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

Emended description.

Basidiomes stipitate-pileate with tubular-poroid hymenophore, epigeal, xerocomoid, small or medium-sized. Pileus surface dry, matt, subtomentose to finely squamulose pileus, in some species, gradually cracking with age and becoming areolate-rimose along the margin or overall, exposing the pale or reddish context in the cracks, colours ranging from ochraceous, pinkish-red, scarlet red, vinaceous red, reddish-brown to brown, not showing a blue-green reaction with NH 3. Stipe fibrous, surface dry, pruinose or fibrillose, yellow to pinkish-red or brownish. Context whitish to light yellow, sometimes flesh pink or reddish-pink in the upper part of the pileus, pale to dull yellowish-brown in the stipe, usually bruising pale blue or, less frequently, deep blue in the pileus and the connection zone with the stipe; the stipe base context sometimes showing minute orange to orange-red dots. Taste mild. Spore print olive-brown. Pileipellis a palisadoderm or physalo-palisadoderm of more or less encrusted hyphae, or rarely a trichoderm. Basidiospores smooth, never striate nor truncate at apex, ellipsoid-fusiform, with an average spore quotient (Qm) lower than or up to 2.6. Hymenophoral trama bilateral divergent intermediate between the “ Boletus - type ” and the “ Phylloporus - type ”. Lateral stipe stratum absent or rarely present of the boletoid type. Fertile caulohymenium. Hymenophore tissue and basidiospores inamyloid or with a fleeting amyloid reaction; and negative amyloid reaction at stipe base according to Imler’s procedure. Basidiomes grow preferably in light, grassy forests, forest edges, meadows, and disturbed places, such as urban or rural areas.

The etymology of the generic name Hortiboletus derives from the Latin word hortus (garden) and refers to the growth of these boletes in gardens.

Generic type.

Boletus rubellus Krombh.

Notes.

According to the original description, the genus Hortiboletus is close to Xerocomellus, differing by the smooth basidiospores without any ornamentation (both under light microscopy and SEM, see Fig. 3), the average spore quotient (Qm) that is lower than or equal to 2.5 and the presence of tiny carrot-orange to orange-red dots in the context at the stipe base (Vizzini 2015). Nevertheless, recent studies show that not all representatives of the genus Hortiboletus share the aforementioned morphological features, based on which they were separated from Xerocomellus. For example, the average spore quotient of some collections of H. bubalinus and H. subpaludosus may be higher than 2.5 (Ladurner and Simonini 2003; Wu et al. 2016). In addition, Hortiboletus species do not have peculiar microchemical reactions (iodine, metachromatic, cyanophilic). Amyloid reaction of the stipe base trama is negative according to Imler’s procedure (Imler 1950).

Possible and fickle appearance of orange-red dots (“ ponctuation de Redeuilh ”; Simonini (1998)) in the context of the stipe base is a feature which discriminates European and Levantine members of the genus Hortiboletus from those of Xerocomellus and Rheubarbariboletus. However, it is probably a character shared by only a few species in this genus. For the majority of Asian and American Hortiboletus species, such as H. amygdalinus, H. arduinus, H. campestris, H. coccyginus, H. flavorubellus, H. napaeus, H. rubroreticulatus, H. rufosquamosus, H. rupicapreus, H. sinorubellus, H. subpaludosus, and H. tomentosus, no red dots are reported to occur in the stipe base (Thiers and Smith 1966; Smith and Thiers 1971; Wu et al. 2016; Frank et al. 2020; Xie et al. 2020; Lebeuf et al. 2021; Mao et al. 2023; Pham et al. 2024; Wang et al. 2024). Moreover, in many cases (especially in wet conditions), they are not observable even in H. rubellus, H. engelii, and H. hershenzoniae. In most specimens of H. hershenzoniae, they are visible only using a stereomicroscope. As already pointed out by Taylor and Eberhardt (2006) and Gelardi (2010), our present investigation shows that dispersed red dots may be rarely present in some collections of H. bubalinus.

Hortiboletus exsiccata, like those of Xerocomellus, gradually lose their bright colour becoming brownish upon drying; but red dots in the context of the stipe base, if present in fresh basidiomes, retain their bright colour for decades. In addition, the stipe base context in Rheubarbariboletus remains yellow-orange to orange. Probably there are similar pigments present in the stipe base of both genera, but further investigation will be necessary to confirm it.

Hortiboletus species have a palisadoderm or physalo-palisadoderm pileipellis, as is the case in the genus Xerocomellus, although H. arduinus, H. campestris, H. coccyginus, H. flavorubellus, and H. napaeus appear to display a trichoderm pileipellis (Thiers and Smith 1966; Smith and Thiers 1971; Frank et al. 2020; Xie et al. 2020). According to literature and our current observations, the hymenophoral trama of Hortiboletus species is intermediate between the “ Boletus - type ” and the “ Phylloporus - type ” and an intermediate structure is also typical of stipitate-pileate Xerocomellus species (Šutara 2008; Vizzini 2015). When compressed under a cover glass, the lateral strata separate easily, as in the “ Boletus - type ” trama (Singer 1965). In the descriptions of H. arduinus, H. napaeus, H. rufosquamosus, H. rupicapreus and H. tomentosus, the trama is reported to be of the boletoid type (Xie et al. 2020; Lebeuf et al. 2021; Mao et al. 2023). However, we assume that their trama is also intermediate, although the authors of these species likely did not recognise this transitional type.

Species of this genus develop ectomycorrhizal associations with plants of the families Betulaceae, Cistaceae, Corylaceae, Fagaceae, Malvaceae, Pinaceae, and Salicaceae, growing preferably in grassy forests, forest edges, meadows, riverbanks, and disturbed places in general, such as urban or rural areas (urban parks, public and private gardens, lawns, flowerbeds, ruderal places, pastureland, open heaths, abandoned agricultural fields, sport fields, parking lots, roadsides, clearings, drainage ditches, etc.) (Ladurner and Simonini 2003; Gelardi 2009; Das et al. 2016; Wu et al. 2016; Naseer et al. 2019; Frank et al. 2020; Lebeuf et al. 2021). Hortiboletus species are edible, but similarly to other xerocomoid boletes, not particularly palatable. Accordingly, they do not have economic importance and are rarely collected for consumption by mushroom hunters.

Maps of distribution of four target species (H. rubellus, H. bubalinus, H. engelii, and H. hershenzoniae), based on the current study and sequence data from public repositories and associated publications, are given below (Figs 4, 5).