Hortiboletus rubellus (Krombh.) Simonini, Vizzini & Gelardi

Biketova, Alona Yu., Svetasheva, Tatyana Yu., Taylor, Andy F. S., Simonini, Giampaolo, Gelardi, Matteo, Morozova, Olga V., Polemis, Elias, Muñoz, José A., Albert, László, Saitta, Salvatore, Wasser, Solomon P., Nevo, Eviatar, Zervakis, Georgios I., Vizzini, Alfredo & Dima, Bálint, 2025, Morphological and molecular re-assessment of European and Levantine species of the genus Hortiboletus (Boletaceae), IMA Fungus 16, pp. e 144731-e 144731 : e144731-

Hortiboletus rubellus (Krombh.) Simonini, Vizzini & Gelardi , in Vizzini, Index Fungorum 244: 1 (2015)

Figs 3 a , 6 , 7 , 8

≡ Boletus rubellus Krombh. , Naturgetr. Abbild. Beschr. Schwämme 5: 12 (1836)

≡ Suillus rubellus (Krombh.) Kuntze , Revisio generum plantarum 3 (3): 536 (1898)

≡ Leucobolites rubellus (Krombh.) Beck , Z. Pilzk. 2 (7): 142 (1923)

≡ Tubiporus rubellus (Krombh.) S. Imai , Trans. Mycol. Soc. Japan 8 (3): 113 (1968)

≡ Xerocomellus rubellus (Krombh.) Šutara , Czech Mycol. 60 (1): 50 (2008)

= Xerocomus rubellus Quél. , Compt. Rend. Assoc. Franç. Avancem. Sci. 24 (2): 620 (1896) [1895]

= Boletus versicolor Rostk. , in Sturm, Deutschl. Fl., 3 Abt. (Pilze Deutschl.) 5: 55 (1844), nom. illegit., Art. 53.1 (Shenzhen), non Boletus versicolor L. , Sp. Pl. 2: 1176 (1753) = Trametes versicolor (L.) Lloyd (1921) [1920]

= Versipellis versicolor (Rostk.) Quél. , Enchir. Fung.: 158 (1886), nom. illegit., Art. 53.1 (Shenzhen)

= Versipellis pruinata var. versicolor Quél. , Enchir. Fung.: 158 (1886), replaced synonym of Boletus versicolor Rostk. (1844)

? = Boletus sanguineus With. , Syst. Arr. Brit. Pl., Edn 4 4: 313 (1801), nom. illegit., Art. 53.1 (Shenzhen), non Boletus sanguineus L. , Sp. Pl., Edn 2 2 (2): 1646 (1763) = Pycnoporus sanguineus (L.) Murrill (1904)

? = Viscipellis sanguinea (With.) Quél. , Enchir. Fung. (Paris): 156 (1886), nom. illegit., Art. 53.1 (Shenzhen)

? = Boletus rutilus Fr. , in Fries & Hök, Boleti, Fungorum generis, illustratio: 5 (1835)

Holotype.

CZECHIA • tab. XXXVI, figs 21–24 ( Krombholz 1836).

Epitype designated here

( MBT 10022257 ). ITALY • Emilia-Romagna: Reggio Emilia, Viano, Pulpiano , 44°30'23"N, 10°33'24"E, 507 m, several young to mature basidiomes growing in a cultivated field beside a track, close to a Quercus cerris and Q. pubescens wood, 19. 06. 2011, leg. & det. G. Simonini, MCVE 31743 (GS 10212 – collector’s number, K-M 001435705 – isoepitype), GenBank: ITS – PV 094851 .

Description.

Basidiomes pileate-stipitate, xerocomoid, epigeal, small to medium-small sized. Ontogenetic development gymnocarpic.

Pileus (2 –) 3.5–10 (– 15) cm broad, nearly hemispherical at first, then convex, pulvinate to applanate, also slightly depressed at centre, sometimes irregularly shaped; surface matt, dry, ± finely tomentose to sometimes rugulose, but later smooth, sometimes areolate towards the peripheral zone or with minute to coarse cracks especially in dry weather and showing a yellowish or somewhat pinkish subcuticular layer resembling some areolate Xerocomellus species; margin initially involute then extended, regular to wavy-lobed, acute and slightly exceeding the pileal context; rather variable in colours, but generally with bright red tints, initially blood red, wine red, cherry red, rose red, orange-red, pink, fading in mature basidiomes from ochraceous red, ochraceous pink to reddish-brown, with a margin at the very beginning whitish (due to a narrow band 1–2 mm broad), then gradually concolourous with age, unchanging or becoming slightly darker on handling or when injured.

Tubes up to 12 mm long, at first adnate, later emarginate or slightly decurrent with a tooth, initially lemon or chrome yellow, later greenish-yellow to olive brown, slightly bluing when injured.

Pores large, wider than 1 mm (up to 1.5–2 mm) in old basidiomes, roundish to angular, concolourous with the tubes, in mature basidiomes with orange-rusty tinges, weakly bluing when bruised and later becoming dirty brownish.

Stipe (2 –) 2.5–7 (– 12) × 0.4–2.5 (– 3.5) cm, usually as long as or slightly longer than pileus diameter, central or slightly off-centre, solid, firstly versiform, clavate or somewhat bulbous, later more cylindrical or sometime inflated in the lower part, always tapering at the very base and slightly rooting; surface with fine reddish punctuations or fibrils on an ochraceous yellow background, later often with rather coarse longitudinal striations; surface usually concolourous with the pileus or paler, but yellow or pale yellow at apex; sometimes the stipe can be entirely yellow or entirely red, or a mix of both colours; basal mycelium whitish-yellow.

Context rather firm when young, later soft, becoming fibrous in the stipe (up to 1.5 cm in the centre of the pileus); pale yellow, dirty ochraceous yellow in the lower half of the stipe and eventually pale pinkish underneath the cuticle; in the stipe base with a typical orange-red pigment in the form of small dots, which are sometimes scarce and hardly noticeable or conversely abundant and then visible as a large flame-orange area at the stipe base; light blue on exposure, especially in the pileus-stipe connection zone, above the tubes and in the peripheral layers of the stipe, later fading to dirty yellow; discolouration less pronounced in dry and old basidiomes.

Spore print olive brown.

Odour pleasantly mushroom-like to fruity or inconspicuous. Taste mild.

Macrochemical spot-test reactions: Melzer’s reagent: a weak fleeting-amyloid reaction in the stipe context and exceptionally also in the hymenophoral trama of dried basidiomes sometimes occurs, but most collections show a strong dextrinoid reaction in all tissues; 20 % KOH: reddish-brown on pileus and hymenophore, pinkish on pileal context, bright orange on stipe surface and context and progressively darker downwards; 25 % NH 4 OH: negative to pale yellow everywhere; 10 % FeSO 4: sordid green everywhere, particularly on hymenophore.

Basidiospores [561 / 19 / 19] (10.3 –) 11.6 ± 0.6 (– 12.8) × (4.7 –) 5.0 ± 0.2 (– 5.5) μm, Q = (2.07 –) 2.31 ± 0.12 (– 2.59), Vm = 155 ± 17 μm 3, inequilateral, broadly ellipsoid or fusiform to ellipsoid-fusiform in side view, ellipsoid to amygdaliform or slightly fusiform in face view, smooth under both light microscope and SEM, apex rounded, with a short apiculus, usually with a weak to pronounced suprahilar depression and with a slightly pronounced adaxial swelling, moderately thick-walled (up to 1 μm), straw yellow to honey yellow in water and 5 % KOH, having one or two large oil droplets when mature, rarely pluri-guttulate, inamyloid and orthochromatic.

Basidia [17 / 4 / 4] (24.8 –) 33.9 ± 6.1 (– 46.3) × (7.8 –) 10.8 ± 1.3 (– 12.0) μm, subclavate to clavate, predominantly 4 - spored, sterigmata 3.1–5.0 μm, hyaline to yellowish in 5 % KOH, and containing concolourous oil guttules; basidioles subcylindrical to faintly clavate, similar in size to basidia.

Cheilocystidia [17 / 4 / 4] (26.0 –) 33.8 ± 7.0 (– 43.8) × (8.7 –) 9.3 ± 0.4 (– 10.0) μm, relatively common, scattered, slender, ventricose-fusiform, often with elongated straight or flexuous neck, with rounded tip, smooth, hyaline to pale yellowish in water and 5 % KOH, without epiparietal encrustations. Pleurocystidia [10 / 2 / 2] (21.9 –) 37.8 ± 10.2 (– 51.3) × (6.0 –) 8.4 ± 0.7 (– 11.3) μm, infrequent, slender, ventricose-fusiform, often with elongate neck, mostly hyaline, rarely with yellowish content in 5 % KOH. Pseudocystidia not observed.

Hymenophoral trama bilaterally divergent, intermediate between the “ Phylloporus - type ” and the “ Boletus - type ”: lateral strata consisting of poorly divergent, tightly arranged, non-gelatinous hyphae almost touching each other, hyaline to very pale yellow in water and 5 % KOH; mediostratum consisting of tightly adpressed, non gelatinous, parallel running hyphae; in Congo red the mediostratum is slightly darker than the lateral strata.

Pileipellis a physalo-palisadoderm consisting of moderately long, more or less cylindrical, septate hyphae; terminal elements [539 / 15 / 15] (21.4 –) 25.6 ± 4.2 (– 36.1) × (6.3 –) 8.6 ± 1.7 (– 12.1) μm, Q = (2.03 –) 3.12 ± 0.55 (– 4.19), versiform, mainly broadly cylindrical with rounded apex to cystidioid, sometimes also with elongate neck, but also slender, cylindrical, moderately clavate, slightly tapering towards the tip, lageniform, utriform, acorn-shaped, sometimes bifurcate, apex mainly rounded-obtuse, but sometimes pointed, not encrusted (especially in young basidiomes) or poorly and finely encrusted (fine granular or rarely zebra-like epiparietal encrustations) at base, hyaline to pale yellow in water and 5 % KOH; subterminal elements subcylindrical or somewhat inflated and then broader than terminal ones (up to 18 μm), sometimes branched, more frequently encrusted, pale yellowish-brown in 5 % KOH. Subpileipellis elements usually distinctly encrusted by a fine, pale yellow, granular pigment (5 % KOH).

Stipitipellis consisting of slender, subparallel to loosely interwoven, smooth-walled, hyaline to pale yellow hyphae 3–5 μm broad, forming a partially developed caulohymenium; in the upper third of the stipe, bundles of caulohymenium consist of clavate, hyaline to yellow caulobasidia (30–50 × 8–15 μm), caulobasidioles, and broadly ventricose-fusiform or clavate caulocystidia (24–50 × 7–11 μm), with an often extremely elongated neck (to 30 μm long). Stipe texture formed of parallel running, ± hyaline, 2–4 mm broad hyphae; scattered to scarce orange-red crystals (hyphal excretions) dissolving in 5 % KOH found in the stipe base context. Lateral stipe stratum under the caulohymenium poorly differentiated from the stipe trama, consisting of divergent, inclined and running towards the external surface, densely arranged hyphae.

Stipe trama inconspicuous, formed of parallel running, filamentous, 4–13 μm broad hyphae, ± hyaline in water and 5 % KOH, inamyloid.

Clamp connections absent in all tissues.

Ecology and phenology.

Solitary, gregarious in small groups (2–3 basidiomes) or even subcaespitose, growing on bare soil, on litter or in the grass, often in anthropogenic environments, such as gardens, urban parks, lawns, meadows, disturbed roadsides, light grassy forest and planting edges, shrublands, sometimes in undisturbed forests, mostly under broadleaved trees including preferably Quercus spp. ( Q. cerris , Q. coccifera , Q. faginea , Q. frainetto , Q. petraea , Q. pubescens , Q. pyrenaica , Q. robur ), Castanea sativa , Fagus spp. ( F. sylvatica , F. orientalis ), and Tilia spp. ( T. cordata , T. × europaea ). It was also collected in mixed forest with the presence of Carpinus betulus , Corylus avellana , Betula pendula , and sometimes Pinus sylvestris and Salix spp. , but there are no reports in monodominant forests consisting of these species. There are some molecularly unconfirmed data ( Engel et al. 1996; Ladurner and Simonini 2003; Assyov in litt.; Klofac in litt.; pers. obs.) about a possible ectomycorrhizal association of H. rubellus with Abies , Alnus , Cedrus , Picea , Populus ; however, they are not confirmed in the present study. The main fruiting period is spring (late April) to autumn (November to early December).

Known distribution (see Fig. 4 a).

Proven area of this species distribution is Europe and Western Asia (Fig. 4 a ). Asia: Abkhazia * (this study, GP), Armenia ( Nanagulyan 2008), Azerbaijan ( Mustafabayli and Aghayeva 2019), and Turkey ( Sesli and Denchev 2009); Europe: Austria ( Breitenbach and Kränzlin 1991; Ladurner and Simonini 2003), Belgium ( Vadthanarat et al. 2019, GP), Bulgaria ( Assyov and Denchev 2004, 2010), Croatia ( Tkalčec and Mešić 2003), Czechia ( Pilát and Dermek 1974; Šutara et al. 2009; Mikšík 2017), Denmark ( Knudsen and Taylor 2008, 2012; Kjøller unpubl., GP), Finland ( Ladurner and Simonini 2003; Knudsen and Taylor 2008, 2012), France ( Lannoy and Estadès 2001; Ladurner and Simonini 2003; Eyssartier and Roux 2011; this study), Germany ( Engel et al. 1996; Ladurner and Simonini 2003; this study, GP), Greece ( Zervakis et al. 1998; Konstantinidis 2014; this study, GP), Hungary ( Dima et al. 2013, Kaposvári 2013; this study, GP), Italy ( Alessio 1985; Simonini 1998 a, 1998 b; Lunghini and Perrone 2001; Papetti et al. 2001; Taylor et al. 2001; Cazzoli 2002; Ladurner and Simonini 2003; Boccardo et al. 2008; Galli 2013; Vasquez 2014; this study, GP), Malta ( Mifsud 2017; Sammut 2021), Montenegro ( Perič and Perič 2006; Kasom and Karadelev 2012), Netherlands ( Ladurner and Simonini 2003; Noordeloos 2007), North Macedonia ( Karadelev et al. 2007), Norway ( Knudsen and Taylor 2008, 2012, this study, GP), Poland ( Łuszczyński 2007), Portugal ( Calzada Domínguez 2007), Romania ( Pál-Fám and Benedek 2010), Russia ( Bolshakov et al. 2021; this study, GP), Serbia ( Uzelac 2009; Atlagić et al. 2013; Petrović et al. 2019), Slovakia ( Vasas and Locsmándi 2013), Spain ( Ladurner and Simonini 2003; Pardo et al. 2004; Calzada Domínguez 2007; Muñoz et al. 2008; this study, GP), Sweden ( Taylor and Eberhardt 2006; Knudsen and Taylor 2008, 2012; this study, GP), Switzerland ( Breitenbach and Kränzlin 1991), Turkey (East Thrace) ( Asan and Gücin 1990), Ukraine ( Zerova and Rozhenko 1988; Akulov and Prydiuk 2007; this study, GP), and United Kingdom ( Watling and Hills 2005; Hills 2008; Kibby 2017; this study, GP). This species is also recorded in North Africa ( Algeria, Morocco, Tunisia) ( Singer 1967; Malençon and Bertault 1970; Bertault 1979; Haimed et al. 2013; Nounsi et al. 2014; Outcoumit et al. 2014; El Mokni and El Aouni 2019; Ouali et al. 2021; Youcef Khodja et al. 2021), Macaronesia (Canary islands) ( Bañares Baudet 1988), East Asia (Russian Siberia, China, Japan, Thailand) ( Chiu 1957; Singer 1967; Vasiljeva 1978; Bi et al. 1994; Engel et al. 1996; Nagasawa 1997; Zhuang 2001; Zang 2006; Mao 2009; Bolshakov et al. 2021; Thongkantha et al. 2021); and North America ( Guatemala, Mexico, USA) ( Singer 1945, 1947, 1967; Smith and Thiers 1971; Grund and Harrison 1976; Both 1993; Bessette et al. 2000, 2016, 2019; García-Jiménez 2013; Flores Arzù 2020; Saldivar 2021). However, extra-European and extra-Western Asian records, perhaps except for north African samples, may represent different Hortiboletus species and their identity should be verified by DNA sequencing. Based on the description and illustration (p. 101; Pl. 12, Fig. 2 ) of Binyamini and Avizohar-Hershenzon (1973), a record of this species (as Xerocomus rubellus ) from Israel represents Xerocomellus redeuilhii A. F. S. Taylor, U. Eberh., Simonini, Gelardi & Vizzini.

Additional examined material.

ABKHAZIA * • Gagra District: near Pitsunda, vicinity of Ptitsefabrica, on a bank of Bzyb River near the mouth , 43°11'53.5"N, 40°18'49.0"E, on soil near a path in mixed Quercus-Fagus forest, 02. 10. 2007, leg. A. Kiyashko, det. A. Kiyashko & T. Svetasheva, LE F- 265201 ( GP) ; FRANCE • Île-de-France: Paris, Thoiry , 48°51'48"N, 10°31'09"E, 165 m, five basidiomes in the grass, under Quercus sp. , 30. 07. 1993, leg. & det. G. Redeuilh, GS 1116 ; GERMANY • Hesse: Giessen, old graveyard , under Quercus sp. , 10. 06. 1998, leg. & det. A. Taylor, AT 1998115 ( GP) ; Rhineland-Palatinate: Mainz, Ober Olmer Wald , mixed woodland, but mainly Quercus sp. , 05. 08. 1998, leg. & det. A. Taylor, AT 1998046 ( GP) ; GREECE • Central Greece: Trikala, Logga , 39°48'60"N, 21°55'54"E, under F. sylvatica , 02. 07. 2012, leg. G. Konstantinidis, det. G. Konstantinidis, B. Dima & A. Biketova, GK 6074 ( GP) ; • West Macedonia: Kozani, Vouchorina , 40°12'34"N, 21°15'59"E, under Quercus sp. , 03. 07. 2012, leg. G. Konstantinidis, det. G. Konstantinidis, E. Polemis & A. Biketova, GK 6705 ( GP) ; HUNGARY • Csongrád-Csanád: Szeged, Elisabeth Park , 46°14'43.9"N, 20°09'49.6"E, on the ground in the grass, with various deciduous trees in vicinity, five basidiomes, 19. 07. 2020, leg. A. Biketova & B. Dobrić, det. A. Biketova, K-M 001435676 (AB B 20-358; GP) ; • Somogy: Darány , under Q. robur , C. betulus , Tilia sp. , 18. 09. 2013, leg. & det. L. Albert, AL 13-125 ( GP) ; • Vas: Őrség NP, Szalafő , on a forest path in mixed forest dominated by Q. petraea , F. sylvatica , C. betulus , P. sylvestris , 29. 06. 2022, leg. & det. D. Varga & B. Dima, DB- 2022-06 - 29 - 3 ( GP) ; • Komárom-Esztergom: Vértes Mts, Csákányospuszta, Mária-szakadék , in mixed forest with Quercus sp. , F. sylvatica , C. betulus , 21. 07. 2018, leg. & det. Gy. Vrba, VGy- 2018-07 - 21 - 1 ( GP) ; ITALY • Emilia-Romagna: Reggio nell’Emilia, Carpineti, Pantano , 44°29'23"N, 10°31'09"E, 670 m, a single basidiome at the edge of the wood, with Quercus pubescens and C. sativa , 23. 10. 1993, leg. & det. G. Simonini, GS 1037 ; • Reggio nell’Emilia, Viano, Pulpiano , 44°30'35"N, 10°33'22"E, 515 m, nine basidiomes in a field cultivated with Medicago sativa , close to Q. cerris and Q. pubescens wood, 03. 07. 1994, leg. & det. G. Simonini, MCVE 17716 (GS 1180) ; • ibid., 44°30'33"N, 10°33'20"E, 512 m, four basidiomes in a field cultivated with M. sativa , close to a Q. cerris and Q. pubescens wood, 10. 07. 1994, leg. & det. G. Simonini, MCVE 17717 (GS 1196) ; • ibid., 44°30'31"N, 10°33'25"E, 526 m, two basidiomes in the path in the furrow of a tractor wheel, in Q. cerris and Q. pubescens wood, 23. 07. 1994, leg. & det. G. Simonini, GS 1206 ; • ibid., 44°30'30"N, 10°33'15"E, 507 m, five basidiomes in a field cultivated with M. sativa , close to a Q. cerris and Q. pubescens wood, 30. 07. 1994, leg. & det. G. Simonini, MCVE 17773 (GS 1241) ; • ibid., 44°30'37"N, 10°33'21"E, 517 m, several young to mature basidiomes growing on a lawn near a track, close to a Q. cerris and Q. pubescens wood, 31. 08. 1994, leg. & det. G. Simonini, MCVE 17903 (GS 1424; GP) ; • ibid., 44°30'32"N, 10°33'20"E, 511 m, several young to mature basidiomes growing in a ploughed meadow, close to a Q. cerris wood, 07. 06. 1998, leg. & det. G. Simonini, MCVE 18231 (GS 1894; GP) ; • ibid., 44°30'35"N, 10°33'24"E, 516 m, 9 basidiomes in a cultivated field close to Q. cerris and Q. pubescens wood, 23. 08. 2015, leg. M. Comuzzi, det. G. Simonini, GS 10226 ; • Lazio: Manziana (RM), Macchiagrande di Manziana , 42°07'24"N, 12°07'19"E, 320 m, several young to mature basidiomes collected on acidic soil in a very moist grassy clearing bordering a mixed broadleaved woodland dominated by Q. cerris with the presence of Q. frainetto , Acer sp. and Crataegus sp. , 12. 10. 2019, leg. M. Gelardi & F. Costanzo, det. M. Gelardi, F. Costanzo & A. Biketova, MG 781 ( GP) ; • Sicily: Messina, Cesarò, Portella dei Bufali , 37°52'17"N, 14°41'17"E, 1196 m, five basidiomes in the grass close to Q. cerris and Quercus sp. trees, 21. 10. 2013, leg. G. Vasquez, det. leg. G. Vasquez & G. Simonini, K-M 001437348 (GS 10116) ; NORWAY • Ostfold Co.: Rygge, Fuglevikasen 12 , 59°23'17.8"N, 10°39'29.8"E, 10. 09. 2014, leg. E. W. Hanssen & R. Braathen, det. B. Dima, O-F- 76041 ( GP) ; • Telemark Co.: Kragerø, Berg Museum , 58°53'02"N, 9°22'59"E, 06. 08. 2000, leg. T. E. Brandrud, det. B. Dima, O-F- 168828 ( GP) ; RUSSIA • Bryansk Oblast: Bryansky Forest Reserve , on soil in broadleaf forest dominated mostly by Q. robur , T. cordata , and Ulmus glabra , 07. 08. 2015, leg. & det. T. Svetasheva, LE F- 332230 ; • Penza Oblast: Gorodishchensky District, vicinity of Nikonovo , floodplain Quercus forest, 30. 07. 2018, leg. A. Ivanov, det. A. Ivanov & T. Svetasheva, LE F- 316026 ; • Tula Oblast: Shchekinsky District, Museum-estate of Lev Tolstoy “ Yasnaya Polyana ” , on a grassy edge in mixed Quercus-Tilia forest, 14. 08. 2002, leg. & det. T. Svetasheva, LE F- 234797 ; • vicinity of Lesnoy township , “ Tulskiye zaseki ” broadleaved forest, on soil with Q. robur , T. cordata , Acer platanoides , U. glabra , and C. avellana , 27. 06. 2012, leg. & det. T. Svetasheva, LE F- 332231 ; • Kurkinsky District, vicinity of Khvorostyanka, “ Vodyanoe pole ” protected area , on soil in broadleaved forest with Q. robur , T. cordata , and B. pendula , 09. 07. 2001, leg. & det. T. Svetasheva, LE F- 315853 ; • ibid., margin of a broadleaved forest, on soil in the grass, 17. 07. 2001, leg. & det. T. Svetasheva, LE F- 332082 ; • Udmurt Republic: Votkinskji District, 4.6 km south of Perevoznoe , 56°51'36"N, 53°54'07"E, 75 m, on the ground in a Q. robur forest, with Salix , 09. 07. 2015, leg. V. Kapitonov, det. V. Kapitonov & G. Simonini, GS 10512 ( GP) ; • Votkinskji district, Perevoznoe , 56°53'13"N, 53°55'31"E, 98 m, on the ground in a Q. robur forest, 13. 07. 2015, leg. V. Kapitonov, det. V. Kapitonov & G. Simonini, GS 11020 ; SPAIN • Bizkaia: Urduliz, Mendiondo , 43°21'23"N, 2°57'00"W, 100 m, under Q. robur , 03. 09. 2005, leg. E. Fidalgo & J. Muñoz, det. J. Muñoz & A. Taylor, JAM 0556 ( GP) ; • Burgos: Valle de Mena, Barrasa , 43°06'16"N, 3°19'39"W, 430 m, locality of the holotype of Xerocomus erubescens , growing among the grass in a cleared area, under Q. faginea , 28. 10. 2001, leg. J. Muñoz, det. J. Muñoz, A. Taylor & A. Biketova, JAM 0224 ( BAR 1997082010 ; GP) ; • Segovia: Boca del Asno , on humus soil, with P. sylvestris and Q. pyrenaica , 13. 10. 1999, leg. Soc. Micol. Madrid, det. H. Ladurner & A. Biketova, IB 19991021 ( GP) ; SWEDEN • Blekinge Co.; F. sylvatica and Quercus sp. forest, 04. 09. 2003, leg. A. Ryberg, det. A. Taylor, AT 2003134 ( GP) ; Stockholm Co.: Stockholm, 24. 07. 2001, leg. H. - G. Toresson, det. A. Taylor, AT 2001124 ( GP) ; • Uppsala Co.: Nåsten, Fäbodarna , under Quercus sp. , 30. 07. 2005, leg. & det. A. Taylor, AT 2005022 ( GP) ; • Västra Götaland Co.: Tanum, Greby , mixed forest, 06. 09. 2004, leg. J. Nilsson, det. A. Taylor, AT 2004285 ( GP) ; UKRAINE • Vinnytsia Region: Vinnytsia Forest Park, 50 m from the northern shore of Guralnya Lake, along a footpath , two mature basidiomes on ground near a fallen tree branch, in F. sylvatica and Q. robur forest, 09. 08. 2012, leg. A. Biketova & A. Zamorotskiy, det. A. Biketova, K-M 001435677 (KW 60663 F; GP) ; UNITED KINGDOM • England: Berkshire, Windsor Great Park, Bishops gate area, SU 975723 , under T. × europaea , 23. 08. 2005, leg. A. Hills, det. A. Hills & A. Taylor, K-M 000167799 (AH 2005033; GP) ; • Hampshire, New Forest Wormstall Wood, SZ 350980 , amongst grass under Quercus sp. , 28. 07. 2004, leg. P. Hills, det. A. Hills & A. Taylor, K-M 000168971 (AH 2004055; GP) .

Notes.

This species was first described and illustrated as Boletus rubellus by Krombholz (1836). The original description (in German) is here translated in English: “ Reddish bolete. The pileus is plane-convex, slightly depressed in the centre, smooth, matt, red, with yellow, weakly adnexed hymenophore consisting of tubes having uniform length, medium-sized, with yellow, small pores, having uniform diameter and almost round; the stipe is slender, straight or curved, with round cross section, smooth, rarely longitudinally streaked, equal, slightly tapering towards the base, having the same red colour of the pileus on a yellow-brown background. The pileus context is fleshy, soft, pale yellow, reddish in the middle part of the stipe, dark yellow at the base; no odour, no peculiar taste ”.

A short Latin diagnosis follows: “ Boletus pileo plano-convexo, medio subdepresso, laevi, glabro, opaco, rubro; hymenio luteolo, subadfixo, tubulis subaequalibus, mediocribus, ostiolis flavidis, minutis, aequalibus, subrotondis; stipite longo, erecto flexuoso, tereti, glabro, rarius striato, aequalis, basi fuscescente, flavido; substantia carnosa, mollis, pilei pallide flava, stipitis medio et supra rubella, infra interne lutea. Odore nullo, sapore non speciali ”.

The holotype of Krombholz’ species is an illustration (table 36, figs 21–24) showing four basidiomes, one of them cut in half. The drawings are very schematic, they highlight the bright red pileus and stipe, the stipe tapering at the base and longitudinally streaked, the pale yellow context that is darker and reddish at the stipe base ( Krombholz 1836).

Later in 1896, Lucien Quélet described Xerocomus rubellus . The diagnosis is here translated from French: “ Stipe thin, velvety and red, pale-yellow-coloured at apex and at the base. Pileus convex (0.02–0.03 m), finely tomentose, bright blood red; context daffodil-like coloured (light yellow), then pink, blue and green, having an abundant green juice; taste fruity, smell sweet then sour. Tubes small, lemon creme; pores sinuate, pubescent, daffodil sulphur yellow, turning green similarly to the stipe. Spores fusoid (0.01–0.011 mm), pale lemon. (Pl. VI, fig. 11.).

Summer. – embankments and roads in grassy woods, Nivernais (Mme Daulnoy), Maritime Alps (Barla); it is close to versicolor. The species having the same name by Krombholz is the same as sanguineus With. and rutilus Fr ”.

Although the matter is debatable, we here traditionally assume that the original material of Quélet’s X. rubellus is the drawing ( Quélet (1896), T. XXIV Pl. VI fig. 11), constituted by a single basidiome. Quélet’s drawing is difficult to interpret, the only emerging character being the tapering stipe base.

The combination “ Xerocomus rubellus (Krombh.) Quél. 1896 ”, sometimes still used, does not comply with Art. 41 (ICN Shenzhen, Turland et al. (2018)) about the validity of new combinations, since no reference to a basionym is provided by Quélet. On the contrary, Quélet (1896: 620) wrote “ L’espèce de mème nom de Krombholz est le mème que sanguineus, With. et rutilus, Fr. ” (“ The species with the same name by Krombholz is the same as sanguineus With. and rutilus Fr. ”). Therefore, Quélet assumed that Boletus rubellus Krombh. is a synonym of both B. sanguineus With. and B. rutilus Fr. , excluding “ his ” X. rubellus . Quélet’s standpoint was also expressed in his previous work Enchiridion Fungorum (1886), where he formally assessed the synonyms of B. rubellus Krombh. , namely B. sanguineus With. and B. rutilus Fr. , and accommodated them in the genus Viscipellis , while the dry-pileus boletes were assigned to the genus Versipellis . According to Quélet, B. rubellus Krombh. is definitely different from X. rubellus (dry velvety pileus) and the latter is a valid name for a new species, having a different type from that of B. rubellus Krombh. The citation “ Xerocomus rubellus (Krombh.) Quél. 1896 ” is a mistake introduced by some later authors and then multiplied and has, therefore, to be rejected ( Redeuilh 1990).

However, by reading the original diagnosis of B. rubellus and X. rubellus and by looking at their respective illustrations, we observe no major differences between them. They both have a red pileus and stipe (the latter with a yellowish tint at the apex and at the base) and a matte, tomentose to velvety pileus surface. Assuming that the identities of B. rubellus and X. rubellus are overlapping, Krombholz’ name would have priority, being the older of the two.

It is difficult to understand why Quélet interpreted B. rubellus Krombh. as a bolete with a viscid pileus. Some specimens collected in rainy conditions may effectively show a slight degree of gelatinisation or perhaps he confused them with Aureoboletus gentilis , placing it in “ Viscipellis ” and in synonymy with other boletes with a viscid pileus. Recently, B. rubellus Krombh. was assigned to Hortiboletus and designated as the type of the newly-established genus ( Vizzini 2015). Furthermore, we herein designate an epitype of Boletus rubellus complying with Krombholz’s description and holotype.

We sequenced the ITS region of Hortiboletus specimen JAM 0224 (BAR 1997082010) growing in the type locality of Xerocomus erubescens Cadiñanos & Muñoz , which turned out to be conspecific with H. rubellus . Xerocomus erubescens has the pinkish tint in the pileus context ( Cadiñanos and Muñoz 1992), which is occasionally present in H. rubellus . It is worth noting, however, that X. erubescens superficially resembles H. engelii due to the lack of reddish tints on both pileus and stipe surfaces (except for very young specimens), whereas the pinkish tinge of the upper pileus context and the pileipellis structure ( Cadiñanos and Muñoz 1992) are much more reminiscent of the typical diagnostic features of H. bubalinus ( Oolbekkink 1991) . In addition, the typical orange-red pigment in the stipe base of H. rubellus and H. engelii is not described for X. erubescens , but this character is also often overlooked ( Ladurner and Simonini 2003). Therefore, due to this and to the fact that at least two species of Hortiboletus can grow in the same locality, we cannot treat JAM 0224 as a topotypical collection of X. erubescens . Unfortunately, multiple attempts to sequence (using Sanger sequencing) the holotype collection of X. erubescens have failed.

There are a number of intraspecific taxa of H. rubellus (namely twelve according to Index Fungorum (2025)) that have been described from areas far away from the currently known geographic range of H. rubellus . However, most of them are likely to represent other species. This is the case with Boletus rubellus var. flammeus , which is shown to be a synonym of Boletus (Hortiboletus) flavorubellus (see Extralimital taxa).

Based on the present multilocus phylogenetic analysis (Fig. 1 ), H. rubellus belongs to the crown clade of Hortiboletus and forms a well-supported terminal clade (BS = 97; PP = 0.97), which is sister to H. sinorubellus from China (BS = 100; PP = 1.00). However, this sister relationship was not shown in the rpb 2 analysis (Fig. 2 ). Using the ITS barcoding region for comparison, the closest European species to H. rubellus is H. engelii with a remarkable 9 % dissimilarity (> 50 nucleotide and indel differences). The ITS of H. rubellus has been difficult to sequence with the classical Sanger method due to tandem repeat insertions in ITS: 1) short AC repeats in the ITS 1 region that vary in length within the species from 14 to 26 bp and even within the same genome with maximum observed difference of 2 bp (in GS 1424 and K-M 000167799); and 2) long tandem repeats in the ITS 2 estimated to be from 166–167 bp (2-7 - 7) to 165–170 bp (2-3 - 5), based on different alignment parameters.

Figures

Figure 1. ML phylogenetic tree of Hortiboletus generated from a multilocus (ITS + LSU + tef 1 - a + rpb 2) dataset. BS values ≥ 50 % and PP values ≥ 0.8 indicated at nodes. Thickened branches indicate high statistical support (BS ≥ 70 % and PP ≥ 0.95). Species names of collections follow current identification, except original names of types of Boletus harrisonii and B. rubellus var. flammeus. Hortiboletus species present in Europe and Levant are indicated by light red-coloured fields. Collections with newly-generated sequences are indicated in bold. Two-letter country codes (ISO 3166-1 alpha- 2) denote the origin of specimens. At the end of the annotations to some collections, abbreviations are indicated in brackets: e – epitype, h – holotype, p – paratype, env – environmental sample.

Figure 2. Phylogenetic trees of Hortiboletus from separate ML analyses of each locus: ITS, LSU, tef 1 - a, and rpb 2. BS values ≥ 50 % and PP values ≥ 0.8 are indicated at nodes. Thickened branches indicate high statistical support (BS ≥ 70 % and PP ≥ 0.95). Pie charts underneath each label are the percent of total characters in the alignment that are parsimony-informative.

Figure 3. Scanning electron micrographs of Hortiboletus basidiospores: a H. rubellus (MCVE 31743, epitype); b H. engelii (K-M 000170238); c, d H. hershenzoniae (K-M 001435594, holotype). Photos: (a) A. Yu. Biketova and Y. Pollak; (b) A. Yu. Biketova and B. Dobrić; (c, d) A. Yu. Biketova and M. Kalina.

Figure 4. Distribution map of a H. rubellus and b H. bubalinus, based on the current study and sequence data from public repositories and associated publications.

Figure 6. Hortiboletus rubellus basidiomes: a epitype collection MCVE 31743; b MG 781; c DB- 2022-06 - 29 - 3; d AL 13-125, e LE F- 316026; f K-M 001435677; g basal mycelium of Vgy- 2018-07 - 21 - 1; h orange-red dots in the stipe base context of GK 6705. Photos: (a) G. Simonini; (b) M. Gelardi; (c) B. Dima; (d) L. Albert; (e) A. I. Ivanov; (f) A. Yu. Biketova; (g) G. Vrba; (h) G. Konstantinidis.

Figure 7. Microscopical features of H. rubellus: a basidiospores of the epitype collection MCVE 31743 in L 4; b basidia in Congo red, left collection K-M 001437348, right – MCVE 31743; c pileipellis in Congo red, left collection MCVE 31743, centre – GS 10226, right – GS 11020; d pleurocystidia in Congo red (K-M 001437348); e cheilocystidia in Congo red (GS 11020); f end cells of pileipellis in Congo red (GS 10226). Scale bar: 10 μm. Photos: G. Simonini.

Figure 8. Size distributions of a spores and b terminal cells of pileipellis in Hortiboletus using the “ isoprobability ellipse ” method. The distributions of average spore sizes in collections are shown with one standard deviation.