Click Here for Main Page (Photo © TFL)

About Boletes

The boletes are members of the large group of fungi characterized primarily by the formation of spores, or reproductive bodies, on a highly specialized, microscopic structure known as a basidium (plural, basidia). The presence of these basidia separates this class of fungi, the Basidiomycetes, from all others. There are two sublcasses or groups of Basidiomycetes. One of these, the Heterobasidiomycetes, is characterized by a basidium composed of two or more cells, often four, and is represented by such common fungi as the rusts, smuts, and jelly fungi. In the second subclass, the Homobasidiomycetes, the basidium is single-celled. Since the basidium of the boletes is unicellular, they belong to the Homobasidiomycetes along with the mushrooms or toadstools, polypores, and puffballs that are so commonly observed in California woodlands. The fruit body of the boletes is similar in appearance to the typical mushroom except that, in the boletes, tubes have replaced the gills, or lamellae, on the under surface of the cap. Because of the presence of these tubes and their pores, or openings, boletes are often called fleshy pore fungi.

Since the fruit body of mushrooms and boletes is a specialized structure whose major function is the production of basidia, it will hereafter be called the basidiocarp (basidium-bearing body). Even the most cursory examination of the bolete basidiocarp reveals its similarity in general appearance and structure to the typical mushroom, or agaric, basidiocarp. The cap or pileus of the two is similar, and so is the stem or stipe. On some boletes, as in some gill fungi, there is a layer of tissue extending from the cap margin to the stem, thereby enclosing the tube cavity. This layer is the partial veil which, when the cap expands, is torn free from the margin of the cap, leaving a ring or collar of tissue attached to the stipe. This ring of tissue constitutes the annulus. The internal composition of the two types of basidiocarps is likewise similar. Each is composed of minute filaments woven together in such a way as to form a pseudotissue. In contrast to the dry, woody polypores, these basidiocarps characteristically have a high content of moisture and are putrescent; that is, they decompose readily after reaching maturity.

Like mushrooms, the bolete basidiocarp represents only one stage in the life cycle of the fungus. This large, fleshy structure is so intimately involved in the reproduction of these organisms that it is often considered to be the major reproductive structure of the fungus. It must be remembered that this role, however, properly belongs to the spore, more precisely called the basidiospore. The vegetative phase of most boletes and of many mushrooms exists in close association with the roots of many common forest trees, such as Douglas fir, oaks, and pines, as well as in the adjacent soil and humus. The vegetative unit is a microscopic, thread-like filament known as a hypha (plural, hyphae); collectively, the hyphae constitute the mycelium. The mycelium is often white or brightly colored and can frequently be seen adhering to the base of the stipe of the basidiocarp. The individual filaments are septate; that is, they have cross walls which divide the filament into a series of cells arranged end to end. The walls of the hyphal cells are most frequently thin; however, thick-walled hyphae are sometimes developed. The contents of the cells of the hypha, like most other plant cells, characteristically include the nucleus and cytoplasm. However, as in most fungi these structures are so small that they are extremely difficult to observe. The hyphae and mycelium of the boletes are similar in appearance and structure to those of other Basidiomycetes.

The major difference between the boletes and gill fungi, as suggested above, is that in the boletes the basidia are located on the inner surface of numerous tubes, which are typically vertically arranged on the lower surface of the pileus (except in Gastroboletus). These tubes, or gills in the case of mushrooms, are commonly designated as the hymenophore, or the part of the basidiocarp bearing the hymenium. The hymenium, in turn, is a layer of rather closely packed basidia plus distinctive sterile cells called cystidia. Another difference noted in the field is that, although some mushrooms grow on logs or other woody substrates, only a few boletes are found consistently on such substrates, and most occur in the soil or humus in the vicinity of woody plants.

The Basidiocarp

Figure I
Longitudinal section through bolete basidiocarp
  1. Pileus
  2. Pileus Trama
  3. Tubes (Hymenophore)
  4. Annulus
  5. Stipe Cuticle
  6. Stipe Trama

The Pileus

Bolete pilei are typically large, often reaching 15 centimeters (cm) or more in diameter, and are rarely as small as 2-4 cm in diameter. Characteristically, they are more or less convex or bulbous in outline when young, becoming plane or plano-convex when mature. Colors range from almost black to many different shades of brown, pink, or bright red. The colors, with a few conspicuous exceptions, are relatively constant for a given species and change only slightly as the basidiocarp matures. Although there are no known white species in California, there are a few in which the color is constantly pallid. When the surface of the pileus is bruised, many boletes show some type of color change.

The nature of the surface of the pileus is of considerable taxonomic significance in the boletes. In most species of Suillus and in a few belonging to other genera it is viscid, or "sticky" or "slimy," to the touch. This condition results most commonly from the gelatinization of the walls of the hyphae of the cuticular or surface layer and is particularly noticeable in wet weather. In the nonviscid species the surface may be either moist or dry to the touch. The surface may vary from glabrous, or bare, to fibrillose, or covered with a layer of fibrils that may be closely appressed or loosely and irregularly arranged. In some species the arrangement of the fibrils may give a tomentose, or velvety, appearance. There are several boletes in which the fibrils become agglutinated into scales. Most commonly these scales remain closely attached to the surface, but in some boletes the tips may break free. In a few they become quite large and give the pileus a squarrose or squamulose appearance. Boletes, as they grow older, often show a tendency to become split, or rimose, on the margin and to become checked, or areolate, elsewhere on the surface. The areolations sometimes become deep and strongly pronounced in prolonged periods of dry weather, which results in a frustose condition of the pileus.

The margin or edge of the young pileus in most boletes is entire, that is, more or less smooth and even and with no ornamentations. In the genus Suillus, however, there is frequently a noticeable cottony roll of veil tissue closely attached to the margin of the young pilei. This roll, commonly referred to as a false veil, is typically white or whitish and usually disappears as the pileus matures. Also in the genus Suillus and in some other genera, a partial veil is sometimes present which, when breaking free from the pileus, often leaves fragments, or appendiculations, hanging from the margin. Typical of the members of the section Leccinum of the genus Leccinum is the continued growth of the outer layer or cuticle beyond the edge of the pileus, resulting in the formation of a band of tissue around the margin. This band usually breaks into fragments, or "flaps," as the pileus expands. These fragments seem to serve no function and eventually become either inconspicuous or completely disappear.

The flesh or context of the pileus is usually relatively soft and putrescent, and with a high water content. It varies in thickness from 0.5-2 cm, but in exceptional cases may be as thick as 4-5cm or as thin as 2-4 millimeters (mm). Most commonly, the context is some shade of yellow or white, with other colors such as pink or tan rarely evident. The context of many boletes changes color when exposed to the air or damaged. The characteristic blue discoloration, or bluing, seen in a number of species may result from the oxidation by an enzyme of a compound known as boletol. Other color changes, such as the reddening, browning or blackening seen in many species, are probably similar oxidative reactions, but the compounds and chemical pathways are largely unknown.

The taste and odor of the context of most boletes are mild and inoffensive. Boletus piperatus is the only species known from California that has the peppery or acrid taste so frequently encountered in species of Russula and Lactarius of the gilled fungi. Boletus calopus, B. rubripes, and possibly a few others have a noticeably bitter taste, and some, such as Suillus pungens and S. acerbus, have somewhat harsh and unpleasant tastes. No boletes yet known from California have a highly distinctive odor.

The Hymenophore

The hymenophore, which consists of soft, moist, putrescent tubes, is the most distinctive feature of the boletes. The hymenium forms the inner lining of these tubes. There is a superficial resemblance between the hymenophore of polypores and boletes. The basidiocarps of polypores, however, are tough, dry, and woody.

In most boletes the hymenophore separates readily and cleanly from the pileus, and in many the tubes are easily separated from each other; however, as has been pointed out by Snell, in some cases, best exemplified by the section Subtomentos, of the genus Boletus and by the genus Suillus, the tubes appear to develop in a different manner and can be separated only by tearing the walls apart. Except for Gastroboletus, the tubes are more or less vertically arranged in an orderly fashion, and a spore print is readily obtainable. Most often the tubes are either shallowly or deeply depressed at the stipe, but in some boletes, especially species of Suillus, they extend down the surface in a decurrent fashion.

The pores of the tubes are worthy of special mention. Most are angular, varying from almost square to somewhat rectangular, and range in size from 0.5-2 mm in diameter. In some boletes the arrangement of the pores is radial, or boletinoid, and they radiate from the stipe much like the spokes of a wheel. Large pores compounded with internal partitions are seen in several species of Suillus. In a few boletes the pores are so large and elongated that they resemble lamellae and are described as lamellate. In Boletus edulis and related species the pores are "stuffed" when young; that is, they are filled by intrusive hyphae which with maturity of the basidiocarp collapse and disappear. The pores are usually concolorous with the tubes; however, in some species they are pink or red in contrast to the yellow color of the body of the tube or, less commonly, they may be a contrasting shade of brown or black. The pores ordinarily show the same color changes as the remainder of the hymenophore.

The Stipe

The boletes are stipitate and no sessile species are known. The stipe is central. An eccentric attachment is found rarely, except in Gastroboletus, and species with truly lateral stipes are still unknown. The overall shape of the stipe is sometimes equal or more or less the same size from apex to base. More commonly, it is clavate with a gradual enlargement toward the base. In some boletes, best exemplified by Boletus satanas, there is a conspicuous and abruptly bulbous base, which is sometimes as much as 5 cm in diameter. There are others in which the stipe is ventricose, or larger in the midportion than at the apex or base. Generally, the range in length is from 4-5cm to a maximum of 14-15 cm, although Suillus brevipes, for example, may have a stipe as short as 1 cm and that of Boletus mirabiis may sometimes reach 20 cm. The diameter of the apex of the stipe, on an average, varies from 0.5-2 cm, but the more massive species, such as Boletus regius and B. edulis, may exceed 3-4 cm, whereas B. piperatus, Gyroporus castaneus and some Suillus species may be less than 0.5 cm in diameter.

The color of the bolete stipe ranges from white to yellow to pink to darker colors, such as red, brown, or almost black. Frequently the background color is overlain by some other pigment, resulting in a blending of colors. As in the pileus, some colors show a tendency to fade or, conversely, become more intense with age. Often the fading or darkening is uneven, which results in the formation of bands or blotches of color. Ornamentations on the surface of the stipe are often a contrasting color, and usually darken with age. Usually, the same series of color changes occurring in the pileus upon bruising or exposure takes place in the stipe, except that the changes may be more intense.

The surface of the stipe in a major percentage of boletes is either dry, moist, or, less commonly, viscid. Typically, it is glabrous, tomentose, fibrillose, or fibrillose-scaly. Sometimes it is reticulate, perhaps best demonstrated by Boletus edulis or B. eastwoodiae, in which there is a noticeable network of raised lines on the surface. A reticulum at the apical portion may be formed by the extension of the tubes down the stipe. In such cases the ridges forming the reticulum are composed of basidia and cystidia. In Leccinum the surface is furfuraceous, that is, covered with squamules, or scales. These are usually white or pallid when young, but change to dark brown or black with age, and are composed largely of caulocystidia, or large sterile cells. In Suillus the surface often has glandulae, or small raised dots, that vary considerably in size, are typically colored some shade of brown, and may stain the fingers when handled. These glandulae are composed of clusters, of fascicles, of caulocystidia. Frequently, the basal portion of the bolete stipe is clothed with coarse hairs, which may be distinctively colored. Another important taxonomic feature associated with the surface of the stipe is the annulus, or remnants of the partial veil. It is sometimes evanescent; that is, it disappears as the basidiocarp matures or is represented merely by a fibrillose zone. On the other hand, it may be massive and constitute a conspicuous part of the mature basidiocarp. In some boletes, particularly in the genus Suillus, the stipe may be peronate, a condition in which the lower part of the stipe is completely surrounded by velar tissue. In Pulveroboletus ravenelii the veil and annulus are dry, copious, brilliant yellow, and floccose in texture. In some species of Suillus, the annulus is white or pallid, frequently inconspicuous, and may be noticeably viscid.

The context of the stipe is typically similar in structure and texture to that of the pileus. The sequence of color changes in the stipe apex when exposed is of considerable taxonomic value in the genus Leccinum. The stipe is typically solid, but in a few species, such as Gyroporus castaneus, it is hollow, at least in the basal portion.

The Gastrocarp

Figure II
Longitudinal section through gastrocarp
  1. Peridium
  2. Gleba
  3. Stipe-columella

The gastroboletes have been classified in the past as puffballs rather than as boletes because they do not give a spore print. However, they are now included with the boletes because of the morphological similarity of the basidiocarps.

Because of the similarity of the gastroboletes to puffballs, different terms are used to describe the macroscopic features of the basidiocarp in the genus Gastroboletus. The basidiocarp is called a gastrocarp, the hymenophore is the gleba, and the stipe is the stipe-columella. The surface layer of the cap that in some species encloses the gleba is called the peridium or peridial membrane. The arrangement of the tubes in the gastrocarp is irregular; they are not vertically oriented and often radiate at various angles under the cap. The stipe-columella is often poorly developed, frequently eccentric, and often surrounded by the gleba.

The Basidiospore

Figure III
Bolete basidiospores
  1. Boletus
  2. Leccinum
  3. Suillus
  4. Tylopilus

The basidiospores are of considerable diagnostic value in the taxonomy and systematics of the boletes and must be checked before positive determinations can be made. At the generic level the color of the spores in mass is of significance, and spore prints are often necessary. The spore print varies in color from a shade of brown in such genera as Boletus, Leccinum, and Suillus to yellow in Gyroporus and flesh or dark pink in Tylopilus. The individual spore ranges from hyaline or colorless to brown. The shape of the spores is, in a broad sense, more or less similar for all California species. Generally, in face view they appear elongate and cylindric to fusoid (spindle-shaped) or ellipsoid in outline. When seen in profile, however, the two sides are often unequal, with one showing a slight bulge, a condition described as ventricose. The bulge is located near the apiculate end, that is, the end attached to the basidium. Occasionally, oddly shaped or pleomorphic spores are produced, often noted when the basidium is not four-spored. Since the germination of bolete spores is exceedingly difficult, it is not known whether these atypical spores are viable or not. In Boletus truncatus and sometimes in other species the apical end is often abruptly terminated and appears as if it had been cut off or truncated. A germ pore or thin spot in the wall is sometimes apparent in this region.

The spore length averages between 9-1 5 micrometers (µm) and the width from 4-5 µpm. Smaller spores are characteristic of Boletus orovillus and several species of Suillus. On the other hand, spores are as long as 17-20 µm in species of Leccinum and in some boletes, such as Boletus mirabilis. "Giant" spores are sometimes seen in mounts from many different species and are presumably produced by one- or two-spored basidia. The spore wall is generally thin, averaging about 1 µm, but some spores, such as in Boletus mirabilis, possess thickened walls. In all known California species the outer wall appears smooth with the light microscope except that the walls of spores of Boletus zelleri and B. mirabiis have sometimes been reported as very weakly punctate or roughened.

The bolete spores show little or no change in color when mounted in water, dilute solutions of potassium hydroxide, or similar basic compounds. When mounted in Melzers reagent, however, they may either remain unchanged or one of several color changes may occur. Most commonly, the brown color of the walls is merely intensified or becomes a deep rusty brown. In some the spores give a dextrinoid reaction in which they become bright rust red or tawny. The chemical reaction responsible for this color change is not fully understood, especially since rarely do all the spores show the same reaction. Recently, several species have been found in which the spore wall gives a typical blue-black, or amyloid, reaction when mounted in Melzers reagent. Spores of Boletus amyloideus and Tylopilus amylosporus show this reaction. Like the dextrinoid reaction, the chemistry of the amyloid reaction is poorly understood, and all spores from a given species usually will not react positively.

The Hymenium

Figure IV
Longitudinal section through the bolete hymenophore
  1. Basidium
  2. Hymenial cystidium
  3. Basidium with sterigmata and basidiospores
  4. Subhymenium
  5. Bilateral trama showing central strand and divergent hyphae

The hymenium is the palisade or layer of basidia and associated cells that forms the inner lining of the tubes and, in some instances, may extend down the stipe if the surface is reticulate. The most important cells in this layer are the basidia, which develop as terminal cells of the hyphae that form the tissue of the tube. Nuclear fusion (karyogamy), reduction division (meiosis), and subsequent spore formation occur in the basidia. The basidia are rather large, club-shaped cells ranging from 20-30 µm in length and from 7-12 µm in width. At the apex four short, somewhat hooked branches develop. These are sterigmata, and eventually each will bear a single basidiospore.

Basidioles or brachybasidioles are also commonly seen in the hymenium. These may be undeveloped basidia or basidioid cells that never produce basidiospores. They appear to have the same origin as the fertile basidia, but their true function is not understood. Perhaps they serve as lateral support for the spore-bearing basidia.

A third type of cell commonly found in the hymenium of most boletes is the cystidium. These cells are sterile and, like the basidia, arise as differentiated hyphal tips. They are often highly distinctive in size and shape, and frequently extend well beyond the basidial layer into the tube cavity. Most often they are clavate to fusoid or ventricose in shape with an elongated to obtuse or mucronate apex. Cystidia reach 50-75 µm in length and 10-i5 µm in diameter. These cells are usually thin-walled, but may become noticeably thickened or incrusted with amorphous materials. When mounted in water, the cystidia are typically hyaline or only weakly pigmented; however, in potassium hydroxide or in Melzers reagent there is often a marked color change to reddish brown or black. Most species of Suillus in California have at least some of the cystidia grouped together in large clusters or fascicles. These clustered cystidia when mounted in potassium hydroxide become very dark brown to black or purplish, at least in the basal portion. In the gilled fungi the cystidia occurring on the gill edge, that is, the cheilocystidia, are often quite distinct in shape and size from those on the face of the gill, the pleurocystidia. However, in most boletes there is little or no distinction between those on the pores and those on the tube lining. Therefore, they will be referred to as hymenial cystidia to distinguish them from the cystidia that sometimes occur on the stipe or pileus cuticle. The role of the cystidia is uncertain, but they may act as organs of excretion, serve as air traps, or perhaps aid in the evaporation of moisture from the hymenial surface.

Figure V
Crosssection of the bolete hymenophore
  1. Tube (hymenophoral) trama
  2. Hymenium
  3. Pore

The Trama

The body of the pileus, hymenophore, and stipe is composed of filamentous, thin-walled hyphae that are undifferentiated, except for the surface, or cuticular, hyphae. These internal hyphae are referred to collectively as the trama. In the tubes the hyphae are characteristically arranged in a compact central strand with noticeable filaments diverging from it, except in those few boletes in which the hyphae are arranged parallel to one another. This divergent, or bilateral, trama is often apparent only in young basidiocarps and at maturity may appear tangled or interwoven. The tube trama is generally hyaline when mounted in potassium hydroxide, but may change to some shade of brown, especially in the central strand. The tramal hyphae are often relatively large and range from 4-6 µm in width. In several species the walls of the hyphae seem to gelatinize or dissolve when mounted in water or weak base solution (KOH). Occasionally, thick-walled laticiferous hyphae are irregularly distributed in the trama. They may be secretory hyphae similar to those in Lactarius, but no latex-bearing species of boletes are known from California except for those which secrete droplets of resinous material on the pores when young.

The pileus trama is generally similar in all bolete species and of little taxonomic or phylogenetic value. Its typical appearance is one of tangled hyphae, which are more or less loosely interwoven and usually homogeneous. Laticiferous hyphae are sometimes irregularly interspersed, but are not seen as frequently as in the tube trama. The stipe trama in all boletes is composed of masses of more or less parallel hyphae that may be loosely or tightly packed, except in Gyroporus where the hyphae are transversely arranged.

The Cuticle

Figure VI
Types of bolete cuticle
  1. Trichodermium composed of interwoven hyphae and hyphal tips
  2. Trichodermium composed of erect hyphal tips
  3. Ixotrichodermium in which the walls of the hyphae are gelatinizing

One of the most important and useful anatomical features from a taxonomic point of view in the boletes is the microscopic structure of the cuticular or external layer of the pileus, and to a lesser extent of the stipe. The cuticle is usually easily distinguished from the pileus trama. In some California boletes a second layer under the outer layer, called the subcutis or hypodermium, is present; it is characterized by rather tightly interwoven hyphae and often a distinctive staining reaction in KOH and Melzer's reagent.

The simplest and perhaps most primitive type of cuticle is the cutis in which there is a layer of appressed, filamentous hyphae over the surface of the pileus. The cells of these hyphae are more or less similar in shape and size and seldom break free from one another. Such a cuticle is only rarely seen in the boletes.

More characteristic of the boletes is the trichodermial type of cuticle, which is composed of interwoven hyphae or hyphal tips that may be arranged in an upright palisade or turf. However, there is considerable variation in the trichodermial cuticle, and noticeable changes may occur as the basidiocarp matures. If at all possible, young specimens should be checked, for the hyphae of a tangled or interwoven trichodermium may collapse with age in some boletes and give the appearance of a cutis. In others the hyphal tips may remain more or less erect and highly differentiated, forming a hymeniform cuticle. If the hyphal tips become clustered or aggregated, the surface of the cap appears tomentose or scaly. Another variation in the trichodermium occurs in many species of Suillus and occasionally in other genera: the walls of the hyphae gelatinize or partially dissolve; the resulting slimy or viscid layer is referred to as a pellicle or an ixotrichodermium.

In many species of Leccinum the individual cells of the cuticular hyphae are quite large, loosely held together, and may disarticulate or separate from one another. The walls of these cells are sometimes faintly roughened or asperulate, but this feature is usually visible only at rather high magnifications. The pigments in the cells of the cuticular hyphae of some species of Leccinum become aggregated into noticeable globules when mounted in Melzers reagent. The chemistry of this reaction is not understood and neither is its significance in establishing species relationships within the genus; however, since the reaction is constant for a given taxon, it is used as a taxonomic character. In Boletus chrysenteron and B. zelleri the external surface of the cuticular hyphae become noticeably incrusted. These incrustations may be irregularly dispersed over the surface or arranged in the form of spirals. Often they become rust brown when mounted in potassium hydroxide or Melzers reagent. Pileocystidia or sterile, highly differentiated cells somewhat similar to the cystidia in the hymenium are very rarely present in the cuticle of the boletes.

The cuticle of the stipe shows less variation than the pileus cuticle. Most commonly, the surface is differentiated as a cutis, but trichodermial or ixotrichodermial types also occur. In Suillus the surface of the stipe of many species is noticeably dotted with differently colored, often irregularly shaped, resinous globules. Because of their appearance, these globules are designated as glandulae; however, structurally they are composed of masses of caulocystidia similar to the cystidia in the hymenium and give similar color changes when mounted in potassium hydroxide.

Clamp connections are short, inconspicuous branches located at the cross walls or septa of the hyphae and form a by-pass around the septations. They are present in some of the boletes and are most readily found in the cuticular hyphae, at the base of the basidium or, less frequently, in the basal tomentum of the stipe. The significance of clamp connections is not fully understood since they are often not present at every septation. Their significance has been further confused by the recent observation that the vegetative hyphae of at least some species of Suillus when grown in pure culture develop clamps, but in the basidiocarps of the same species all hyphae are devoid of such structures. In the California flora, clamp connections have been found consistently only in Gyoporus castaneus.

Mycorrhizal Associations

In California the boletes are exclusively forest-inhabiting fungi and none are known that fruit only in fields or open meadows. The restriction to the forests is due to the necessity of the formation of mycorrhizal associations in order to survive. This type of association is an intimate relationship between the vegetative mycelium of the fungus and the young roots of the associated tree. The mycelium forms a rather dense layer or mantle on the external surface of the root. This relationship seems to be mutually beneficial. The mushroom profits by obtaining nutrients and perhaps water from the host tree. The benefits afforded to the tree by the fungus are not so readily apparent, but indications are that the fungus accumulates considerable quantities of certain minerals from the soil that are subsequently available to the tree and increase the absorptive surface of the roots. Obviously, if such associations are essential for the survival or development of the tree, these fungi assume considerable significance in reforestation practices.

Since the fungus hyphae are microscopic except in mass, the difficulty of tracing the connection between the basidiocarp and the roots of the tree is readily apparent. It is understandable, therefore, that most mycorrhizal associations are only presumed and cannot easily be confirmed except by cultural practices. Such assumptions are based upon repeated observations that a specific bolete always occurs in close proximity to the same kind of tree. In the boletes such associations may be formed with hardwood trees or conifers. Some species appear capable of forming mycorrhizal associations with a rather wide range of trees; others are seemingly highly restricted. In California there are numerous examples of extreme host restriction. Suillus pungens, for example, is confined to Monterey and knobcone pines. Suillus glandulosipes has been found only under Bishop pine, S. fuscotomentosus only with ponderosa pine, and S. caerulescens, S. lakei, and S. ponderosus only with Douglas fir. Similar associations have been observed with other genera. Leccinum manzanitae is known only from under madrone or manzanita and L. montanum, L. californicum, L. insigne, L. aurantiacum, and L. discolor have been found only under aspen. Boletus dryophilus, B. aereus, and B. regius have been found only under oaks, and B. edulis only under pines. The accompanying table lists several species of boletes from California and their probable mycorrhizal hosts.

Some California Boletes and Their Probable Mycorrhizal Host

The California Flora

The taxonomic and floristic aspects of the bolete flora of California received comparatively little attention prior to 1960. One of the earliest reports is "A Catalogue of the Pacific Coast Fungi" by Harkness and Moore, issued in 1880, in which, as an addendum, sixteen different species of California boletes were listed, mostly from the vicinity of San Rafael in the San Francisco Bay Area. Their list included several European species, which apparently have not been reported since. Unfortunately, most of their collections were destroyed by fire in 1906, and it is strongly suspected that some of the European species recorded by them would now be placed in other taxa. Among those in their list and also in ours are Boletus piperatus, B. subtomentosus, B. edulis, B. satanas, B. erythropus, Suillus granulatus, and Gyroporus castaneus. Harkness, the senior author, was a physician who lived in the San Francisco Bay Area and pursued his interests in the fungi as a hobby. Most, if not all, of his work was conducted at the California Academy of Sciences in San Francisco. He did an enormous amount of highly professional work not only on the mushrooms of the state but also on other fleshy fungi, particularly the hypogeous species.

Although studies on the fungus flora of California intermittently appeared following the Harkness and Moore report, it was not until after the turn of the century that the next significant papers on boletes were published. In 1905 Earle published descriptions of Boletus flaviporus and B. tomentipes. Boletus flaviporus is common in the San Francisco Bay Area, but B. tomentipes is apparently rare since no recent collections have been made. From 1909 to 1912 Murrill published several papers in which additional species of boletes from the state were included. In 1910, in his treatment of the boletes in North American Flora, he described Boletus eastwoodiae as a new species from California. Suillus lakei and Boletus zelleri were first reported by him from California in 1912. Zeller, from Oregon State University, published frequently on the fleshy fungi of Oregon and occasionally included comments on, or descriptions of, California species.

Following the reports by Murrill, the bolete flora of California was mentioned only incidentally until the appearance of a series of highly significant papers on North American boletes by Snell and Dick and their associates. The first of this series appeared in 1932. Subsequently, many collections of California boletes were sent to them for identification. Some proved to be either new species, including for example, Suillus megaporinus and Boletus atrofuscus, or species not previously known from North America, such as Boletus appendiculatus and B. regius. Snell and Dick did not, however, publish any papers devoted exclusively to the California flora.

Alexander H. Smith of the University of Michigan Herbarium spent part of two collecting seasons in the vicinity of Crescent City. He collected numerous boletes as well as other fungi and, although he has not published specifically on his California bolete collections, they are very important. His data and collections were made available. William Bridge Cooke collected fungi at intervals in the Mount Shasta region from 1940-1960, during which time he made the type collection of Boletus abieticola. RoIf Singer has also collected in the vicinity of the San Francisco Bay Area, and described Suillus lithocarpi-sequoiae from collections made in Muir Woods National Monument.

The total number of bolete species known from California at this time is approximately 80. In comparison, Michigan has 244 (Smith and Thiers); New England, 129 (Snell and Dick); North Carolina, 75 (Coker); Florida, 58 (Singer); Texas and Louisiana, 40 (Thiers); Washington, 35 (Hoffman); Great Britain, 66 (Watling); Central Europe, 65 (Singer). The rather wide numerical discrepancy between these different areas can, in part, be explained by the presence or absence of suitable mycorrhizal hosts. Climate, soil types, and center of origin are other factors that may also play an important role in the distribution patterns and diversity of species. There are in California several cosmopolitan species such as Boletus edulis, B. piperatus, B. subtomentosus, Leccinum insigne, Suillus granulatus, S. brevipes, and Tylopilus pseudoscaber. As in other bolete floras, Suillus, Boletus, and Leccinum are the most common genera within the state.

One of the most notable features of the California bolete flora is the absence of species belonging to Strobilomyces and Boletellus. No taxa belonging to either of these rough-spored genera are presently known west of the Rocky Mountains. Furthermore, there is a relatively small number of species of Leccinum and Boletus and only a single species of Gyroporus. Species of Fuscoboletinus and Boletinellus have not yet been found. An explanation is not readily available for the absence of the rough-spored species and the reduced number of species of Boletus and Gyroporus. The scarcity of birches (Betula spp .) probably accounts for the reduced number of Leccinum species. The absence of species of Fuscoboletinus and Boletinellus may be similarly explained by the lack of suitable mycorrhizal hosts such as larch (Larix spp.). On the other hand, the number of species of Suillus found in California is equal to or exceeds that reported for any other bolete flora. This is perhaps due to the great diversity of conifers within the state. The percentage of endemic species of boletes is high, again surpassing that of many other regions. At present there are eleven species of Boletus, three of Gastroboletus, seven of Leccinum, nine of Suillus, and two of Tylopilus known only from California.


Many boletes are well known for their quality as edible fungi. Boletus edulis is one of the best of all mushrooms and is excellent when used either fresh or dried. Fortunately for the mycophagist, it is abundant in the state and large collections are made annually in the northern coastal and Sierra Nevada pine forests. Other boletes considered to be top quality are Boletus zelleri, B. mirabiis, Leccinum aurantiacum, L. manzanitae, and L. insigne. Some species of Suillus are also acceptable but generally less popular.

When collecting fleshy fungi for food, care should be taken to avoid including large amounts of soil or substrate. Do not collect older basidiocarps since they are likely to be less palatable and are often insect-riddled. Wrap the collections separately in wax paper to protect them and to avoid possible contamination from other collections. Wax paper is preferable to plastic bags since it permits the air to circulate more freely, thus avoiding sweating or waterlogging. It is not within the scope of this book to recommend cooking methods. There are a number of good recipe books available with directions for preparing mushrooms for the table. Worthy of special mention is the one published by the San Francisco Mycological Society entitled Kitchen Magic with Mushrooms.

Most boletes are nonpoisonous but some are regarded as mildly toxic. In this category are those in which the pores are red in contrast to the yellow color of the tubes. Boletus eastwoodiae, for example, is known to cause stomach disorders. In some parts of the world there is a feeling against eating any bolete in which the context turns blue when exposed to air; however, some species with this characteristic are not poisonous. It is strongly advised that anyone interested in eating unfamiliar boletes do so in small amounts at first so that any unfavorable reactions may be detected.

Field and Laboratory Procedures

In this study traditional procedures for the handling of specimens were followed. Individual collections were wrapped in wax paper to minimize fading or other color changes due to loss of moisture and to prevent damage to the basidiocarps. In the laboratory suitable specimens were photographed and detailed notes taken on the macroscopic features. Spore prints or masses of basidiospores were obtained from mature caps placed pore surface down on white paper. To maintain high humidity and to avoid air currents while obtaining the prints, the caps were either covered with a glass dish or wrapped with wax paper. Spore prints were thoroughly air dried before the colors were recorded.

The reactions of the various parts of the basidiocarp to different chemical reagents were also determined whenever possible. In the species descriptions the chemical reactions reported for the context and cuticle always refer to the pileus unless otherwise specified. Among the reagents used were the following:


KOH: 3 percent solution of potassium hydroxide in distilled water.
NH4OH: 10-14 percent solution of ammonium hydroxide in distilled water.


HCl: Concentrated solution of hydrochloric acid.
HNO3: Concentrated solution of nitric acid.
H2S04: Concentrated solution of sulfuric acid.


Melzer's reagent, composed of:
FeSO4 or FeCl3: 10 percent solution of either ferrous sulfate or ferric chloride in distilled water.
Guaiac: saturated alcoholic solution.

The collections were subsequently dried and placed in small storage boxes until the microscopic data were taken. Microscopic examination of the various parts of the basidiocarp was accomplished by the rehydration of selected portions of the dried basidiocarps and the preparation of freehand sections using a straightedge razor or razor blade. The sections were mounted in 3 percent potassium hydroxide solution (KOH) and in Melzer's reagent. Most microscopic data were taken from the potassium hydroxide preparations. Melzer's reagent was used primarily to detect color changes, including amyloid reactions (dark-blue to blackish discolorations). Color changes were also recorded when observed in the potassium hydroxide mounts. Microscopic measurements were usually made using the oil-immersion objective. Eventually the collections were identified and permanently accessioned into the herbarium.

All the colors recorded in the species descriptions that appear in quotes and parentheses were taken from Ridgeways Color Standards and Color Nomenclature. The type collections as well as all additional collections cited in the species descriptions are deposited in the Cryptogamic Herbarium at San Francisco State University. Duplicate collections have been distributed to several herbaria in the United States, Canada, and Europe.

Classification of Boletes

Synopsis of the California Boletaceae

The classification of the boletes "officially" begins with the publication of Systema Mycologicum by Elias Fries in 1821. In that publication he placed all species of fleshy pore fungi in the single genus Boletus. Soon thereafter various systems for the division of this large taxon into smaller units were proposed, beginning with the appearance, also in 1821, of S. F. Grays Natural Arrangement of British Plants. He placed all the boletes in Suillus, Leccinum, or Pinuzza. Since the time of these two historic works, other arrangements or systems for the classification of boletes have been presented. Among these was the contribution by Quélet, in which he erected several additional genera. Several years later, Gilbert in Les Bolets recognized eleven different genera. Peck was among the first to appreciate the distinctiveness of the bolete flora of the United States, although he placed a majority of the species in previously established European genera. Murrill, on the other hand, recognized few European genera and species as occurring in the United States and, as a consequence, established several new genera native to North America.

Perhaps the most elaborate classification system of the boletes is the one advanced by Singer in 1962. He recognized two families, the Boletaceae and Strobilomycetaceae, and a total of eighteen genera. This system was based on the worldwide bolete flora and has been rather widely adopted, especially in European countries. In 1971, Smith and Thiers, in The Boletes of Michigan, followed a system which differed from Singer's in that all species were placed in the family Boletaceae and only eleven genera were recognized. The reduction in the number of genera resulted, in part, from the combining of Suillus and Boletinus, the elimination of other genera, such as Xerocomus, Xanthoconium, Porphyrellus, and Paragyrodon, and the transfer of the genus Phylloporus, a lamellate genus often classified with the boletes, to the family Paxillaceae, as has been recommended by Watling. Furthermore, because of the obvious affinities of species of Gastroboletus to the boletes, this genus was placed in the Boletaceae. The arrangement of genera and species of California boletes as given next is based to a large extent upon the classification system of Smith and Thiers.

The Boletes of California
Copyright © 1975 by Dr. Harry D. Thiers
Additional content for the online edition © 1998 by Michael Wood, Fred Stevens, & Michael Boom
A MykoWeb Page