Hygrophorus olivaceoalbus

Hygrophorus olivaceoalbus, commonly known as the olive wax cap or sheathed waxy cap, is a species of fungus in the genus Hygrophorus. The mushrooms have olive-brown, slimy caps with dark streaks and a dark umbo; the caps measure 3 to 12 cm (1+1⁄8 to 4+3⁄4 in) in diameter. Other characteristic features include a slimy stem up to 12 cm (4+3⁄4 in) long that is spotted with ragged scales up to a ring-like zone. As its name implies, the mushroom has a waxy cap and gills. The species appearing from midsummer to late autumn under conifers in mountain forests of North America and northern Eurasia. According to a publication by the Council of Europe, the fungus is nearly extinct in France. Although it is edible, opinions are divided regarding its taste. Besides its usage as an edible mushroom, the fungus possesses antibiotic-like compounds.

Taxonomy The species was first officially described as Agaricus olivaceoalbus by Elias Fries in 1815. It had earlier been published as Agaricus adustus by August Johann Georg Karl Batsch in 1783, but this was an illegitimate renaming of Agaricus brunneus published in 1774 by Jacob Christian Schäffer. It received its current scientific name when Fries transferred it to the genus Hygrophorus in 1838. Paul Kummer moved the species to Limacium in 1871, but this genus has since been sunk into synonymy with Hygrophorus. Several varieties of H. olivaceoalbus have been proposed:

Together with H. pustulatus, H. persoonii, H. mesotephrus and H. latitabundus, H. olivaceoalbus form the section Olivaceoumbrini within the genus Hygrophorus. The fungi of this section have greasy to slimy caps and stems. Their caps are darkish brown grey, olive or orange, and their stems are nattered or somewhat distinctly ringed. Common names that have been used for the mushroom include the "slimy-sheathed waxy cap", the "olive hygrophorus", the "sheated waxgill" and the "olive wax cap". The specific epithet olivaceoalbus is derived from the Latin words for olive-brown (olivaceus) and white (albus).

Description

Fruit body

The cap of H. olivaceoalbus is 3–12 cm (1+1⁄8–4+3⁄4 in) wide and is hemispherical in young fungi; they become flatter and wider with age, but they keep their characteristic dark umbo. Underneath the slimy grey to sooty-brown surface, the cap cuticle is streaked with fine, dark grey radially arranged fibers. Young fruit bodies are covered by two velum layers; the inner velum, composed of dark fibrils, becomes a ring or sheath (annular zone) on the stem that is covered by the gelatinous outer layer. The fruit body has a long stem ranging from 3 to 12 cm (1+1⁄8 to 4+3⁄4 in), a diameter of 1–3 cm (3⁄8–1+1⁄8 in) and a somewhat slimy surface in wet weather. It is often wavy or bent. The base of the stem is sometimes slimmer than near its apex. Above the annular zone, the stem is smooth and whitish. It is covered by two layers of tissue: the exterior sticky layer, and the comparatively thin interior layer that consists of flaky fibres, similar to those under the cap's slime layer, with which they are initially linked. As the stem grows and increases in length, the interior layer becomes ripped, and breaks up into ragged dark concentric bands. The gills of H. olivaceoalbus are thick, widely spaced and adnate (broadly attached) to decurrent (extending slightly down the stem); they are white (slightly greyish on the base), and have a waxy surface. The flesh of the mushroom is smooth, thin and white. The taste and odor are mild and have no distinct smell. When treated with a dilute solution of sodium hydroxide (NaOH) or sulfuric acid, the flesh turns reddish. The spore print of H. olivaceoalbus is white.

Microscopic characteristics The spores are 9–12 × 5–6 μm, ellipsoid, and are not amyloid; their surface is smooth. They are yellow in Melzer's reagent. The basidia have dimension of 46–62 × 7–10 μm, and are tetrasporic with short, stubby sterigmata. They have neither pleurocystidia nor cheilocystidia. The cap cuticle has a width of 250 to 450 μm and consists of loopshaped, dark hyphae with a width from 2 to 3 μm, which form an ixocutis (a horizontal layer of hyphae embedded in slime) and possess clamp connections; the fungus has no hypocutis. The gill trama consists of hyphae about 3 to 8 μm thick; the cap tissue comprises radial hyphae. The mycorrhiza, formed from H. olivaceoalbus as a fungal partner, such as the Piceirhiza gelatinosa, is white and has a smooth, waxy surface, with several layers of hyphae layered around the tree's roots; sometimes this mycorrhiza shows hypertrophy. The hyphae are covered with a jellylike mass that is secreted from the outer walls of the hyphae. The ectomycorrhizae can reach lengths of up to 10 mm (3⁄8 in) and have few side branches; many older ectomycorrhizae have a cavity at the tip that resembles a hole.

Similar species

Hygrophorus olivaceoalbus shows similarities between other closely related fungi of the genus Hygrophorus, some of which have only minor differences in physical features. Examples include H. pustulatus, H. inocybiformis, H. tephroleucus or H. morrisii. In the field, H. olivaceoalbus is distinguished by a combination of features including the double velum, the dark streaks on the slimy cap, the nattering of the stem, and growth under pines, as well as by microscopic characteristics. There is no risk of confusing it with toxic fungi. Hygrophorus persoonii and H. olivaceoalbus produce different mycosterine (sterol) and their flesh react differently with the addition of sodium hydroxide (red on H. olivaceoalbus versus olive green on H. persoonii). Furthermore, H. persoonii favours oaks as a mycorrhizal partner. The North American species H. inocybiformis produces a smaller fruit body with caps measuring 3–6 cm (1+1⁄8–2+3⁄8 in) wide, and dry stems that measure 3–6 cm (1+1⁄8–2+3⁄8 in) long by 0.5–1.2 cm (1⁄4–1⁄2 in) thick.

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