|Major Groups > Boletes > Leccinum|
The Genus Leccinum
by Michael Kuo
Once you have collected them a few times, Leccinum species are for the most part pretty easy to identify to genus. Like other boletes, they are soft-fleshed mycorrhizal partners with trees (and shrubs), featuring tubes and pores on the underside of a cap that sits atop a central stem. Unlike most other boletes, however, their stems are punctuated with scabers which typically become brown or black by the time the mushroom is mature (though in a few species the scabers are light in color, reddish, or nearly invisible to the naked eye). If you are new to bolete identification, compare scabers carefully with glandular dots and reticulation. Aside from the scabrous stems, there is something particularly Leccinumesque about most of the mushrooms in the genus; their stature and colors are somehow distinctive. But while recognizing that a bolete is a Leccinum is usually relatively easy, figuring out what species you have found can be truly frustrating. In fact, if you are a North American collector at this point in time, it is probably not possible to identify most Leccinum species with scientific certainty.
Current research, supported by DNA evidence, points to the probability that the methods we have traditionally used to identify Leccinum species--namely, observation of the mushrooms' physical features with and without a microscope--do not necessarily identify actual genetic species with success. If this reality frustrates you, I'm sorry--but try looking at it this way: this is an exciting time to be collecting Leccinum, and amateur mushroomers and mushroom clubs are in a position to make substantial and important contributions to mycology. Careful documentation of many, many Leccinum collections from across the continent is the key to an eventual mycological understanding of the genus; see Kuo, 2007 ("Mushrooming in the Age of DNA: Now Comes the Fun Part") for further discussion and suggestions.
My treatment of Leccinum (which is far from complete) is more "mycological" and less "field-guide-ish" than other treatments at MushroomExpert.Com, so I apologize to readers who become frustrated with Mycologese, dense technical descriptions, and long lists of references and documentation. I have included a brief, nontechnical "Synopsis" at the top of each species page--but it was, in part, my frustration with the inadequacies in existing North American mycological treatments of the genus that led me to this project, and we will never reach a point where the field-guide descriptions correspond to scientific reality without a revision of the genus on mycological terms.
A Contribution toward an Understanding of Leccinum in North America
In much of the northern spotted owl's range, old growth conifer forests provide relatively high densities of their preferred prey, the northern flying squirrel and the red-backed vole. Ninety percent of these rodents' diet consists of ectomycorrhizal, underground, truffle-like fungi in these large, dense forests of the Pacific Northwest . . . Thus, in this situation, we see a tight, four-part harmony in which a bird species preys on rodent species that eat truffle-like mycorrhizal fungi which, in turn, are responsible for maintaining the health of Douglas fir trees whose branches and trunks provide prime roosting and nesting sites for the bird. As is often the case, charismatic macrofauna, like the owl, become the icon for conservation of unique and pristine ecosystems, yet experimental evidence verifies that ectomycorrhizal fungi are the actual keystone organisms.
I have based my treatment of Leccinum in North America on the idea that the mushrooms cannot be adequately understood or classified without reference, first and foremost, to their ecology. This idea has been suggested by many DNA studies in mycology over the last decade or so, and with reference to Leccinum in particular, by the studies of den Bakker and collaborators (2004a, 2004b, 2005). I consider the monograph of European Leccinum by den Bakker & Noordeloos (2005) to be the best starting point for a consideration of the genus in North America--not because I expect all (or even, necessarily, any) of the European species to be found on our continent, but because this monograph effectively incorporates well supported DNA results with ecology and morphology, providing a good template to help guide further investigation. The template should not be enforced too rigidly, however, and once the North American Leccinum taxa begin to be sufficiently documented, collected, and subjected to analysis we will undoubtedly find many patterns that do not fit the European template very well.
The Genus Leccinum S. F. Gray
The fungi in Leccinum are ectomycorrhizal symbionts with members of the Fagales, Pinales, and Ericales (as these orders are currently defined; see Stevens, 2007), primarily with Quercus, Carpinus, Corylus, Betula, Populus, Pinus, Picea, Abies, Tsuga, Pseudotsuga, Arbutus, and Arctostaphylos--and possibly with Salix in the Malpighiales. The vast majority of Leccinum species appear to be very host-specific, limited to association with a given species, genus, family, or, at most, order. However, since data about mycorrhizal association is glaringly insufficient, the precise contours of host specificity are not currently known. At least one species (Leccinum aurantiacum) is known to be a "generalist," able to form mycorrhizae with fairly diverse hosts. Leccinum is best known from the northern hemisphere, but the genus is present in Australia, New Zealand, and Africa and it would be premature to make proclamations about its range before distribution is more thoroughly documented. Collections have come from diverse ecosystems in alpine, boreal, temperate, subtropical, and tropical regions.
On the basis of preliminary evidence (den Bakker and collaborators, 2004b), species of Leccinum appear to have evolved from a common ancestor that was most similar, genetically, to today's oak- and hornbeam-associated species. This anecestral Leccinum may have then split into birch-associated and oak/hornbeam-associated lineages, with the conifer-, Ericales-, and aspen-associated lineages deriving from the birch line. Host switches in the evolutionary history of Leccinum appear to have been followed by periods of rapid speciation--a phenomenon also observed in molecular studies of Suillus, Hebeloma, and Pisolithus (Kretzer and collaborators, 1996; Aanen and collaborators, 2000; Martin and collaborators, 2002).
A generalized morphology of the genus Leccinum has become more difficult to characterize with the addition of DNA data since species of Chamonixia, along with a few species placed by some researchers in Boletus and Tylopilus, must now be characterized as well. However, morphologists have noted affinities between these mushrooms and Leccinum for decades and, excluding Chamonixia, the concept I am using here corresponds almost entirely to that of Singer, published in 1947 and stubbornly reiterated in 1986.
Species of Leccinum are boletoid or gasteroboletoid. With the exception of illustrations and a brief description of Chamonixia caespitosa, I have not treated the gasteroboletoid species here. The boletoid species of Leccinum have a glabrous, fibrillose, or fibrillose-squamulose pileus surface; in some species the margin of the pileus extends substantially to create overhanging flaps of tissue. One rare North American species (Leccinum potteri) has a veil that leaves powdery remnants on the pileus surface. The tube layer is usually depressed around the stipe, and is often swollen in its middle portions. The pores are small and sub-circular. The pore surface is initially whitish or yellowish before the spores mature. The stipe is tapered to the apex, and fairly tough in comparison to the pileus. Its surface is adorned with scabers which may be pale or pigmented initially; pale scabers usually (but not always) darken with maturity. The context is white or pale yellow and may (or may not) discolor when sliced and exposed to air. The pileipellis is disposed as a cutis or a trichoderm, sometimes with inflated or otherwise distinctive elements. Pleurocystidia and cheilocystidia are usually present on the tubes. Scabers are composed of bundled tufts of filamentous hyphae, terminating in caulobasidia and caulocystidia. The spores are smooth and subfusiform.
By this morphological definition, boletoid species of Leccinum are separated from the other bolete genera primarily on the presence of scabers, regardless of whether the scabers darken to brown or black, placing a handful of taxa whose generic position has been hotly debated over the years in Leccinum rather than Boletus or Tylopilus--including Leccinum chromapes, Leccinum longicurvipes, and Leccinum subglabripes.
Figure 1 represents taxonomic divisions within Leccinum, based on molecular data (Binder & Besl, 2000; Binder & Hibbett, 2004; den Bakker and collaborators, 2004b; den Bakker & Noordeloos, 2005), mycorrhizal association, and morphological features (the disposition of the pileipellis and the presence or absence of overhanging marginal flaps). The basic divisions are more or less supported by all of the cited papers, but precise alignment of subsections within section Leccinum has yet to be firmly resolved.
North American Leccinum Research
Our understanding of Leccinum in North America is unfortunately still in its early childhood, despite over 100 years of hard work on the part of North American mycologists. In some areas of North American Leccinum taxonomy the waters have been muddied so thoroughly that they may be unswimable, forcing us to return to the starting line--focused, this time, on ecological factors as well as morphology, and supported by molecular evidence. Elsewhere (see 2007) I have argued that mycology's attempt to understand and classify mushrooms on the basis of morphology alone has been a largely unscientific endeavor, doomed to fail from the onset--and that future scientific success will depend on seeing mushrooms as parts of ecosystems, not figurines displaying morphological features.
The bulk of North American Leccinum taxonomy is based solely on morphology, and comes from three publications by Smith, Thiers & Watling (1966, 1967, 1968), from the Smith & Thiers (1971) treatment of the Boletes of Michigan, and from the Thiers (1975) treatment of the Boletes of California. The authors planned a monograph of Leccinum in North America, but never completed it. Thus the publications we have were written as a "provisional treatment, in a sense, to enable our species concepts to be widely tested" and the authors were uncharacteristically lax about "citation of all the material studied, or drawings of the microscopical structures" (1967). Documentation of host preference in these publications is at best imprecise--and at worst, absent. I have examined a smattering of Smith's collection notes and accession book entries (held in MICH), and found no hidden ecological data. I chose Smith's notes for Leccinum longicurvipes as a test case, but there are hundreds of Smith's Leccinum collections in the herbarium. While I doubt that examination of all the collection notes would contribute much in the way of previously hidden significant ecological data, it is of course a remote possibility.
An additional problem for North American Leccinum studies is that many taxa are represented by very few collections--sometimes by only one. While a 30- or 40-year-old exsiccatum from a herbarium can still be studied for micromorphological data, such study is only rarely necessary since Smith and Thiers usually documented the microscopic features of their collections with more rigor and skill than many current researchers (aside from those trained by Smith and Thiers) could possibly duplicate. In the few instances where microscopic data is missing, gaps can be filled in; for example, since Smith & Thiers neglected to describe the caulocystidia of what they were calling "Leccinum griseum," I have examined their collections and added this detail to the record (see Leccinum pseudoscabrum). However, the missing molecular and ecological data may be more difficult or even impossible to recapture. It is not always possible to extract DNA from decades-old herbarium exsiccata with standard (and affordable) methods. As far as recapturing ecological data, the prospects are frequently even more remote. Even when we can make fairly good guesses about collection locations, it is by no means a given that the woods still exist--especially in northern Michigan, where many of the Smith & Thiers Leccinum collections were made and where logging and real estate development have since altered the landscape significantly.
All of this means that a substantial amount of collecting and rigorous documentation must now be done to advance understanding of Leccinum in North America. There is not much to be gained, at this point, in describing "new" species on the basis of whether the context stains pink, then gray when sliced, or whether the hyphae of the pileipellis develop pigment globules in Melzer's reagent. Neither will further alignments of poorly documented GenBank sequences contribute much to our understanding--despite the understandable temptation for molecular biologists to produce such papers in today's publishing climate. The work that is needed is not glorious: collecting specimens; painstaking documentation of ecology; thorough morphological description of collections; the accumulation of hundreds of well documented vouchers in public herbaria for molecular analysis and monographic study. Obviously, this is too much work for one or two Leccinum researchers to accomplish--but it could be accomplished in relatively short order with the help of our continent's mushroom clubs and mycological societies. Several seasons' worth of careful collection and documentation in relatively limited geographic areas where clubs are popular (among them California and the Pacific Northwest, the Rocky Mountains, and the Gulf Coast) would advance Leccinum studies immeasurably; see Kuo 2007 for recommendations.
At present, accurate identification of most North American Leccinum collections is not possible, and collectors must accumulate the much-needed data that will allow a set of identification characters to be constructed once molecular results support the division of taxa. Especially important will be documentation of mycorrhizal host, but careful, "traditional" study of morphological features will obviously also be required.
The surest way to determine a mushroom's mycorrhizal host with certainty is to discover physical evidence of mycorrhizae and support the identification of the rootlet and the fungal hyphae with DNA sequencing and morphological analysis. To my knowledge this painstaking process has only been undertaken in Leccinum once, by Osmundson and collaborators (2007), who provided molecular support for the mycorrhizal relationship between Comarostaphylis arbutoides and Leccinum monticola. However, while I believe this method of host determination should ultimately be attempted with most Leccinum species, it is beyond the scope of normal field mycology. In the meantime, careful documentation of potential mycorrhizal hosts (i.e., every plant known or suspected to be mycorrhizal with Leccinum within plausible mycorrhizal distance of the basidiocarp) for many collections would result in a data set that could reliably be mined to infer mycorrhizal relationships through the process of elimination.
Collections in apparent mycorrhizal isolation, while rare in nature, are potentially very valuable and should be painstakingly documented. Mushrooms appearing under isolated trees (for example, in a yard or a cemetery) or in apparent "pure stands" provide relatively rare opportunities to eliminate mycorrhizal variables--though collectors should be aware of and document the potential for bushes in the Ericales (including huckleberry and kinnikinnick) to be mycorrhizal associates, as well as the potential for recently removed trees to have still-living root systems.
Plausible mycorrhizal distance is often assumed to correspond roughly to a tree's "drip line" but my observations of Suillus americanus, an obligate mycorrhizal associate of Pinus strobus, lead me to believe that a more accurate calculation can be made by mentally toppling trees over and assuming that rootlets capable of forming mycorrhizae may be present anywhere within the radius circumscribed by the top of the "fallen" tree. In other words, mycorrhizal mushrooms can appear much farther away from the tree than is often supposed, and collectors documenting potential hosts should bear this in mind. Beech and (especially) aspens, both known to host Leccinum, often clone themselves via their eager root systems, and it would not surprise me to discover that rootlets of these trees could extend even farther than my "toppling calculation" would allow.
Many species of Leccinum occur in ecosystems that seem maliciously designed to frustrate precise documentation of mycorrhizal hosts. The northern counties of Michigan's Lower Peninsula represent one such area, and Smith's concept of Leccinum speciation is based in large part on extensive collecting in this region. But separating aspen, birch, and various conifers as potential hosts in Emmet and Cheboygan counties can be a monumental task. I have assured myself that I collected the birch-associated Leccinum holopus under aspen and hemlock in Emmet County, only to return the next year and notice the tiny birch sapling hidden in bushes a few feet from the collection location. Similar host determination problems occur in transitional "aspen zones" in montane ecosystems, though the list of tree species is often somewhat shorter.
Ideally we would have, for example, 100 collections of Leccinum rugosiceps with identification supported by molecular and morphological data, and a long list of potential mycorrhizal associates for each collection--which a computer could sift through for common denominators, arriving at a list of a few well supported hosts. I use Leccinum rugosiceps as my example because, while present data suggests fairly convincingly that the species is mycorrhizal with Quercus and that the rigorous ecological documentation I am proposing is not needed, there might easily be a precise host (Quercus rubra? "red oaks" in general?) that has not yet been determined.
In the present study I have consulted Leccinum collections in the extensive online databases of five herbaria (TENN, OSU, MICH, NY, and BPI) for ecological data, but this strategy should be seen as a desperate substitute for working with reliably recorded, carefully documented ecological data--and conclusions drawn from existing herbarium data on ecology should be read as extremely tentative. Even the best and most prominent collectors, many of whose Leccinum collections are in these online databases, did not hold ecology to be as important as it has turned out to be; as a consequence the data regarding potential mycorrhizal associates is minimal and unreliable.
Most macromorphological characters in Leccinum should be noted in the field--especially the color of the scabers and the staining reactions of the sliced context. Ideal collections include specimens in all stages of development. In situ photographs from all angles, using diffused flash (see Kuo 2004 for basic techniques with digital cameras) should be accompanied by photographs taken in the laboratory or at home.
Pileus. Color of the pileus, while not negligible, may not be as informative as traditional treatments have supposed. In the European context, den Bakker & Noordeloos (2005) argue that molecular results demonstrate many species are capable of displaying a range of light to dark colors. Texture of the pileus surface also demonstrates a certain amount of variability, though some species are consistently fibrillose-squamulose, glabrous, etc. The margin of the pileus usually projects at least a millimeter or so beyond the tubes when the mushroom is young, but by maturity species in subsection Leccinum display substantial overhanging marginal flaps of tissue. One North American taxon, Leccinum potteri, features veil remnants on the young pileus surface.
Hymenium. The color of the tubes and pore surface in both young and mature specimens should be recorded, together with bruising and staining reactions. Standard measurements (depth of the tubes, number of pores per mm) should be taken from mature specimens.
Stipe. Since proportions of the stipe may have some relevance in the North American context (for example, in separating Leccinum crocipodium), diameter measurements should be taken from young and mature specimens at the apex, the widest point, and the base (the point of insertion into the substrate). The "ground color" beneath the ornamentation should be recorded, together with textural surface details (many species develop soft ridges or even quasi-reticulation, aside from the scabers). Some species demonstrate bluish or olive stains on the stipe surface, usually in the basal region. The size ("fine" or "coarse") and density of the scabers should be assessed for the upper stem, the midportion, and the basal area. Scaber color across the developmental stages has proved to be very important in the European context, and the present treatment holds this character (together with its microscopic corollary, the staining reaction of caulocystidia in KOH) as a primary hypothesis for potential morphological separation of North American material. Since the contemporary concept of Leccinum has freed us from the debate over whether darkening of the scabers to dark brown or black is indicative of the genus, we may now be able to describe scaber color more objectively; with no disrespect intended, I suggest that some of the Smith & Thiers descriptions of scaber color (especially those supported by one or a few basidiocarps) may have been unconsciously loaded with the need to justify their position in their argument with Singer, resulting in darker mature scaber colors (or assumed darker colors). The color of the scabers in both buttons and mature specimens should be recorded--but
Context. The color of the context in Leccinum ranges from white to pale yellow. Staining of the sliced context, while not irrelevant, has probably been given more taxonomic priority in the North American context than variation (and consistency in methodology) should allow. In particular, the presence of a pinkish staining stage before the stained surface resolves to grayish or black has likely been overemphasized. In the European context, staining and the eagerness of the staining reaction have been found to be occasionally informative, but generally so only in limited taxonomic areas (for example the consistently weak staining of Leccinum vulpinum). Some species usually display bluing flesh in the stipe base or in the pileus, but the bluing may not be consistent enough to justify taxonomic priority. Staining reactions should be documented at the time of collection when the mushrooms are fresh.
Spore Print. While spore print color has been used as a predictive character frequently in traditional treatments of Leccinum, it has also been the source of considerable confusion resulting from the difference between a "wet" and a "dry" print--and the character has occasionally been "fudged," as in Smith, Thiers & Watling (1966), where spore print color plays a major role in the key but taxa with undocumented print colors are found subsequent to choices emphasizing the character. In the European context den Bakker & Noordeloos (2005) do not even bother to record or discuss spore print color. While I doubt that the character has any predictive value in the North American context, either, it may be worth continuing to record the print color and preserving prints along with exsiccata--at least until substantial research documents the potential futility of the effort.
Odor and Taste. To my knowledge odor and taste, which are almost universally "not distinctive" in the genus, have not been found to have substantial variation or predictive value within Leccinum.
Exsiccata. Examination of the macromorphology of exsiccata falls into two categories: attempting to recapture details the specimen manifested in the fresh state, and assessing macromorphological features that result from the drying process. As for the first category, it is obviously preferable to examine a fresh specimen or photographs of the specimen in the fresh state, and macromorphological observations based solely on examination of exsiccata should be seen as tentative. In the second category, features resulting from the drying process that may have some predictive value in limited taxonomic areas include the color of the pileus, hymenium, scabers, and context as dried--but variation in drying methods and the age of herbarium specimens may influence these features.
Micromorphological characters that appear to have predictive value in some taxonomic areas of Leccinum (when placed in ecological and molecular context) are derived from observation of the anatomy of the pileipellis, the hymenium, the stipitipellis, and the basidiospores. For the latter, a spore print is the ideal source of material and a water mount will suffice. A radial section of the pileus surface containing about 1 mm of the pileus context for visual orientation should be mounted in unstained, 2-5% KOH or in water. The hymenium should be sectioned perpendicular to the tubes and mounted in unstained KOH. The stipitipellis should be sectioned by slicing a paper-thin section in the mid-portion of the stipe, parallel to the stipe surface, including at least one scaber (in specimens where scabers are clearly defined), and mounted in unstained KOH. If a spore print is unavailable, the hymenium and stipitipellis sections will provide spores for observation if the material was mature. When examining exsiccata, I have found these sections to be more easily accomplished without preliminary rehydration in alcohol and water--and I have compared results from rehydrated sections and straight-to-KOH sections without finding any differences.
Pileipellis. In the European context, den Bakker & Noordeloos (2005) found "two basic types" of pileipellis in Leccinum: a "trichoderm with erect chains of elements" and a "complex type of intricate trichoderm, often with a cutis-like suprapellis." For the sake of convenience the authors call these types "trichoderm" and "cutis"; I have also used this terminology. Additionally I have used the admittedly imprecise term "epithelium" to refer to a trichoderm-like pileipellis in which terminal elements are substantially swollen. Precise documentation of the anatomy of the terminal elements of a trichoderm-like pileipellis may have some value in the North American context; Leccinum rugosiceps, for example, appears to manifest swollen penultimate and antepenultimate elements with consistency. However, the presence or absence of "cylindrocysts" (see Lannoy & Estades 1995), together with the formation of pigment globules in a Melzer's reagent mount (see Smith & Thiers 1966), represent characters that appear not to occur with reliable consistency, and can now safely go unrecorded.
Hymenial Cystidia. Den Bakker & Noordeloos (2005) found "no diagnostic value" in the morphology of hymenial cystidia among European Leccinum species; they found shapes and sizes to vary without consistent correlation to genetic species. However, data on hymenial cystidia is inconsistently recorded in the den Bakker & Noordeloos treatment, and the color of the cystidia in a KOH mount (particularly the cheilocystidia on the pore surface, which tend to display more pigmentation) may turn out to have limited predictive value (perhaps paralleling the KOH reaction of the caulocystidia) and should not yet, in my opinion, be discarded as a potentially valuable character.
Basidiospores. While careful documentation of basidiospore morphology should be continued in North American Leccinum studies, we should probably not expect the data to reveal much for potential species diagnosis. Spores in Leccinum are often quite variable in size, and exceptional "monster" spores are not infrequently encountered. Additionally, the current data on spore morphology in North American Leccinum cannot be relied upon, due to inconsistencies in methods: immature spores have undoubtedly been included; little or no effort has been made to account for potential differences in dimensions of spores produced in the tubes or on the stipe; mounting media have been inconsistent and poorly documented. In the European context den Bakker & Noordeloos (2005) found spore shape (not length, in and of itself) to have some potential value as a character; researchers who want to generate basidiospore morphology data that can successfully be compared to the data of den Bakker & Noordeloos should follow the same methods:
Caulocystidia. Caulocystidia in Leccinum vary substantially, and in most species a wide variety of cystidia shapes can be demonstrated. However, in some species one or a few types of caulocystidia represent the overwhelming majority, and are easily demonstrated without a painstaking search. Additionally, the color of caulocystidia as mounted in KOH appears to have some consistency and, consequently, diagnostic value in limited taxonomic areas. Relatively little attention has been paid to caulocystidia by North American (or other) researchers--but since caulocystidia represent, in part, a microscopic corollary to the scabers, which den Bakker & Noordeloos (2005) have found to be substantially more worthy of attention than was previously suspected, it is worth maintaining the hypothesis that careful documentation of caulocystidia may help support identification--a hypothesis used by den Bakker & Noordeloos with success in the European context.
Whether or not macrochemical reactions have any predictive value in Leccinum cannot currently be assessed, since data is inconsistently and sparsely recorded throughout even the morphology-based literature. Apparently distinctive reactions--like the bright red reaction of the pileus surface to KOH in Leccinum longicurvipes--which prompted M. E. Bigelow to write "fantastic!!" when annotating a 1969 collection (HEB 15796 in NY)--must now be documented again and again, and confirmed by ecology- and molecular-based research. Reactions to potassium hydroxide (a 5-20% aqueous solution), ammonia, and iron salts should be recorded in the field, and the chemicals should be applied to the pileus surface, the context (in the stipe, in the pileus), the pore surface, and the stipe surface (including the scabers). Ideally, reactions should be recorded for both young and mature specimens. I believe the data for macrochemical reactions is worth recording, at least for the time being--though the subtle gradations in the reaction of the context to iron salts were not found to be informative (and possibly subject to "age and humidity of the fruit-body") for European taxa in subsection Scabra by den Bakker & Noordeloos (2005).
Draft Key to Leccinum in North America (April 2007)
[Gasteroboletoid species not treated; see Chamonixia caespitosa for an example.]
As of April, 2007, I have not finished investigating most of the remaining North American taxa, which constitute the bulk of Leccinum. I have placed a few taxa (linked below) in the "uncertain" category, and I have treated the white, birch-loving Leccinum holopus--but most of the work remains to be done. I apologize to readers who are disappointed and are eager to identify collections made under manzanita, madrone, birch, and aspen--but in my defense I will say that the work published here represents most of my available time in the winter of 2006-2007, and I hope to return to Leccinum after another season of collecting mushrooms. Below is a taxonomic digest of North American Leccinum (taxa that I have treated are linked, and links are provided when texts are online), and the references list.
Partial Taxonomic Digest of Leccinum for North America
aberrans Smith & Thiers (1971 p. 211)
den Bakker & Noordeloos, 2006.]singeri Smith & Thiers (1971, p. 204)
snellii Smith, Thiers & Watling (1967, p. 120) [Online in Smith & Thiers, 1971, p. 207.]
solheimii Smith, Thiers & Watling (1966, p. 142)
subalpinum Thiers (1976, p. 264)
subatratum Smith, Thiers & Watling (1968, p. 260) [Online in Smith & Thiers, 1971, p. 129.]
subfulvum Smith, Thiers & Watling (1966, p. 141)
subglabripes (Peck) Singer (1945, p. 799)
subgranulosum Smith & Thiers (1971, p. 210)
subleucophaeum Dick & Snell (1960) [Online in Smith & Thiers, 1971, p. 189.]
sublutescens Smith, Thiers & Watling (1966, p. 139) [Online in Smith & Thiers, 1971, p. 164.]
subpulchripes Smith & Thiers (1971, p. 205)
subrobustum Smith, Thiers & Watling (1968, p. 261) [Online in Smith & Thiers, 1971, p. 145.]
subspadiceum Smith, Thiers & Watling (1968, p. 263) [Online in Smith & Thiers, 1971, p. 133.]
subtestaceum Smith, Thiers & Watling (1966, p. 145) [Online in Smith & Thiers, 1971, p. 156.]
var. angustisporium Smith & Thiers (1971, p. 157)
tablense* Halling & Mueller (2003, p. 496) [Online at MCR: Brief or Technical.]
talamancae* Halling, Gómez & Lannoy (in Halling, 1999, p. 750) [Online at MCR: Brief or Technical.]
testaceoscabrum Singer (1947, p. 123) [Online in Smith & Thiers, 1971, p. 140.]
truebloodii Smith, Thiers & Watling (1968, p. 265)
uliginosum Smith & Thiers (1971, p. 131)
variabile Smith, Thiers & Watling (1967, p. 130) [Online in Smith & Thiers, 1971, p. 184.]
vinaceo-pallidum Smith, Thiers & Watling (1968, p. 265) [Online in Smith & Thiers, 1971, p. 136.]
vulpinum Watling (1961, p. 197)
* Central American taxa that may be extralimital but might be expected, for example, in the cloud forests of Mexico.
NOTE: Technical sources only; field guide and Web site references can be found on the site's References Page.
Aanen, D. K., T. W. Kuyper, T. Boekhout & R. F. Hoekstra (2000). Phylogenetic relationships in the genus Hebeloma based on ITS1 and 2 sequences, with special emphasis on the Hebeloma crustuliniforme complex. Mycologia 92: 269-281.
Binder, M. & H. Besl (2000). 28S rDNA sequence data and chemotaxonomical analyses on the generic concept of Leccinum (Boletales). In: Associazione Micologica Bresadola, ed. Micologia 2000. Brescia, Italy: Grafica Sette, 75-86.
Binder, M. & Hibbett, D. S. (2004). Toward a global phylogeny of the Boletales. Retrieved May 1, 2007 from the Clark University Web site: http://www.clarku.edu/faculty/dhibbett/boletales_stuff/Global_Boletales_2004_28S.gif
Both, E. E. (1993). The boletes of North America: A compendium. Buffalo NY: Buffalo Museum of Science. 436 pp.
Bruns, T. D. et al. (1998). A sequence database for the identification of ectomycorrhizal basidiomycetes by phylogenetic analysis. Molecular Ecology 7: 257-272. This paper is available online here, at T. D. Bruns's Web site.
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I wish to express my gratitude to the following individuals and institutions, who helped make this treatment possible: Kate Klipp, John David Moore, Bob Zordani, Carol and Ray Schmudde, John Plischke, Sören Gutén, Irene Andersson, Pamela Kaminski, Richard Nadon, Dianna Smith, Koukichi Maruyama, Jay Justice, Andrew Methven, James Trappe, Dan Luoma, Patricia Rogers, Roy Halling, Patrick Leacock, Barbara Thiers, the University of Michigan Herbarium, the New York Botanical Garden Herbarium, and the Stover-Ebinger Herbarium at Eastern Illinois University.
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Kuo, M. (2007, May). The genus Leccinum. Retrieved from the MushroomExpert.Com Web site: http://www.mushroomexpert.com/leccinum.html