the root collar and white mycelial sheets under the bark. Often A. ostoyae produces increasing gaps in pine stand. The fungi: Pileus 5—10 cm in diameter, broad, hemispherical-campanulate or obtusely parabolic, then convex and finally plane. Surface dry, sometimes of pale colours like orange white or caramel (brown) but usually darker, with distinct scales in shades of dark reddish colors or dark brownish colour, distributed over the surface but more densely toward the centre. Margin inrolled at first then down turned, sometimes striated, entire, usually concolorous to cap but sometimes paler, often cream, flesh or camel. Flesh firm, 1-2 cm thick at the apex of the stipe, context white. Lamellae white or cream when young, to greyish orange, cinnamon later, rather close, adnate to slightly decurrent becoming sinuate in mature specimens, thick at the point of attachment to the stipe but thinner toward the margin. Stipe central, 5-20 cm long, 1.5 cm thick at the apex, clavate when young becoming more or less equal, fibrous, context white to alabaster, often with bright yellow to greenish yellow mycelial fibres at base only, covered with fibers ranging from greyish orange, light brown, burnt sienna and hazel, staining black or sometimes mahogany, sometimes deeply striated. Annulus thick, membranous, usually white to alabaster, circled with a thick fluffy band at margin ranging from tan to chestnut, broken in many places leaving the stipe marked and covered at many locations with dark fluffy remmants concolours to ring. Spores ivory in mass, broadly elliptical to ovate, 5.5-7 x 8-11 µm, smooth, apiculate and nonamyloid. Basidia clavate, four-spored, some with a clamp connection(*). Pleurocystidia absent. Margin of lamellae composed of three layers: a suprapellis consisting of a loose array of more or less parallel, regular, thick-walled hyphae, 5-20 x 18-55 µm; a mediopellis formed of a tight array of parallel, almost regular, very thick walled cells 4-9 x 9-64 µm, staining blue in toluidine blue; a subpellis composed of a loose array irregular, thin-walled globular cells, 9-15 x 25 µm, arranged in a complex network. Subhymenial hyphae filamentous, unclamped (**), binucleate. Lamella trama bilateral (Bérubé & Dessureault 1988). Notes * In natural specimens that the senior author has investigated the basidia had regularly a clamp connection (on the other hand, basidiocarps of A. ostoyae produced in pure culture have usually clampless basidia). ** In natural fruit bodies, some clamped binucleate hyphae occur in the subhymenium at the base of basidia. 1. Additional information about the colour of the basidiocarps is provided by Bérubé & Dessureault (1988). 2. According to Termorshuizen & Arnolds (1987), the names Armillariella obscura sensu Romagn., Armillaria obscura sensu Marxmuller and Armillariella polymyces sensu Sing. & Clemencon were misapplied to this pathogen. Diagnosis: Until the late 1970s, most plant pathologists referred to all annulate Northern Hemisphere species of Armillaria as A. mellea (sensu lato). The 'mellea complex' has been split in many species, including A. ostoyae. Reviews about the taxonomy of Armillaria were provided by Watling et al. (1991) and Crop Protection Compendium (2005). Currently there are approximately 36 species of Armilla

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