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The Genus Cortinarius
by Michael Kuo
Cortinarius is the largest genus of mushrooms in the world, containing an astounding number of species (often estimated well over a thousand). Although the mushrooms in Cortinarius are very diverse, it is usually pretty easy to figure out when you are looking at one, once you get the hang of it. First, they have cortinas covering their gills when young (hence the name of the genus); sometimes the tiny fibers of the cortina are ephemeral and disappear, but they frequently collapse against the stem to create a ring zone. Secondly, the spore print is rusty brown and, as a result of the rusty brown spores, the mature gills of Cortinarius mushrooms are usually also rusty brown. Finally, they are terrestrial and mycorrhizal, so you will only find them in association with trees.
But while the genus may be more or less easy to identify, figuring out what species of Cortinarius you have found may be one of the most difficult challenges in mushroom mycology, compounded by the fact that there are few (if any) comprehensive, current and reliable keys available for North America. One is often forced to use European treatments (see especially Hansen & Knudsen, Moser, and Breitenbach & Kränzlin in the references list below) and adopt a "best-fit" attitude. In short, identifying a Cortinarius to species is often best described as a Quixotic endeavor--or maybe not; Don Quixote did manage to accomplish a thorough assault on a windmill, which is more "accomplishment" than one often achieves with Cortinarius identification (see my unidentified collections, for example).
Traditionally the genus Cortinarius has been divided into a handful of subgenera, based on physical features like the sliminess of the cap and/or stem, the shape of the stem base, and whether or not the cap is hygrophanous; see the extended discussion of these features below. However, DNA-based study of the genus has begun to question at least some of the divisions between the traditional subgenera, and it appears that while examination of these physical features will undoubtedly assist in future Cortinarius identification, correspondence among these features may not necessarily indicate genetic relationships. Importantly, however, DNA-based study of the genus is in its infancy; as the authors of one fairly recent study carefully point out, "[r]esearch on the molecular systematics of Cortinarius, a species-rich mushroom genus with nearly global distribution, is just beginning" (Peintner and collaborators, 2004). A DNA study by Niskanen and collaborators (2011) of the Cortinarius armillatus group in northern Europe sets a wonderful example for future researchers about the diligence and patience required to draw conclusions about species and their distribution; the authors examined and sequenced 1,000 specimens before drawing any conclusions!
Cortinarius Identification Features
Since members of the genus Cortinarius are mycorrhizal, they have evolved with their symbionts and therefore occupy limited ecological niches. Because some, if not all, species are partial to particular trees or groups of related trees, careful ecological field notes and documentation are a must when collecting Cortinarius specimens for identification, especially when collecting in notoriously "mixed" woods like montane, boreal, or coastal ecosystems. In northern Michigan, for example, I have found Cortinarius armillatus, which is an obligate associate of birch, in woods that appeared to be composed only of aspens, hemlock, and balsam fir . . . only to return to the location and discover a tiny, three-foot birch sapling a few feet away from my collection location.
Most keys to the genus begin with slime. Does your mushroom have a slimy cap? A slimy stem? Your answers to these questions lead you to two of the traditional subgenera: Myxacium (cap and stem slimy) and Phlegmacium (cap slimy, stem dry). When you are collecting in normal conditions and your specimens are fresh, assessing the "slime factor" is usually pretty easy. Species in Myxacium have a "slime veil"--a universal veil that consists of slime, kind of like the volva on an Amanita but more disgusting. As the mushroom grows, the slime veil breaks, leaving slime on the cap and on the lower stem.
The problems come when you are collecting in dry conditions, or your specimens have dried out after being picked. In this case it is sometimes possible to judge a mushroom's former sliminess by inspecting the debris that may have adhered to the cap surface as a result of being embedded in gluten that later dried out. Ultimately, however, microscopic examination may be required: Cortinarius species with slimy caps have an "ixocutis" (Mycologese for "slimy skin") in which the hyphae of the cap surface are embedded in a gelatinous matrix.
Yeah, that's not a word. Hygrophanity? Anyway, a "hygrophanous" cap is one that fades markedly as it dries out, often resulting in a substantial color change and, if you're there at the right time, "two-toned" specimens in a transition stage. The traditional subgenus Telamonia is (in part) separated on the basis of the hygrophanous caps of the species.
By maturity most species of Cortinarius develop the characteristic rusty brown gills of the genus. But in the button stage gill color is very variable, and determining the color of the young gills is often essential in identifying Cortinarius collections. This means you will need to have young specimens in your collection--perhaps even specimens in the button stage, before the cap has opened.
The base of the stem is another important feature in Cortinarius identification--which means you will need to be careful to preserve it when collecting; it is often necessary to dig up Cortinarius specimens with a knife. The stem may be more or less equal, or club-shaped ("clavate" in Mycologese), or swollen dramatically and suddenly at the base, so that the basal bulb has a rim (in which case the bulb is said to be "marginate"). Additionally the disposition of the cortina on the mature stem can be important in identification: has a ring zone resulted from the collapse of the cortina, or did it disappear? Universal veil remnants are also sometimes present on the stem, creating a sheathed ("peronate") appearance, as though the stem is wearing a tight-fitting sock.
Several species and species groups in Cortinarius possess distinctive odors. This is best assessed with fresh collections (preferably in the field) that have been sliced in half. Odors in the genus include: radishlike ("raphanoid"), sweetly and strongly fragrant, iodine-like, and flat-out foul. Taste is occasionally important as well--but only experienced mushroom identifiers should taste species of Cortinarius, since many species are seriously toxic. Distinctive tastes include a radishlike taste and, especially, bitterness.
Many species in the genus have distinctive reactions to potassium hydroxide. Apply a fairly strong (5-15%) aqueous solution to both the cap surface and the sliced flesh in order to assess any color changes. Meinhard Moser (see 1983, for example) used the reaction of the flesh to KOH to help arrange the genus taxonomically--and while most mycologists are not willing to go quite this far, there is no denying the utility of a KOH test in much of Cortinarius identification.
Microscopic examination is generally required for identification of all but a few "field guide species" in the genus. Spore morphology is probably the most crucial among the microfeatures; spore shape, size, and the degree of ornamentation (from nearly smooth to strongly verrucose) can all be important in identification. Ideally, spores should be measured from a spore print so that they are unquestionably mature. A Roman aqueduct section, mounted in 2% KOH, will suffice for studying other microscopic features important in Cortinarius, which include the pileipellis (see the discussion and illustration above, under "Sliminess"), the presence or absence of clamp connections, and cystidia on the gills. Impressive, well defined cystidia are almost never heard of in Cortinarius (Cortinarius violaceus is virtually the only exception), but some species possess cylindrical to club-shaped cheilocystidia--sometimes designated as "marginal cells" when they are so inconspicuous as to be unworthy of a full-blown "cheilocystidia" designation (yes, the distinction is frustrating, and no, authors do not use these terms consistently and universally).
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Cite this page as:
Kuo, M. (2011, December). The genus Cortinarius. Retrieved from the MushroomExpert.Com Web site: http://www.mushroomexpert.com/cortinarius.html