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Page number:404 
Remarks (public):Oligoporus caesius can be recognized in the field by the bluish tints on the pileus and pore surface. O. subcaesius is usually distinctly more whitish and does not become blue when bruised. Furthermore, its spores are narrower (1-1.4 µm). However, there are some disturbing intermediate specimens on hardwoods. 
Description type:Non-original description 
Description:Oligoporus caesius (Schrad.:Fr.) Gilb. & Ryvarden - Mycotaxon 22:365, 1985. - Boletus caesius Schrad., Spic. Flora Germ., p. 167, 1794. Polyporus caesius Schrad.:Fr., Syst. Mycol. 1:360, 1821.
Basidiocarps annual, sessile to effused-reflexed, usually solitary, dimidiate to narrow, up to 5 x 6 x 1.5 cm; upper surface greyish to bluish, often in spots or streaks, sometimes bruising intensely blue, finely tomentose to strigose, sometimes glabrous; pore surface white, pale grey to bluish, becoming bluish when bruised, dull, the pores angular, 3-6 per mm, with thin dissepiments, these becoming lacerate; context up to 1 cm thick, white to bluish, soft; tube layer white to gray, soft, fragile when dry, up to 6 mm thick.
Hyphal system monomitic; contextual hyphae thin- to thick-walled, hyaline, often
branched, with abundant clamps, 2.5-7 µm in diam; gloeoplerous hyphae also present, staining brightly in phloxine.
Cystidia and other sterile hymenial elements absent.
Basidia clavate, 4-sterigmate, 16-25 x 4.5-7 µm, with a basal clamp, weakly amyloid when fresh.
Basidiospores cylindric to allantoid, hyaline, smooth, negative in Melzer's reagent or weakly amyloid in masses, 4.5-6 x 1.5-2 µm; spore print bluish.
Type of rot. Brown rot of conifer and hardwood logs and slash.
Cultural characteristics. See Nobles 1958; Stalpers 1978.
Sexuality. Heterothallic and tetrapolar (David 1974).
Substrata. Dead conifers such as Abies, Cryptomeria, Cupressus, Juniperus, Larix, Picea, Pinus, rarely on hardwoods as Acer, Alnus, Arbutus, Betula, Castanea, Ceratonia, Corylus, Cratageus, Erica, Eucalyptus, Fagus, Fraxinus, Pittosporium, Populus, Prunus, Quercus, Salix, Sambucus, Sorbus and Viburnum.
Distribution. Widely distributed throughout European coniferous forests and known north to 70° in Norway. Circumglobal species.
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