The Genus Pluteus  

[ Basidiomycota > Agaricales > Pluteaceae . . . ]

by Michael Kuo

The mushrooms in Pluteus are wood-decomposing saprobes with gills that are free from the stem and pink spore prints (though what mycologists call "pink" is not always what might come to your mind or mine; "brownish pink" or even "pinkish brown" might be more accurate). There is no volva at the base of the stem, which separates them from volvarielloid mushrooms. The spores, under a microscope, are smooth and round or ellipsoid—separating them from the spores of entolomatoid mushrooms, which are angular and/or ridged. Entolomas do not typically grow on wood, however, so a microscope shouldn't be necessary to identify a Pluteus to genus.

Identifying a Pluteus to species, however, ranges from fairly easy to very difficult, and microscopic analysis is often needed. Important macroscopic features include the color and texture of the cap and stem surfaces, sizes and proportions, and an assessment of whether the specimen was growing on the wood of conifers or hardwoods. Under the microscope, a Roman aqueduct section, mounted in 2% KOH, should be used to observe the pileipellis, pleurocystidia, cheilocystidia, and spores. These microfeatures are used extensively in Pluteus identification, and the genus is fairly easily broken up into groups of species based on them. The pileipellis can be "cellular" (containing inflated, round-ish elements) or it can be a cutis (containing long, cylindric elements)—or, in a few cases, it can contains both types of elements. The cystidia are variously shaped, and can feature thick or thin walls, as well as apical "prongs" that are reminiscent of antlers or horns. And, in a few cases, measuring spores and assessing their shapes (globose to subglobose to ellipsoid) is necessary for identification.

Researchers have made big strides, recently, in assessing the Pluteus species in many parts of the world, including North America. Papers by Alfredo Justo and collaborators (2011a, 2011b, 2011c, 2014) and by Andrew Minnis and collaborators (2006, 2010) have made important contributions. But we are only beginning to get a contemporary picture of what our continent's species are, and large gaps remain. In some cases, previously named species have turned out to represent the same thing, simplifying the picture . . . but in other cases (notably the Pluteus cervinus group), "cryptic" species have been uncovered, some of which are apparently virtually impossible to separate without DNA analysis.

The key below treats 40 North American species of Pluteus, and it is based in large part on the more recent treatments mentioned above. For the most part, I have eliminated poorly documented species that were named over a hundred years ago (usually by William A. Murrill) and never collected again—but I have included some of the contemporary species that have not been collected much, in order to provide modern names and concepts and to encourage further documentation of these species. The result, however, may be frustrating to those who don't want to learn mushroom microscopy, and/or who want "Pluteus cervinus" to be the easily applied name that it used to be.