Hortiboletus engelii (Hlaváček) Biketova & Wasser

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 engelii (Hlaváček) Biketova & Wasser , Index Fungorum 257: 1 (2015)

Figs 3 b , 8 , 11 , 12

≡ Boletus engelii Hlaváček, Mykologický Sborník 78: 67 (2001)

≡ Xerocomus engelii (Hlaváček) Gelardi , Boll. Assoc. Micol. Ecol. Romana 24–25 (75–76): 18 (2009) [2008]

≡ Xerocomellus engelii (Hlaváček) Šutara , Czech Mycol. 60 (1): 49 (2008)

= Xerocomus quercinus H. Engel & T. Brückn. , in Engel, Dermek, Klofac, Ludwig & Brückner, Schmier- und Filzröhrlinge s. l. in Europa: 205 (1996), nom. inval., nom. prov., Arts 36.1 (a), 39.1, 40.1 (Shenzhen)

? = Boletus communis Bull. , Herb. Fr. 9: tab. 393 (1789), s. auct.

= Xerocomus communis (Bull.) Bon , Docums Mycol. 14 (56): 16 (1985) [1984], s. auct.

= Ceriomyces communis (Bull.) Murrill , Mycologia 1 (4): 155 (1909), s. auct.

= Boletus communis Sowerby , Coloured Figures of English Fungi 2: 94, pl. 225 (1799), nom. illeg., s. auct.

? = Boletus subtomentosus subsp. declivitatum C. Martín , Matériaux pour la Flore Cryptogamique de la Suisse 2 (1): 18 (1904), s. auct.

= Boletus declivitatum (C. Martín) Watling , Edinb. J. Bot. 61 (1): 43 (2004), s. auct.

= Xerocomus declivitatum (C. Martín) Klofac , Öst. Z. Pilzk. 16: 258 (2007), s. auct.

Holotype.

GERMANY • Thuringia: Gera, MTB 5138/1, Weinberg (vineyard), on the ground, 250 m, 1989, leg. T. Brückner.

Description.

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

Pileus (4 –) 5–13 (– 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, finely areolate towards the peripheral zone or with minute to coarse cracks over the whole surface especially with dry weather and showing a yellowish or somewhat pinkish subpellis layer resembling some areolate Xerocomellus species ( Simonini 1998 b); margin initially involute then extended or even uplifted, regular to wavy-lobed, acute and slightly exceeding the pileal context, whitish at the very beginning for a thin strip, but soon concolourous; of extremely variable colours, but generally dark brown up to black-brown at the beginning, then fading to ochraceous, ochraceous yellow, buff, yellowish-brown, olivaceous brown, greyish-brown, olive grey, grey or also pinkish-red, orange-red, ochraceous red, ochraceous pink, reddish-brown, typically fading in age, although some basidiomes maintain a carmine red tint over the entire life-cycle; unchanging or becoming slightly darker on handling or when injured.

Tubes up to 15 mm long, at first adnate, later emarginate or slightly decurrent with a tooth, initially lemon or chrome yellow, later greenish-yellow to olive brown, unchangeable to weakly bluing when injured.

Pores large, wider than 1 mm (up to 1.5–3 mm) in old basidiomes, roundish to angular, concolourous with the tubes, in mature basidiomes with orange-rusty spots, unchangeable to weakly bluing when bruised and later becoming dirty brownish.

Stipe (2 –) 2.5–7 (12) × 0.6–3.5 (– 4.5) cm, usually as long as the pileus diameter or slightly longer, central or slightly off-centre, solid, firstly versiform, clavate or somewhat bulbose, later more or less cylindrical or sometime inflated in the lower part, not infrequently stout, bulbose to ventricose-subfusiform, always tapering at the very base and slightly rooting; surface with very fine fibrils or scales or, occasionally, very finely ribbed, which may be reddish on an ochraceous-yellow background or concolourous with the background, later often with rather coarse longitudinal striations; yellow at apex; sometimes the stipe can be entirely yellow, entirely red, or a mix of both colours; some basidiomes exhibit a thin, lemon-yellow cortex shaded of carmine red at base; unchangeable or very slowly bruising pale brown on handling; basal mycelium whitish-yellow, rarely bright yellow.

Context rather firm when young, later soft in the pileus (up to 2.4 cm thick in the central zone, up to 1.0 cm thick halfway to margin and gradually becoming thinner towards the edge), becoming fibrous in the stipe; generally pale yellow, but dirty ochraceous yellow in the lower half of the stipe and eventually (especially in collections recorded late in the season and with low temperatures) reddish for a few mm underneath the cuticle, in the stipe base with a typical carrot-orange or orange-red pigment in the form of small dots (“ ponctuation de Redeuilh ”), which are sometimes scarce and hardly noticeable or conversely abundant (especially in dry conditions) 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; discoloration less pronounced in dry and old basidiomes.

Spore print olive brown.

Odour pleasantly fungoid 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 pileus context, bright orange on stipe surface and context and progressively darker downwards; 25 % NH 4 OH: none to pale yellowish everywhere; 10 % FeSO 4: sordid green everywhere, particularly on the hymenophore.

Basidiospores [1691 / 62 / 61] (10.0 –) 12.1 ± 0.8 (– 14.2) × (4.7 –) 5.1 ± 0.3 (– 5.9) μm, Q = (2.04 –) 2.35 ± 0.13 (– 2.57), Vm = 171 ± 30 μm 3, inequilateral, broadly ellipsoid or fusiform to ellipsoid-fusiform in side view, narrowly ellipsoid to amygdaliform or subfusiform in face view, smooth under light microscope, 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 [153 / 7 / 7] (22.9 –) 36.6 ± 5.4 (– 54.8) × (8.6 –) 11.3 ± 1.3 (– 15.2) μm, subclavate to clavate, predominantly 4 - spored, sterigmata (1.5 –) 3.2 ± 1.1 (– 5.0) μm long, hyaline to yellowish in 5 % KOH and containing concolourous oil guttules, without basal clamps; basidioles subcylindrical to faintly clavate, smaller than basidia.

Cheilocystidia [18 / 2 / 2] (49.0 –) 69.8 ± 9.7 (– 85.8) × (7.7 –) 10.0 ± 2.2 (– 16.0) μm, scattered, slender, ventricose-fusiform, often with elongated straight or flexuous neck, with rounded tip, smooth, mostly hyaline to pale yellowish in water and 5 % KOH. Pleurocystidia [87 / 8 / 8] (39.1 –) 65.5 ± 13.6 (– 99.3) × (7.5 –) 10.3 ± 1.9 (– 15.9) μm, infrequent, slender, ventricose-fusiform, often with an elongated straight neck, with rounded tip, smooth, mostly hyaline to pale yellowish in water and 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 yellowish 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 [660 / 22 / 22] (24.7 –) 36.5 ± 11.0 (– 63.3) × (8.4 –) 11.3 ± 2.2 (– 17.3) μm, Q = (1.93 –) 3.47 ± 1.58 (– 7.46), versiform, mainly broadly cylindrical with rounded apex to cystidioid, sometimes also with elongated neck, but also slender, narrowly cylindrical, moderately clavate, slightly tapering towards the tip, utriform, acorn-shaped, apex mainly rounded-obtuse, but sometimes pointed, finely encrusted, more intensely and coarsely downwards (with fine granular or rarely zebra-like epiparietal encrustations), hyaline to yellowish in water and 5 % KOH; subterminal elements subcylindrical, clearly restricted at septa, equal or broader than terminal ones, more frequently and coarsely encrusted, yellowish-brown in 5 % KOH. Subpileipellis elements usually distinctly encrusted by a fine, yellowish, granular pigment (5 % KOH).

Stipitipellis consisting of slender, subparallel, smooth walled, hyaline to pale-yellow hyphae in 5 % KOH, 4.5–6 (– 9) μm broad, caulohymenium hardly observable and not well developed; in the upper third of the stipe, infrequent bundles of clavate caulobasidia (25–35 × 8–13 μm), hyaline to yellow in 5 % KOH and broadly ventricose to spherical caulobasidioles with a thin neck, (8 –) 9–12 (– 15) μm broad are usually present; furthermore, finely encrusted, hyaline, cylindrical, broad terminal cells with a round tip, (8 –) 10–12 (– 14) μm, similar to those of the pileipellis appearing here and there. Stipe texture apparently not differentiated from the stipe trama. Lateral stipe stratum not observed.

Stipe trama consisting of parallel running hyphae, 5.0–8.5 μm broad, ± hyaline in water and 5 % KOH, inamyloid; infrequent thromboplerous hyphae 6.0–8.5 μm broad are observed.

Clamp connections absent in all tissues.

Ecology and phenology.

solitary, gregarious in small groups or even subcaespitose, growing on bare soil, on litter or in the grass, often in anthropogenic environments, shrublands, less frequently in undisturbed forests, under a large array of ectomycorrhizal broadleaved trees and shrubs, including preferably Quercus spp. ( Q. cerris , Q. coccifera , Q. frainetto , Q. ilex , Q. incana , Q. robur , Q. petraea , Q. pubescens , Q. suber ), Fagus sylvatica , Tilia spp. ( T. cordata , T. platyphyllos ), Carpinus spp. ( C. betulus , C. orientalis ), sometimes also Betula spp. , Castanea sativa , Corylus avellana , Halimium halimifolium , Pinus spp. ( P. halepensis , P. pinea , P. sylvestris , P. wallichiana ), and Populus spp. ( P. alba , P. tremula ) ( Engel et al. (1996); Ladurner and Simonini (2003); Peintner et al. (2003), GP; Krpata et al. (2008), GP; Sarwar et al. (2016), GP; Leonardi et al. (2020), GP; pers. obs., GP). Some collections of H. engelii have been found in mixed forests with the presence of Picea abies and Ostrya carpinifolia , but there are no findings in monodominant forest communities formed by these tree species alone. Moreover, there are some indications of a possible association with Cedrus deodara (Kjøller in litt.; Szmidla et al. in litt.; pers. obs.), but these data have not yet been confirmed by molecular studies nor by our examined material. The main fruiting period is spring (late April) to autumn and early winter in the Mediterranean area (November to early December).

Known distribution (see Fig. 5 a).

Asia: Pakistan ( Sarwar et al. (2016), as H. rubellus, GP ); Europe: Austria ( Engel et al. (1996), as X. quercinus ; Krpata et al. (2008), as X. rubellus, GP ; Gelardi (2009)), Bulgaria (Assyov in litt.), Croatia ( Peintner et al. (2003), as X. rubellus, GP ; this study, GP), Czechia ( Hagara et al. 1999; Šutara et al. 2009; Mikšík 2017; this study, GP), Denmark (Kjöller unpubl., as X. communis, GP ), France ( Engel et al. (1996); Lannoy and Estadès (2001), as X. quercinus ; Suz et al. (2014), as Boletus sp. , GP; Van Vooren et al. (2023), as X. communis ; this study); Germany ( Engel and Brückner (1989); Engel et al. (1996), both as X. quercinus ; Engel (1999), as X. communis ; this study, GP), Greece ( Konstantinidis 2014; this study, GP), Hungary ( Albert (2003); Dima et al. (2013), both as X. communis ; this study, GP), Italy ( Simonini (1998 a); Consiglio and Papetti (2001); Taylor et al. (2001); Cazzoli (2002); Galli (2013); Vasquez (2014), all as X. communis ; Ladurner and Simonini (2003), included in X. rubellus ; Leonardi et al. (2020), as H. rubellus, GP ; this study, GP), Netherlands ( Noordeloos 2007), Malta (Mifsud and Mifsud unpubl., GP), Montenegro (Lazarevic et al. unpubl., as X. rubellus, GP ), Norway ( Knudsen and Taylor 2008), Poland (Szmidla et al. unpubl., as B. rubellus, GP ), Romania ( Suz et al. (2014), GP); Russia (this study, GP), Serbia ( Uzelac (2009), as X. communis ; this study, GP), Spain ( Calzada Domínguez (2007); Muñoz et al. (2008), both as X. communis ; this study, GP), Sweden ( Taylor and Eberhardt (2006); Knudsen and Taylor (2008), all as X. communis ; this study, GP), Ukraine (this study, GP), United Kingdom ( Taylor et al. (2002); Hills (2008), both as X. communis ; Watling and Hills (2005); Hills (2007), both as Boletus declivitatum ; Ainsworth et al. (2013), as B. declivitatum ; Kibby (2017); Ainsworth et al. (2024), as Hortiboletus, GP ; this study, GP); North America: USA ( Murrill 1909; D’Elia et al. unpubl., GP). Records from eastern Asia ( Zang 2006), usually referred to as Xerocomus communis or B. communis should be carefully re-evaluated.

Examined material.

CROATIA • Lošinj: Veli Lošinj , under Q. ilex and Olea europaea , 08. 10. 1999, leg. H. Ladurner, det. E. Polemis & A. Biketova, IB 19990917 ( GP) ; CZECHIA • South Moravian Reg.: Brno, hills above Dubnany , on soil under Quercus sp. , 12. 08. 2008, leg. A. Hills, det. A. Biketova, K-M 000170238 (AH 2008092; GP) ; FRANCE • Auvergne-Rhône-Alpes: Allier, Montluçon, the city park , 46°20'11"N, 02°37'01"E, 240 m, three young to mature basidiomes close to Quercus sp. trees, 10. 10. 1986, leg. G. Redeuilh, det. G. Simonini, MCVE 17457 (GS 1121) ; • Bretagne: Côtes-d’Armor, Plévenon, Cap Fréhel , 48°39'30"N, 02°19'17"E, 58 m, five mature basidiomes among the grass under Q. robur , 02. 08. 1987, leg. G. Reduilh, det. G. Simonini, GS 1117 ; • Île-de-France: Paris, Pré Catelan , 48°51'53"N, 02°15'09"E, 44 m, a group of young and mature basidiomes close to Quercus sp. , 22. 08. 1984, leg. G. Redeuilh, det. G. Simonini, MCVE 17456 (GS 1118) ; • Yvelines, Achères, forest east of the city , 48°57'42"N, 02°04'33"E, 27 m, three mature basidiomes in the grass at the edge of the Quercus sp. forest, 25.07. 88, leg. G. Redeuilh, det. G. Simonini, GS 1122 ; • Yvelines, Versailles, Château de Versailles , 48°48'10"N, 02°07'57"E, 133 m, a couple of one young and one mature basidiome, in the grass, under Quercus sp. , 18. 07. 1993, leg. G. Redeuilh, det. G. Simonini, MCVE 17679 (GS 1123) ; • Essonne, Massy, Parc de la Tuilerie , 48°43'36"N, 02°16'28"E, 70 m, one basidiome in the grass close to Quercus sp. trees, 03. 06. 1994, leg. G. Redeuilh, det. G. Simonini, MCVE 17836 (GS 1338) ; • Nouvelle Aquitaine: Creuse, Guéret, the city park , 46°09'46"N, 01°51'50"E, 544 m, in the grass, close to Quercus sp. trees, 11. 10. 1986, leg. G. Reduilh, det. G. Simonini, MCVE 17458 (GS 1120) ; GERMANY • Hesse: Giessen, old graveyard , under Quercus sp. , 30. 05. 1998, leg. & det. A. Taylor, AT 1998122 ( GP) ; • Wiesbaden, Park “ Unter den Eichen ” , 50°05'42.2"N, 08°13'05.9"E, 212 m, one mature basidiome under old Quercus sp. tree, 18. 06. 1998, leg. & det. A. Taylor, MCVE 18563 (AT 1998001, GS 2230; GP) ; • Rhineland-Palatinate: Mainz, Ober Olmer Wald , mixed woodland, but mainly Quercus , 26. 06. 1998, leg. & det. A. Taylor, AT 1998032 ( GP) ; • Mainz, Kloster Ebersbach , under Quercus , 28. 06. 1998, leg. & det. A. Taylor, AT 1998037 ( GP) ; GREECE • Attica: Tatoi , 38°09'39"N, 23°47'47"E, 500 m, under Q. ilex and Q. coccifera , on calcareous soil, 10. 11. 2012, leg. E. Polemis, det. E. Polemis & A. Biketova, ACAM 2012-105 ( GP) ; • Ionian Islands: Corfu, Sinies , 39°46'16"N, 19°56'38"E, under Quercus , 17. 11. 2018, leg. G. Konstantinidis, det. G. Konstantinidis, E. Polemis, B. Dima & A. Biketova, GK 11644 ( GP) ; • South Aegean: Cyclades, Amorgos Island , 36°50'57"N, 25°53'30"E, 300 m, under Q. coccifera , on acidic soil with goat manure, 06. 12. 2005, leg. E. Polemis, det. E. Polemis & A. Biketova, EP 05 - M 224 ( GP) ; • West Macedonia: Grevena Town , 40°4'25"N, 21°25'42"E, under C. orientalis and Robinia pseudoacacia , 28. 09. 2009 and 06. 10. 2015, leg. G. Konstantinidis, det. G. Konstantinidis, E. Polemis, B. Dima & A. Biketova, GK 4141 ( GP) and GK 8378 ( GP) ; • Grevena, Gorgiani , 39°56'41"N, 21°18'17"E, under Q. frainetto , 02. 07. 2012 and 24. 09. 2015, leg. G. Konstantinidis, det. G. Konstantinidis, E. Polemis & A. Biketova, GK 6124 ( GP) and GK 8287 ( GP) ; • Grevena, 1 km from Grevena to Elato , 40°5'45"N, 21°26'50"E, under C. orientalis , R. pseudoacacia and C. avellana , 04. 10. 2016, leg. G. Konstantinidis, det. G. Konstantinidis, E. Polemis, B. Dima & A. Biketova, GK 9736 ( GP) ; HUNGARY • Bács-Kiskun: Tompa , 46°10'10"N, 19°32'28"E, under Q. robur , growing together with Russula sp. , 10 basidiomes, 22. 06. 2018, leg. A. Biketova & Z. Merényi, det. A. Biketova, K-M 001435691 (AB B 18-288) ; • Csongrád-Csanád: Sándorfalva , under P. alba , 08. 06. 2013, leg. & det. B. Dima, DB 5077 ( GP) ; • ibid., 46°23'34"N 20°07'36"E, under Q. robur and R. pseudoacacia , 06. 06. 2018, leg. & det. A. Biketova & B. Dima, K-M 001435680 (AB B 18-284) ; • Fejér: Velence , in park under C. avellana , 06. 06. 2016, leg. P. Finy., det. B. Dima, FP- 2016-06 - 06 ( GP) ; • Heves: Tarnalelesz , under C. betulus , 06. 09. 2015, leg. & det. L. Albert, AL 15-56 ( GP) ; • Komárom-Esztergom: Vértes Mts, Tatabánya , under Q. cerris and Q. petraea , 12. 06. 2022, leg. & det. A. Nagy, B. Dima & Gy. Vrba, DB- 2022-06 - 12 - 3 ( GP) ; • Pest: Visegrád Mts, Pilisszentlászló , under C. betulus , 15. 08. 2021, leg. & det. L. Albert, AL 21-73 ( GP) ; ITALY • Emilia-Romagna: Reggio nell’Emilia, Parco San Lazzaro , 44°41'20"N, 10°39'36"E, 48 m, more than 20 basidiomes from young to mature, under the monumental Q. robur , 13. 09. 1993, leg. & det. G. Simonini, MCVE 17470 (GS 986) ; • Reggio nell’Emilia, city centre, Viale Umberto I , 44°41'25"N, 9°49'58"E, 80 m, seven young to mature specimens under T. platyphyllos , 05. 10. 1990, leg. unknown, det. G. Simonini, GS 760 ; • Reggio nell’Emilia, Viale Isonzo , 44°42'13"N, 10°37'43"E, 51 m, two mature basidiomes in a flower bed close to T. platyphyllos trees, 27. 08. 1995, leg. A. Nuccio, det G. Simonini, MCVE 17905 (GS 1417) ; • ibid., 44°42'14"N, 10°37'52"E, 69 m, five young to mature basidiomes growing in an urban flower bed, under T. platyphyllos , 21. 06. 1998, leg. & det. G. Simonini, MCVE 18267 (GS 1896; GP) ; • Reggio nell’Emilia, Parco del Popolo , 44°42'09"N, 10°37'52"E, 52 m, one large mature basidiome under Q. robur , 11. 09. 1993, leg. R. Meschieri, det. G. Simonini, MCVE 17468 (GS 973) ; • ibid., 44°42'13"N, 10°37'49"E, 53 m, in the grass close to T. platyphyllos and P. alba , 22. 08. 2015, leg. & det. G. Simonini, GS 10138 ( GP) ; • Reggio nell’Emilia, via Emilia San Maurizio, close to pharmacy “ Caravita ” , 44°41'37"N, 10°38'42"E, 54 m, six large mature basidiomes in a flower bed, under P. abies and T. platyphyllos , 28. 06. 1992, leg. & det. G. Simonini, GS 885 ; • ibid., one mature basidiome in a flower bed, under P. abies and T. platyphyllos , 17. 05. 1993, leg. G. Fontana, det. G. Simonini, GS 938 ; • ibid., 11 young to mature basidiomes in a flower bed, under P. abies and T. platyphyllos , 16. 06. 1995, leg. G. Fontana, det. G. Simonini, MCVE 17838 (GS 1370) ; • Reggio nell’Emilia, residential district “ Pappagnocca ” , 44°40'54"N, 10°38'38"E, 64 m, a single huge basidiome among the grass, under Q. robur trees, 15. 10. 1991, leg. unknown, det. G. Simonini, MCVE 21790 (GS 857) ; • Reggio nell’Emilia, Viale Ramazzini , 44°42'09"N, 10°38'14"E, 51 m, one young and one mature large basidiomes, among the grass in a garden in urban area near a young Q. cerris tree, 13. 06. 1992, leg. M. Comuzzi, det. G. Simonini, GS 869 ; • ibid., four young to mature basidiomes among the grass in a garden in urban area, near a young Q. cerris tree, 24. 05. 1993, leg. M. Comuzzi, det. G. Simonini, GS 939 ; • ibid., four mature basidiomes with cracked cuticle, among the grass in a garden in an urban area near a young Q. cerris tree, 04. 09. 1993, leg. M. Comuzzi, det. G. Simonini, GS 958 ; • ibid., three young to mature basidiomes, among the grass in a garden in an urban area near a young Q. cerris tree, 05. 09. 1993, leg. M. Comuzzi, det. G. Simonini, GS 961 ( GP) ; • ibid., three mature basidiomes, among the grass in a garden in an urban area near a young Q. cerris tree, 07. 09. 1993, leg. & det. G. Simonini, MCVE 17463 (GS 964) ; • Reggio nell’Emilia, Cadelbosco di Sopra , Cadelbosco di Sotto , 44°48'19"N, 10°37'06"E, 25 m, urban green areas, in the grass close to Quercus sp. trees, 10. 11. 1996, leg. G. Candiani, det. G. Simonini, MCVE 18151 (GS 1759) ; • Albinea, Ca’ de Caroli, via Mattaiano , 44°35'14"N, 10°38'03"E, 350 m, five mature basidiomes at the edge of the cart track, close to Quercus trees, 17. 10. 1993, leg. & det. G. Simonini, GS 1030 ; • Bergogno, via Bergogno , 44°33'02"N, 10°29'14"E, 590 m, two mature basidiomes at the road edge, close to C. sativa wood, 19.09. 03, leg. & det. G. Simonini, GS 1006 ; • Case Bigi, Parco del Rio Coviola , 44°40'55"N, 10°32'59"E, 70 m, in the mixed broadleaved wood along a small river, eight basidiomes from mid-age to mature, close to Q. cerris , 18. 10. 1998, leg. & det. G. Simonini, MCVE 18353 (GS 2028) ; • Codemondo, Parco del Rio Coviola , 44°40'42"N, 10°32'56"E, 94 m, in the mixed broadleaved wood along a small river, two mature basidiomes, close to Quercus sp. , 24.08. 95, leg. L. Vescovi, det. G. Simonini, MCVE 17904 (GS 1410) ; • Gattatico, Praticello di Gattatico , 44°48'25"N, 10°28'21"E, 37 m, five mature basidiomes under Q. robur trees, 7. 10. 1983, leg. G. Bramini, det. G. Simonini, MCVE 17231 (GS 71) ; • ibid., 44°48'17"N, 10°28'18"E, 35 m, green urban area, three mature basidiomes in the grass, close to Q. robur trees, 12. 10. 1998, leg. G. Donelli, det. G. Simonini, GS 2009 ; • ibid., 44°48'21"N, 10°27'58"E, 25 m, six mature big basidiomes in the grass, green urban area close to Acer campestre and Q. robur , 12. 10. 1998, leg. G. Bramini, det. G. Simonini, MCVE 18346 (GS 2022) ; • ibid., 44°48'21"N, 10°27'55"E, 36 m, three mature basidiomes grown in an orchard ( Rosaceae ), apparently far from Tilia or Fagales , 01. 11. 2001, leg. G. Donelli, det. G. Simonini, GS 2148 ; • Quattro Castella, Parco di Roncolo , 44°37'16"N, 10°29'04"E, 360 m, eleven partially dehydrated mature basidiomes, in a Q. pubescens wood, on calcareous soil, 20. 06. 1992, leg. & det. G. Simonini, GS 874 ; • ibid., 44°37'24"N, 10°22'23"E, 350 m, two mature basidiome, in a Q. pubescens wood on calcareous soil, 21. 06. 1992, leg. & det. G. Simonini, GS 878 ; • ibid., 44°37'17"N, 10°29'11"E, 380 m, one mature basidiome, in a Q. pubescens wood, on calcareous soil, 21. 06. 1992, leg. & det. G. Simonini, GS 879 ; • ibid., 44°37'27"N, 10°29'24"E, 320 m, five mature basidiomes in the Q. pubescens wood, calcareous soil, 19. 09. 1993, leg. & det. G. Simonini, GS 1009 ; • ibid., 44°37'28"N, 10°29'24"E, 315 m, three young to mature basidiomes in mixed broadleaved wood, close to Q. pubescens trees, 12. 10. 1996, leg. & det. G. Simonini, MCVE 18129 (GS 1714) ; • ibid., 44°37'19"N, 10°29'15"E, 399 m, six basidiomes mid-age to mature, in two co-grown tufts, in a wood of Q. pubescens and O. carpinifolia on calcareous soil, 20. 10. 2010, leg. & det. G. Simonini, GS 11010 ; • ibid., 44°37'22"N, 10°29'14"E, 359 m, a single mature basidiome grown in the grass at the edge of a wood of Q. pubescens and O. carpinifolia on calcareous soil, 09. 10. 2010, leg. & det. G. Simonini, GS 11011 ; • Reggio nell’Emilia, Quattro Castella, Rocca di Monte Lucio , 44°37'36"N, 10°27'56"E, 250 m, six mature basidiomes in the grass, close to Q. pubescens trees, 26. 09. 1994, leg. L. Vescovi, det. G. Simonini, MCVE 17775 (GS 1278) ; • ibid., 44°37'35"N, 10°27'49"E, 280 m, seven mature basidiomes in the grass, close to Q. pubescens trees, 26. 09. 1994, leg. L. Vescovi, det. G. Simonini, MCVE 17769 (GS 1279) ; • Parma, Sala Baganza, Carrega Woods Regional Park, near Lago della Svizzera , 44°43'49"N, 10°12'16"E, 153 m, a single isolated basidiome on the shore of the lake, with C. betulus and Q. petraea , 09. 10. 2015, leg. T. Svetasheva, det. G. Simonini, GS 10409 ( GP) ; • ibid., 44°43'47"N, 10°12'16"E, 154 m, a single isolated decayed basidiome on the shore of the lake, with C. betulus , Q. petraea , and Taxodium distichum , 09. 10. 2015, leg. T. Svetasheva, det. G. Simonini, GS 10410 ( GP) ; • Lazio: Rome, Via Appia Antica , 41°50'14"N, 12°32'20"E, 56 m, a single mature basidiome and a primordium collected on loamy, very moist grassy soil under Q. ilex and Ligustrum lucidum with the presence of bamboo sticks in the garden of a private property, 14. 07. 2009, leg. & det. M. Gelardi, MG 205 ; • Liguria: La Spezia, Deiva Marina , 44°13'16"N, 09°31'22"E, 12 m, two mature basidiomes in the grass in an urban flowerbed artificially irrigated, close to T. cordata trees, soil pH = 6.9, 20. 06. 1994, leg. & det. G. Simonini, MCVE 17713 (GS 1144) ; • La Spezia, Portovenere , Muzzerone , 44°03'25"N, 9°49'58"E, 250 m, three large basidiomes, from mid-age to mature, in a sparse forest of Q. ilex , Q. cerris , P. halepensis , and Erica arborea , 18. 10. 1987, leg. C. Fantuzzi, det. G. Simonini, MCVE 21796 (GS 530) ; • Lombardia: Milan, Paderno Dugnano, the city park “ Emilio de Marchi ” , 45°34'28"N, 09°09'47"E, 165 m, three young to mature basidiomes close to Q. ilex and Q. robur , together with many other broadleaved and conifer trees, 19. 09. 1993, leg. G. Pina, det. G. Simonini, GS 1166 ; • ibid., ten and seven young to mature basidiomes, 06. 09. 1987 and 15. 07. 1989, leg. G. Pina, det. G. Simonini, GS 1154 and GS 1159 ; • ibid., one mature basidiome, 17. 10. 1990 and 28. 07. 1993, leg. G. Pina, det. G. Simonini, GS 1163 and GS 1165 ; • three basidiomes, 04. 07. 1993. leg. G. Pina, det. G. Simonini, GS 1164 ; • Sardinia: Sassari, Calangianus , “ Le Grazie ”, under Q. suber , 05. 09. 1997, leg. M. Contu, det. A. Biketova, IB 1997735 ( GP) ; • Cagliari, Sinnai, Casula Mua Sanna, city park , 39°18'08"N, 9°12'11"E, 141 m, five young to mature basidiomes growing among the grass close to Q. ilex and Cupressus horizontalis , 01. 12. 2014, leg. A. Mua, det. G. Simonini, GS 10273 ( GP) ; • Sicily: Messina, Monti Peloritani, Mt. Dinnammare, Colle S. Rizzo , 38°13'03.9"N, 15°30'30.2"E, 450 m, seven young basidiomes growing with Q. ilex , P. pinea , and R. pseudoacacia , 14. 10. 2020, leg. & det. S. Saitta, TUR-A 209576 ( GP) ; • Messina, Monti Peloritani, Mt. Dinnammare, Portella Armacera , 38°11'54.4"N, 15°29'28.9"E, 700 m, six young basidiomes growing with Q. ilex and P. pinea , 14. 10. 2020, leg. & det. S. Saitta, TUR-A 209577 ( GP) ; • Ragusa, Monterosso Almo, Bosco Canalazzo , 37°05'12"N, 14°44'14"E, 690 m, four young to mature small basidiomes among grass in clearings, close to Q. ilex , Q. pubescens trees, 09. 11. 2016, leg. G. Vasquez, det. G. Simonini, GS 10924 ; • Palermo Prov., Cinisi, Piano Margi , under Q. ilex , 06. 11. 2003, leg. G. Venturella, det. A. Biketova, SAF 1994 ( GP) ; • Tuscany: Grosseto, Monte Argentario, Poggio Le Piane, Residence “ il Mascherino ” , 42°25'49"N, 11°10'28"E, 43 m, ten young to mature basidiomes, in sparse Q. suber forest, 30. 10. 1993, leg. & det. G. Simonini, GS 1044 ; • Grosseto, Monte Argentario, Poggio Le Piane, road towards the Residence “ il Mascherino ” , 42°25'19"N, 11°09'55"E, 161 m, four large mature basidiomes, in the Q. ilex wood, 31. 10. 1993, leg. G. Nuccio, det. G. Simonini, GS 1048 ; • Livorno, Collesalvetti, near Agriturismo Tenuta Vallelunga , 43°34'36"N, 10°23'45"E, 68 m, in a wood of Q. cerris , Acer sp. , O. carpinifolia , and F. ornus , 03. 10. 2016, leg. M. Raumi, det. G. Simonini, GS 10848 ( GP) ; • Veneto: Camping Village “ Cavallino ”, Venezia , 45°27'24"N, 12°30'11"E, 3 m, four mature basidiomes growing among the grass and the needles, under Pinus pinea , 20. 08. 2009, leg. F. Golzio, det. G. Simonini, GS 2475 ; RUSSIA • Krasnodarski Krai: Sochi City District, vic. of Mamedova Shchel Village, gorge of Lel River , 43°56'47"N, 39°18'23"E, mixed Fagus - Carpinus - Quercus forest along the river, on soil and litter, 31. 08. 2022, leg. T. Svetasheva, det. T. Svetasheva & E. Malysheva, LE F- 332232 ( GP) ; SERBIA • Central Serbia: Mačva District, Krupanj Municipality, Mala Gornja Village, near the peak of Mt. Kostajnik , 44°25'43"N, 19°17'16"E, 600 m, under F. sylvatica and Q. petrea , one basidiome, 02. 08. 2020, leg. A. Biketova & B. Dobrić, det. A. Biketova, K-M 001435694 (AB B 20-362, GP) ; SPAIN • Bizkaia: Laukiz, Talleri , 43°21'28"N, 2°55'10"W, 29 m, under Q. robur , in a roadside, 02. 09. 2005, leg. E. Fidalgo, det. J. Muñoz & A. Taylor, JAM 0605 ( GP) ; • Cantabria: Saja, Cambilla , 43°06'33"N, 4°16'14"W, 759 m, under F. sylvatica , in a roadside, 31. 08. 2005, leg. J. Muñoz, det. J. Muñoz & A. Taylor, JAM 0547 ( GP) ; • Ucieda, Río Bayones , 43°14'04"N, 4°13'01"W, 268 m, under Q. robur , growing among the grass in a cleared area, 06. 09. 2005, leg. J. Muñoz, det. J. Muñoz & A. Taylor, JAM 0591 ( GP) ; • Navarra: Lekunberri, Puerto de Uitzi , 43°03'01"N, 1°55'01"W, 850 m, under F. sylvatica , in a roadside, 09. 09. 2005, leg. M. Babace, J. Cadiñanos, E. Benguría, E. Fidalgo. A. Meléndez, M. Maguregui & J. Muñoz, det. J. Muñoz & A. Taylor, JAM 0611 ( GP) ; SWEDEN • Scania Co.: Lomma, Alnarpsparken , 55°39'04"N, 13°03'53"E, under Quercus sp. , on clayey and base rich soil, 03. 09. 2000, leg. I. Goa, det. A. Taylor, LD 1213132 ( GP) ; • Stockholm Co.: Stockholm, Ulriksdahl , growing in greenhouse, probably associated with Tilia , 16. 04. 2003, leg. & det. A. Taylor, AT 2003001 ( GP) ; • Västra Götaland Co.: Bohuslän, Hjälteby, Sundsbyvägen , Sundsby , 58°04'13.4"N, 11°41'22.5"E, in deciduous forest on shell gravel ground, 12. 09. 1994, leg. S. Jacobsson, det. A. Taylor, GB- 0017989 (AT 1994026, S. Jacobsson 94026; GP) ; UKRAINE • Vinnytsia Region: Vinnytsia Forest Park , one young basidiome growing among moss, under F. sylvatica , with presence of P. sylvestris and C. betulus , 07. 09. 2014, leg. & det. A. Biketova, K-M 001435697 (AB B 14-246; GP) ; UNITED KINGDOM • England: Berkshire, near Ascot, Silwood Park , 07. 11. 1996 and 29. 08. 2006, leg. A. Hills, det. A. Taylor, K-M 000167946 (AH 1996033; GP) and K-M 000168799 (AH 2006031; GP) ; • Oxfordshire, Bagley West Wood, sawmill area , 25. 08. 2004, leg. A. Hills, det. A. Biketova, K-M 000167271 (AH 2004100, GP) ; • Oxfordshire, Oxford, St Thomas the Martyr Churchyard, SP 507603 , with Q. ilex , 17. 08. 2004, leg. A. E. Hills, det. A. Hills & A. Taylor, K-M 000167206 (AH 2004079; GP) ; • Oxfordshire, near Wallingford, Oakley Wood , under Q. robur , 14. 10. 1998, leg. A. L. Warland, det. A. Hills & A. Taylor, K-M 000170839 (AH 1998100; GP) ; • Oxon, near Henley, Crowsley Park , 31. 11. 1996, leg. A. Hills, det. A. Hills & A. Taylor, AH 1996031 ( GP) ; • South Yorkshire, Sheffield, Norfolk Park , under Q. cerris , 08. 08. 1994, leg. C. Hobart, det. A. Taylor, AT 1994006 ( GP) .

Notes.

Hortiboletus engelii is based on Boletus engelii Hlaváček 2001 , which is, in turn, to be based on the earlier description of the invalid Xerocomus quercinus H. Engel & T. Brückn. nom. prov. The validating paper of J. Hlaváček (2001) is not free from mistakes; the new species is cited as a “ nom. nov. ”, whilst it is a “ sp. nov. ” and the earlier X. quercinus H. Engel & T. Brückn. nom. inval. ( Engel and Brückner 1989; Engel et al. 1996) is cited as a basionym, on the type of which the new species had to be based. However, X. quercinus does not have any designated holotype, but only a list of findings. One of them is from the fungarium of Thomas Brückner, collected on an unknown date in 1989 in Thüringen, Gera, Weinberg, MTB 5138 / 1 and was chosen as the holotype by Hlaváček (2001). In this case, “ MTB ” is an abbreviation of “ Messtischblatt ”, which means “ survey table sheet ” or “ topographical code ” that was often used in Germany. Therefore, there is no voucher specimen indicated in the typification, but that is not compulsory, based on Art 40.6. and 40.7 of the ICN ( Turland et al. 2018). Additionally, there is no evidence that Hlaváček ever studied the original material of X. quercinus himself. Despite all inconsistencies, the publication of B. engelii Hlaváček 2001 basically complies with the rules of the ICN (typification, diagnosis ( Turland et al. 2018)) and, therefore, this name is valid and legitimate.

We attempted to find the type of B. engelii and locate the private collection of T. Brückner, but have not succeeded yet. We found out that the private collection of Heinz Engel was relocated to the Bavarian Natural History Collections ( SNSB); however, none of the X. quercinus specimens listed in Engel and Brückner (1989) and Engel et al. (1996) is stored there. In Engel and Brückner (1989), there is a citation that the listed specimens were preserved not only in their private collections, but also in Edinburgh, likely referring to the herbarium of the Royal Botanic Garden Edinburgh ( RBGE), because it was mentioned that they sent dry material to Roy Watling for verification. However, R. Watling and current curators of RBGE were unable to locate these specimens.

The use of the old name Boletus communis Bull. ( Bulliard 1789) for the taxon, which we here describe under the name H. engelii , would be potentially possible ( Redeuilh and Simonini 1999 b) through the designation of a lectotype chosen among the basidiomes depicted in the original plate (pl. 393). However, there are several nomenclatural and taxonomic reasons which should be taken into consideration to discourage this alternative. As far as the nomenclatural reasons are concerned, we have to consider that J. B. F. P. Bulliard (1789), in a page of Herbier de la France (p. 328) belonging to a volume different from the one in which the original diagnosis of B. communis was firstly published, included the description and plates of B. communis under the newly-described species Boletus chrysenteron : “ Il faut aussi rapporter à cette espèce les individus dont nous avons donné la figure, pl. 393, sous le nom vulg. de BOLET COMMUN ... ”. Furthermore, in the description of B. chrysenteron , the author explicitly cites again the plate 393 earlier referred to B. communis . Accordingly, part of the pl. 490 (including its caption) is referred to as B. chrysenteron , while in the description of B. chrysenteron , the French author cites both pl. 393 and pl. 490. In conclusion:

Boletus communis Bull. 1789 is validly and legitimately published, having both a potential lectotype (to be chosen among pictures in pl. 393) and description (caption of pl. 393).

Pl. 393 contains the potential lectotype of B. communis , but it might also contain a possible lectotype of B. chrysenteron .

Since both B. communis and B. chrysenteron were published before 1 Jan 1953, the uniqueness of the type does not invalidate either B. communis or B. chrysenteron (Art. 36.3 of the ICN ( Turland et al. 2018)). Moreover, since B. chrysenteron is a valid, but illegitimate name (cf. also Hansen and Knudsen (1992) and Redeuilh and Simonini (1995)), choosing a shared lectotype (i. e. the basidiome in the lower right of pl. 393) for B. communis and B. chrysenteron could lead to their homotypic and indisputable synonymy and, consequently, to the use of the name B. communis as a substitute for the illegitimate name B. chrysenteron . Summing up, this is a nomenclatural case hard to unravel and potentially provoking nomenclatural instability in case of a proposal for a putative recombination of Hortiboletus communis in substitution of the later H. engelii , since the same epithet could be used for two different taxa.

From the taxonomic standpoint, we have to outline that the original plate of B. communis by Bulliard (pl. 393) could represent two different species, if not three; the five basidiomes labelled “ B ” (on the top) could correspond to X. subtomentosus (coarse interconnecting ribs are clearly visible on the upper half of the stipe of all individuals); three out of the four basidiomes labelled “ A ” (below, lower left and centre) look much like H. bubalinus , whereas the single basidiome on the lower right might well be an additional H. bubalinus (although with an unusual areolate pileus) or more probably a member of the Xerocomellus chrysenteron / X. cisalpinus complex. The pileus of a half basidiome labelled “ C ” (on the right side of the plate) could be any of the European xerocomoid fungi and it appears to be nearly impossible to identify. Accordingly, not one of the specimens reproduced in this plate can be considered an indisputable H. engelii as it is currently delimited and, thus, B. communis can only be doubtfully considered contaxic with the former species. Therefore, B. communis should be regarded as a nomen confusum.

The taxonomic status of the heterotypic synonym Boletus subtomentosus subsp. declivitatum ( Martín 1904) is also in need of further clarification. The original plate of this subspecies, traditionally considered as conspecific with H. engelii , could well depict a mixture of different species. Elements 1 and 2 could be probably considered as H. bubalinus , while elements 6 and 7 are likely H. rubellus . The remaining sketches are unclear and very difficult to recognise. However, as it is the case for B. communis , it appears that there are no specimens in the original plate of B. subtomentosus subsp. declivitatum that can be linked with certainty to the current concept of H. engelii and, therefore, it should also be considered a nomen confusum. Anyway, being the epithet “ declivitatum ” proposed at the rank of variety, it does not compete with “ engelii ” at the rank of species.

Hortiboletus engelii belongs to the crown clade of Hortiboletus in our multilocus phylogenetic analysis (Fig. 1 ) as a well-supported (BS = 70, PP = 1.00) sister species of H. hershenzoniae (see below). They differ by 15 nucleotides and indel positions (3 % dissimilarity). They both form a weakly-supported clade (multilocus: PP = 0.80) sister to the undescribed Hortiboletus sp. 1 from Pakistan (common clade in the multilocus tree: BS = 98, PP = 1.00).

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 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 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.

Figure 11. Hortiboletus engelii basidiomes: a JAM 0547; b epitype collection MCVE 18563; c MCVE 18267; d GS 10273; e GS 10848; f AL 21-73; g TUR-A 209577; h orange-red dots in the stipe base context of GK 6124. Photos: (a) J. A. Muñoz; (b) A. F. S. Taylor; (c) G. Simonini; (d) A. Mua; (e) M. Raumi; (f) L. Albert; (g) S. Saitta; (h) G. Konstantinidis.

Figure 12. Microscopical features of H. engelii: a basidiospores in L 4; b basidia in Congo red; c pileipellis in Congo red; d pleurocystidia in Congo red; e tuft of cheilocystidia in Congo red. All photos from collection GS 10409. Scale bar: 10 μm. Photos: G. Simonini.