Leccinum holopus

Leccinum holopus, commonly known as the white birch bolete, white bog bolete, or ghost bolete, is a species of bolete fungus in the family Boletaceae found in northern Asia, Europe, and northeastern North America. It associates with birch trees and is typically found in boggy or swampy areas, often growing among sphagnum moss.
Fruitbodies (mushrooms) of "L. holopus" have convex caps measuring up to in diameter. Often pure white—especially in young fruitbodies—the caps sometimes become flushed with buff or brownish tints. The whitish surface of the stipe is covered with small, stiff, projecting scales (scabers) that become tan or darker in age. Some varieties of "Leccinum holopus" have been described that vary in cap color or staining reaction, but DNA evidence suggests that most are the same taxon. Although the fruitbodies are edible, opinions vary as to their culinary desirability.
Taxonomy.
Initially named as a species of "Boletus" by German mycologist Friedrich Rostkovius in 1844, the fungus was later transferred to "Leccinum" by Roy Watling in 1960. Synonyms resulting from transfer to different genera include: "Krombholzia holopoda" and "K. holopus" (both published by Albert Pilát in 1951); "Krombholziella holopus" (Josef Šutara, 1989); "Trachypus holopus" (Paul Konrad and André Maublanc, 1952), and "Trachypus scaber" f. "holopus" (Henri Romagnesi, 1939). Other synonyms, according to Index Fungorum, include "Leccinum olivaceosum", described from France in 1994, and "Leccinum aerugineum" (1991). "Leccinum holopus" is classified in section "Scabra" of genus "Leccinum", a grouping that includes Northern Hemisphere species associating exclusively with birch.
The specific epithet "holopus" is Greek for "with perfect stalk". Common names given to the fungus include white birch bolete, white bog bolete, and ghost bolete.
Several subtaxa of "Leccinum holopus" have been described. In form "aerugineum", described by Josef Šutara in 2009, the flesh discolors green after injury. The variety "americanum", described by Alexander H. Smith and Harry Delbert Thiers in 1971 from collections made in Michigan, injured flesh stains reddish. Lannoy & Estadès described "Leccinum nucatum" in 1993, a taxon that was later (2007) published as variety "nucatum" of "L. holopus"; no molecular evidence was found supporting the existence of this as a distinct taxon, and it is therefore placed into synonymy with "L. holopus". "Leccinum holopus" var. "majus", described by Rolf Singer in 1966 (originally published by Singer as "Krombholzia scabra" f. "majus"), is another historical variety without independent taxonomic significance.
Description.
Fruitbodies of "Leccinum holopus" have convex to flattened caps measuring in diameter, with a narrow band of sterile tissue surrounding the margin. The caps are initially whitish, but can develop gray, buff, tan, or pinkish tints during maturity; the color may also darken and become greenish with age. The cap surface is initially covered with very fine hairs, but later becomes more or less smooth, often with a sticky texture in age or in moist conditions. The flesh is white and lacks any distinct odor or taste; it can have either little or no bruising color reaction with injury, or may become light pink in variety "americanum". On the cap underside is a porous surface comprising pores numbering 2 to 3 per millimeter, each of which is the end of a tube that extends to deep. The color of the pore surface ranges from whitish to grayish to dingy brown, and has little color reaction to injury, although it may discolor yellowish or brownish. There is a depression where the pores meet the stipe. The stipe measures long by wide. Its whitish surface is covered with scabers that darken in age to tan or darker. The stipe base often stains bluish.
"Leccinum holopus" produces a brown spore print. Spores are somewhat fusoid (spindle-shaped) and measure 14–20 by 5–6.5 µm. The basidia (spore-bearing cells) are four-spored and measure 28.5–36.5 by 11.5–12.5 µm. Cystidia on the pores are flask-shaped (lageniform) to fusiform, and 39.0–45.5 by 7.5–9.0 µm, while those of the stipe (caulocystidia) are fusiform, club-shaped, or cylindrical, measuring 39.0–54.5 x 9.1–13.5 µm. There are no clamp connections present in the hyphae of "L. holopus". The cap cuticle is arranged in the form of a cutis—with hyphae that run parallel to the cap surface.
Several chemical tests can be used to help verify an identification of "L. holopus". A drop of ammonium hydroxide solution turns the cap cuticle a pinkish color, but has no reaction with the flesh. A drop of dilute potassium hydroxide (KOH) has no reaction on the cap surface, and either no reaction or a brownish reaction with the flesh. Application of iron (II) sulphate solution does not have a reaction on the cap surface, and either no reaction to slightly olive coloration on the flesh.
Similar species.
"Leccinellum albellum" is similar in appearance to "L. holopus", but grows in association with oak and has a more southerly distribution. "L. scabrum" is a widely distributed lookalike that can be distinguished from "L. holopus" by its larger size and generally darker colors.
Edibility.
Although commonly considered edible, opinions vary on the culinary appeal of "Leccinum holopus" fruitbodies. Michael Kuo, writing in "100 Edible Mushrooms", considers it a good edible; Peter Roberts and Shelley Evans in "The Book of Fungi" say "it is edible, but is said to be tasteless and pappy, so is not recommended." Fruitbodies are optimally harvested when they are young, before the flesh becomes too spongy, and before insect larvae establish themselves. Minimal cleaning is required in the field. The mushroom has a mild, somewhat sweet flavor that is enhanced after brief sauteeing. Drying the mushrooms enhances the flavor, but diminishes the sweetness of fresh mushrooms.
Habitat and distribution.
"Leccinum holopus" is a mycorrhizal species. It fruits on the ground (often among "Sphagnum" moss), singly to scattered in wet areas like cedar swamps, bogs, or soggy forests. Like most "Leccinum" species, the fungus is highly host-specific and associates with birch ("Betula"). In North America, the range of "Leccinum holopus" extends from eastern Canada to New York, extending west to the northern Rocky Mountains, roughly coinciding with the distribution of the paper birch ("Betula papyrifera"). In this range, it is common and fruits from August to October. "L. holopus" var. "americanum" is known only from North America. The fungus is rare in southern Europe, but more common in the "Sphagnum" swamps in the north. In Asia, it has been recorded from Taiwan and Qinghai (China).
Fruitbodies of "L. holopus" are a source of food for fly species such as "Pegomya winthemi" (family Anthomyiidae) and "Megaelia pygmaeoides" (family Phoridae).