Hortiboletus flavorubellus (Thiers & A. H. Sm.) Biketova & Tremble comb. nov.

Boletus flavorubellus Thiers & A. H. Sm., Michigan Bot. 5 (3): 117 (1966). Basionym.

= Boletus rubellus var. flammeus A. H. Sm. & Thiers, Boletes of Michigan: 269 (1971)

? = Boletus subfraternus Coker & Beers ex Meyers & Van T. Cotter, Index Fungorum 412: 1 (2019).

Material examined.

USA • Michigan: Washtenaw Co., Ann Arbor, 42°16'31"N, 83°43'51"W, on wet soil in woods, 11. 08. 1960, leg. & det. A. H. Smith, MICH 10015 (A. H. Smith 62872 – collector’s number, holotype), GenBank: ITS – PV 036233, LSU – PV 036238, tef 1 - α – PV 106170, and rpb 2 – PV 106174; • Washtenaw Co., west of Ann Arbor, Saginaw Forest on Liberty Road, 42°16'12"N, 83°48'25"W, scattered on humus in mixed forest of Quercus sp. and Pinus sp., 02. 08. 1966, leg. & det. A. H. Smith, MICH 10035 (A. H. Smith 73026, holotype of Boletus rubellus var. flammeus), GenBank: ITS – PV 036231 and PV 036232, LSU – PV 036236 and PV 036237, tef 1 - α – PV 106171, and rpb 2 – PV 106175 .

Notes.

Present phylogenetic analysis, which included sequences of holotypes of B. flavorubellus MICH 10015 and B. rubellus var. flammeus MICH 10035, proved that these two taxa are conspecific and allowed us to propose a new taxonomic combination Hortiboletus flavorubellus . These two type specimens also cluster together in the same species-level clade in the phylogenomic tree of Tremble et al. (2024). In the current multigene phylogeny, H. flavorubellus forms a strongly-supported terminal clade (BS = 100, PP = 1.00), sister to the H. engelii / H. hershenzoniae / Hortiboletus sp. 1 subclade (BS = 85, PP = 1.00).

This species, described by Thiers and Smith in 1966, is not yet understood by both mycologists and citizen scientists. Based on the sequence data from INSDC, connected publications (Nuhn et al. 2013; Frank et al. 2020; Kuo and Ortiz-Santana 2020), as well as records in iNaturalist (2024) and Mushroom Observer (2024), there are no collections identified either as B. flavorubellus or B. rubellus var. flammeus (see Fig. 1 and Suppl. materials 1, 3). A few of the collections are identified as H. rubellus (e. g. FLAS-F- 60513) or Xerocomellus cf. rubellus (MB 03-033), a synonym of which B. rubellus var. flammeus is considered, based on Index Fungorum (2025). However, most of H. flavorubellus collections were either not identified to the species level or misidentified as H. campestris (e. g. iNaturalist 66900805), B. harrisonii (e. g. PBM 4097), and B. subfraternus (MICH KUO- 08101302) (see Fig. 1 and Suppl. materials 1, 3). However, there is a possibility that B. subfraternus is the actual synonym of this species; therefore, it is necessary to conduct its type study, including analysis of their DNA sequences.