The problem is, Suillus species can't read (nor can they access the Internet), so they have never heard of Michael Kuo's List of Things Suillusy, and most of them don't know they ought to make things easy for us and manifest all of these features at once. This makes some of them a little difficult to identify to genus—but, as a general rule of thumb, you can probably expect a potential Suillus to demonstrate at least three of the features above. Alternatively, if you have a microscope, the presence of bundled, brown cystidia in the tubes (see the illustration) is in itself a reliable indicator of the genus Suillus.

There are some very beautiful and impressive Suillus species—and, after fall rains, they can come up in stunning numbers. My favorites are the species with dry caps and partial veils—Suillus lakei, Suillus spraguei, and Suillus ampliporus in North America, and in Europe, Suillus cavipes. But there are also hoards of slimy, brown and yellow suilluses that can be mind-numbingly similar, especially when they reach old age and become . . . well, the technical term is "splatty."

Given the seemingly tentative way that Suillus is defined as a genus (usually more or less as I have done here, with a checklist of characters that are not all demonstrated in all specimens), it is surprising how well the generic concept has held up over the years—once the separate genus "Fuscoboletinus," consisting mostly of Suillus-like mushrooms associated with larch or tamarack, was folded into Suillus. Contemporary, DNA-based explorations have for the most part upheld the traditional genus, while at the same time revealing two important things: that variability in morphology has led to the naming of more "species" than are supported by phylogenetic differences, and that species of Suillus are highly host-specific.

"Host-specific" denotes the limited mycorrhizal associations made by most Suillus species. While a few species appear capable of associating with a fairly wide range of conifers, most are limited, either to a specific tree species (Suillus placidus associates only with eastern white pine, for example), or to a group of related trees, like "hard" or "soft" pines (the former with needles bundled in twos or threes; the latter with needles bundled in groups of five). Thus Suillus brevipes associates with lodgepole pine, red pine, jack pine, and other hard pines, but not with soft pines.

Tree identification, then, is obviously crucial if you want to identify the Suillus you've collected. Fortunately most conifers are not too difficult to identify, and deciding whether a pine is "soft" or "hard" is a simple matter of counting bundled needles. That said, pines and other conifers are often "mixed," making it difficult to decide which trees the mushrooms are associating with—especially since the distance between a given tree and a mushroom in mycorrhizal association with its root system may be quite a bit more than one might think. The old-school "drip line" radius idea is ridiculously wrong, as I have demonstrated for myself again and again with Suillus species. I have found Suillus americanus 80 feet away from the nearest eastern white pine, otherwise surrounded only by grass and non-mycorrhizal trees. The pine was about 80 feet high—which means that what I call "toppling distance" (Could the tree hit the mushroom if it toppled over?) is the potential distance from a tree's trunk at which associated mushrooms can be expected.

So if you are collecting in an area where conifers are mixed, you will need to take your best guess at which tree species your Suillus is paired with, and be prepared to test the other trees as hypotheses if your first guess doesn't work out. Particularly frustrating ecosystems in this regard include high-elevation forests in the Rocky Mountains, where pines, spruces, and firs are frequently mixed, and Appalachian forests with scattered conifers. Larch trees in the Pittsburgh area, for example, can be scattered among oaks and other hardwoods and, in an attempt to reach the sunlight, have grown so narrowly and so high that they are almost impossible to find without recognizing their bark or finding their small needles among the fallen oak leaves.

As for the North American species of Suillus, my friend Nhu Nguyen has done us all a favor; he investigated a stunning number of Suillus collections from across the continent, including type collections for species named from North America, and published his findings in a magnificent treatise (Nguyen and collaborators 2016). This work is now the standard for North American Suillus, and the identification key I have made below is based primarily on the species included in Nguyen's paper.

Aside from documentation of the host tree, morphological features will need to be closely assessed for successful Suillus identification—and since some of these features are subject to change over the course of a mushroom's development, you will be more successful in your identification attempts when you have several specimens in your collection, representing different stages of development. The presence or absence of a partial veil should be noted (heads up, however, since there are a few species that are confusing in this regard, manifesting a roll of soft tissue around the young cap margin that is debatably an actual veil—sometimes the term "false veil" is applied); also the presence or absence of visible glandular dots on the stem sometimes plays a role in identification. Colors of the cap and pore surface are important, as well as the dryness or stickiness of the cap, and whether or not there are fibrils and/or scales present. Microscopic examination is generally not needed for Suillus identification.