Basidiomata red-orange to red. Pileus innately fibrillose or with small, recurved fibrillose scales on the margin. Exsiccata reddish brown to red. Spores ellipsoid to amygdaloid, 7–8(−8.5) × 4–5 μm.
Typus: USA. North-Carolina: Macon County, Highland, Sunset Road Trail, 21 Aug 1984, gregarious, soil and duff of road cut, hardwoods and Tsuga, A. S. Methven 3128 (EIU, HOLOTYPE). GenBank No. JX045675.
Etymology: The species is named in honor of Kenneth A. Harrison Sr., a mycologist from Wolfville, Nova Scotia.
Pileus 3–6(−9) cm broad, convex to obtusely conic, becoming conic-umbonate or broadly convex to plane in age, innately fibrillose or with small, recurved fibrillose scales on the margin, disk often smooth and nearly glabrous, dry; deep red-orange (Brazil Red) to brownish orange (Sanford’s Brown) or brownish red (Burnt Sienna) overall or disk deeper red (almost Garnet Brown) at times. Lamellae almost crowded, adnate with a slight tooth or adnexed, deep orangered (English Red) to darker red, becoming brownish orange (Xanthine Orange) to orangish brown (Amber Brown). Stipe 4–8 cm long, 0.4–0.8 cm thick, equal, evenly deep rusty orange (Orange Rufous), becoming sordid reddish brown in places with age. Cortina no observations. Universal veil indistinct. Basal mycelium yellow. Context in pileus thick, firm, dull pinkish vinaceous (Old Rose). In stipe solid, dark reddish except for narrow cortex which is ± concolorous with the stipe surface. Odor indistinctive. Exsiccata pileus reddish brown (2.5YR 5/4–4/4), red (2.5YR 5/6) to yellowish red (5YR 5/6), stipe red (2.5 YR 5/6–4/6), reddish brown (2.5YR 4/4) to yellowish red (5YR 5/6) with reddish yellow (7.5YR 6/8–7/8) to almost yellow (10YR 8/8–7/8) mycelium. UV observations negative, whole basidiome.
Spores 7–8(−8.5) × 4–5 μm, av. = 7.2–7.5 × 4.3–4.5 μm, Q = 1.55–1.85(−1.90), Qav. = 1.60–1.76 (five collections), ellipsoid to slightly amygdaloid, moderately verrucose, nondextrinoid to weakly dextrinoid. Lamellar trama hyphae not encrusted, aniline red in KOH. Lamellar edge partly fertile, especially young basidiomata with clavate- to balloon-shaped marginal cells, 10.5–17 × 7–10 μm. Basidia four-spored, 23–30 × 5–7.5 μm, in KOH hyaline or aniline red. Pileipellis a cutis with few ascending terminal elements. Scalp preparation aniline red in KOH. Uppermost hyphae (about 4–6 layers of hyphae) 5–15 μm wide, hyaline, not encrusted. Lower hyphae 5–20 μm wide, often aniline red, some with aniline red granules, not encrusted. Hypoderm not differentiated. Clamp connections present.
Ecology and distribution: In deciduous hardwood and mixed forests, often with Tsuga. August to September. Known only from eastern North America, from Massachusetts into the Appalachian mountain range.
Differential diagnosis: Cortinarius harrisonii is a red-orange to red species with innately fibrillose, to fibrillose scaly pileus and small spores. The pileus in relation to stipe is often wide, as it also can be in C. sanguineus. Cortinarius sanguineus differs from C. harrisonii by deeper red basidiomata, also seen in exsiccata, and the larger spores. Cortinarius sierraensis has more brownish pileus and larger spores. Also the distribution is very different; to date C. sierraensis is known only from Sierra Nevada, California.
Specimens studied: USA. MASSACHUSETTS: Hampshire County, vicinity of Skinner State Park, Route 47, Hadley, 24 Sep 1977, R. Halling 2331 (NYBG). NORTH CAROLINA: Macon County, Highland, Sunset Road Trail, gregarious, soil and duff of road cut, hardwoods and Tsuga, 21 Aug 1984, A.S. Methven 3128 (EIU–HOLOTYPE). Swain County, Deep Creek, under Rhododendron in deciduous woods, wet area, 25 Sep 1971, K. Harrison 11308 (DAOM). TENNESSEE: Elkmont, 17 Aug 1939, A.H. Smith 14870 (MICH). Sevier County, Great Smoky Mountains National Park, Greenbrier, Old Settlers Trail, under mixed hardwood and Tsuga, 25 Sep 2006, R. Tulloss 13394, TENN61657 (TENN).