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Russula michiganensis

by Michael Kuo, 6 September 2024

One of several black staining russulas in North America, Russula michiganensis can be separated from the others with a combination of features: it stains gray and black directly, without staining red first (see for example Russula densifolia); its gills are fairly crowded; its taste is slowly but distinctly acrid; it appears under oaks in eastern North America's oak-based forests; and the amyloid ornamentation on its spores extends up to about 0.5 µm. The cap and stem are initially white, but the gray and black bruising and staining commence almost immediately (as though even the air currents can bruise it), and the surfaces are soon somber.

Russula michiganensis has likely been included in some eastern North American treatments of "Russula albonigra," which is apparently a strictly European species (Adamčík & Buyck 2014) with a menthol-like taste and very low spore ornamentation that projects only up to 0.3 µm. Western North America's "Russula albonigra" is a large, conifer-associated species that may be unnamed; its gills are not as crowded, its cap develops brown colors, and its taste is not distinctive.

Description:

Ecology: Mycorrhizal with oaks and possibly other hardwoods in oak-based forests; growing gregariously; late summer and fall; originally described from northern Michigan (Shaffer 1962); reported in North America from the Great Plains eastward. The illustrated and described collection is from Illinois.

Cap: 5–8 cm; convex, becoming broadly convex with a central depression; appearing bald, but with s sub-felty feel when rubbed; dry; white at first, becoming whitish with a grayish center, then developing gray to black stains and eventually becoming nearly black overall; the skin peeling about one-third of the way to the center; the margin not lined.

Gills: Broadly attached to the stem; crowded; short-gills frequent; creamy, bruising and staining gray, then black.

Stem: 2–5 cm long; 1–2 cm thick; tapered to base; dry; bald; white, bruising gray, then black; basal mycelium white.

Flesh: White; staining gray, then dark gray when sliced.

Odor and Taste: Odor not distinctive; taste slowly acrid.

Chemical Reactions: KOH on cap surface negative. Iron salts on stem surface pinkish.

Spore Print: White.

Microscopic Features: Spores 7–9 x 4–5 µm; ellipsoid; ornamentation as amyloid lines and warts up to about 0.5 µm high, sometimes forming partial reticula. Basidia 30–35 x 6–8 µm; clavate; 4-sterigmate. Pleurocystidia 30–70 x 4–8 µm; fusiform to lageniform; smooth; thin-walled; hyaline in KOH, with refractive contents. Pileipellis an ixocutis; elements 2–5 µm wide, smooth, hyaline to brown in KOH; pileocystidia not found.

REFERENCES: R. L. Shaffer, 1962. (Shaffer, 1962; Kibby & Fatto, 1990; Adamčík & Buyck, 2014.) Herb. Kuo 08012402.

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Spores

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Kuo, M. (2024, September). Russula michiganensis. Retrieved from the MushroomExpert.Com Web site: http://www.mushroomexpert.com/russula_michiganensis.html