Exsudoporus frostii (J.L. Russell) Vizzini, Simonini & Gelardi


Figure 1. Exsudoporus frostii (PLP847_2018_185) fruiting bodies with red caps, deep and raised reticulation on stalks (left picture). Characteristic of the Exsudoporus genus, golden to yellow drops exude from the underside of fresh specimens (right picture).

Exsudoporus frostii (J.L. Russell) Vizzini, Simonini & Gelardi (Basidiomycota, Boletaceae) is a conspicuous bolete found in broadleaf or mixed forest throughout the eastern United States. Mature fruiting bodies (summer and fall) produce notable blood-red to apple-red caps, deeply reticulated stalks and dark red pores that when fresh ooze golden droplets (Fig. 1).  The pileus is convex to nearly plane with age, 5-15 cm wide and often with a lighter yellow margin. Chemical spot tests stain the cap bright orange KOH (Fig. 2) and yellow NH4OH. Tubes stain dark blue to black when cut and extend 6-15 mm deep (Fig. 2). Basidiospores are 11-17 µm by 4-5 µm, subfusiform to ellipsoidial, smooth and pale brown in color (Fig. 2). The stipe is 4-12 cm long and 1 – 2.5 cm wide, slowly stains blue and lacks a partial veil and annulus (Fig. 2). Prominent reticulation is continuous over the entire stipe and the stipe is red and sometime yellow at the base. The raised reticulation and dark red cap E. frostii help to distinguish it from less-prominent reticulation and lighter colored cap of E. floridanus.


Figure 2. Exsudoporus frostii (PLP847_2018_185) stipe and tubes rapidly stain blue after being cut (top and bottom, left). Chemical spot test stains the cap bright orange in KOH (top right). Basidiospores subfusiform, smooth and pale brown in color (bottom right).

Other comments: In the genus name, ‘exsudo’ is latin for “to come out in sweat, exude”. Thus, the genus Exsudoporus is aptly comprised of three species (E. frostii, E. floridanus and E. russellii) that exude “sweat” through their pores, when they are young, in the form of golden to yellow droplets. Further, the species name ‘frostii’ is reported homage to Charles C. Frost (1805 to 1880) the “shoe maker botanist”. Frost cobbled shoes for 49 years (started at age 15) in Battleboro, VT. He became a mycologist under his doctor’s orders that (for treatment of severe heartburn), “he devote an hour each morning and an hour each afternoon to the observation and study of plants in the fields”. Between 1845 and 1875, Frost cataloged numerous New England fungi. He had 60 first reports, 40 of which were boletes and gill-fungi, and all the while maintaining his full-time vocation as a shoe maker.

Taxonomy: Basidiomycota, Agaricomycetes, Boletales, Boletaceae, Exsudoporus, frostii


Syn = Boletellus frostii J.L. Russell

Bessette, A.E., Roody, W.C. and Bessette, A.R. 2000. North American boletes: a color guide to the fleshy pored mushrooms. Syracuse University, Press, Syracuse, NY.

Bessette, A.E., Roody, W.C. and Bessette, A.R. 2016. Boletes of eastern North America. Syracuse University Press, Syracuse, NY.

Murrill, W.A. 1908. Notes on the life and work of Charles C. Frost. Torreya, 8(8), 197-200.

Simpson, D.P. 1977. Cassell’s Latin Dictionary: Latin-English, English-Latin. Macmillan Publishing Co. Inc., New York, NY.

Vizzini, A. 2014. Nomenclatural novelties. Index Fungorum, 183.


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