Suillus subg. Douglasia R. Zhang, X. F. Shi, G. M. Mueller & P. G. Liu, 2025

Shi, Xiaofei, Zhang, Shiru, Mueller, Gregory M., Liu, Peigui, Yu, Fuqiang & Senanayake, Indunil C., 2025, A subgeneric revision of the genus Suillus (Suillaceae, Boletales) and novel taxa from Eastern Asia based on morphology and multigene phylogenies, IMA Fungus 16, pp. e 144260-e 144260 : e144260-

Suillus subg. Douglasia R. Zhang, X. F. Shi, G. M. Mueller & P. G. Liu subgen. nov.

Etymology.

“ Douglasia ” refers from “Douglas fir”, the common name of Pseudotsuga menziesii , which is the only known host for the subgenus.

Typification.

Suillus caerulescens A. H. Sm. and Thiers

Diagnosis.

This subgenus is exclusively associated with Pseudotsuga menziesii . Other features of this subgenus include the glabrous or fibrillose pileus; solid, annulated, stipe lacking glandular dots; stipe often changing color to green or blue when cut and lacking clamp connections.

Morphology.

Basidiomata stipitate-pileate with tubular hymenophore. Pileus develops from hemispherical to convex or applanate, glabrous or covered with fine appressed scales, viscid beneath the fibrillose scales, or viscid but not glutinous, rusty to pale brown or with cinnamon tinge; context yellow, no color change when cut. Hymenophore broadly adnate to subdecurrent, tubes 5–10 mm deep, or up to 15 mm deep, staining brownish when bruised, mouths 1–2.5 mm diameter, angular, and radially arranged. Stipe equal, tapering downwards or upwards, solid, dry and floccose, lacking glandular dots, veil superior, membranous to floccose or gelatinous; reticulate at apex, context of lower stipe sometimes changing color to green or blue when cut. Spore print dull cinnamon or brownish.

Basidiospores smooth, oblong or suboblong, ochraceous in KOH, usually 7–11 μm. Basidia 4 - spored, clavate, hyaline yellow in KOH. Cystidia abundant, typically fasciculate, hyaline, or containing brown content and surrounded by brown amorphous material in KOH. Pileipellis usually with two layers, inner layer composed of gelatinous interwoven hyphae, and the other of ochraceous smooth or incrusted hyphae in KOH. Clamp connections absent.

Habitat.

Scattered to gregarious, ectomycorrizal with Pseudotsuga menziesii .

Known species.

Suillus caerulescens , S. lakei and S. ponderosus

Notes.

The grouping of the three species associated with Pseudotsuga menziesii ( S. caerulescens , S. lakei , S. ponderosus ) could not be classified to any subgenera in previous publications. The three species were not treated as related species in Smith and Thiers (1964) or Singer (1986). Previous section classification generally overlooked the host associations with Pseudotsuga . The three species were supported as monophyletic in past ITS phylogenies, but no higher classification was proposed ( Kretzer et al. 1996; Nguyen et al. 2016). Efforts were undertaken to search for Suillus under Asian Pseudotsuga trees, but none were found from sporophore surveys or ecological root tip sampling ( Murata et al. 2013; Wen et al. 2014; this study). The geographic range of the subgenus is restricted to Western North America and Rocky Mountains in North America delimited by the range of Pseudotsuga menziesii .