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Hypomyces microspermus

by Michael Kuo, 30 September 2023

Like its look-alike, Hypomyces chrysospermus, Hypomyces microspermus is a white-then-yellow, mold-like fungus that covers the surfaces of boletes. While the two Hypomyces species are identical to the naked eye, however, they are different under the microscope—and their choice of victims is different: Hypomyces microspermus attacks only "xerocomelloid" boletes, while Hypomyces chrysospermus attacks many kinds of boletes, xerocomelloid or not.

Xerocomelloid boletes are small or medium-small boletes with slender stems and red to brown, subvelvety caps that often become cracked and mosaic-like; see Xerocomellus chrysenteron, Xerocomellus rubellus, and Boletus harrisonii for examples. So, if your golden yellow Hypomyces is not parasitizing a xerocomelloid victim, it is likely to be Hypomyces chrysospermus—but if the bolete victim is xerocomelloid, you will need to consult microscopic features; the aleuriospores of Hypomyces chrysospermus are larger (16–25 µm across) than the aleuriospores of Hypomyces microspermus (12–16 µm across).

Description:

Ecology: Parasitic on xerocomelloid boletes (see above); summer and fall, or over winter in warm climates; originally described from Massachusetts (Rogerson & Samuels 1989), with paratypes from across the United States and Canada, as well as Indonesia and New Zealand; also reported from Mexico, Central America, and Australia. The illustrated and described collections are from Illinois.

Fruiting Body: A mold-like covering that spreads across the bolete, eventually covering it entirely; white and tissue-like at first, becoming powdery and golden yellow, and, eventually, crustlike and brown to reddish brown.

Microscopic Features: Aleuriospores 11–16 µm including ornamentation; globose; echinate with spines 1–1.5 µm long; thick-walled; golden yellow in KOH. Conidia 6–13 x 4–6 µm; ellipsoid; smooth; hyaline in KOH. Subicular hyphae 2–5 µm wide, septate, smooth, hyaline in KOH.

REFERENCES: C. T. Rogerson & G. J. Samuels, 1989. (Rogerson & Samuels, 1989; Douhan & Rizzo, 2004; Valdez & Douhan, 2012; Siegel & Schwarz, 2016.) Herb. Kuo 05291801, 07282203, 09092205.

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Aleuriospores

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Kuo, M. (2023, September). Hypomyces microspermus. Retrieved from the MushroomExpert.Com Web site: http://www.mushroomexpert.com/hypomyces_microspermus.html