Polypore primer:
An introduction to the characters used to identify poroid wood decay fungi 

Tom Volk
Dept. of Biology
3024 Cowley Hall
University of Wisconsin-La Crosse
La Crosse, Wisconsin 54601
volk.thom@uwlax.edu

TomVolkFungi.net

(This paper originally appeared in McIlvainea 14 (2): 74-82, 2000.) 

Figure 1. Some polypores. A. Laetiporus cincinnatus B. Trametes versicolor C. Bridgeoporus nobilissimus D. Ganoderma applanatum

The polypores are a fascinating group of fungi, although they are usually ignored by most mycophiles because of their typical inedibility, commonly small size, unfamiliar habitat and general obscurity. However, these fungi are very interesting from an ecological, microscopic, and biotechnological standpoint, and are well worth observing and learning to identify. With practice, a great many species can be learned just by their macroscopic features. An added bonus from a collecting viewpoint is that, unlike fleshy mushrooms, most of these fungi can be found even during dry weather or in the winter, since many are tough or perennial and many others produce basidiocarps only beneath the surface of logs lying on the forest floor, where it remains wet most of the year. 

Polypores (family Polyporaceae and similar fungi) can be easily distinguished from the other common poroid fungi, the boletes, by their typically hard exterior, their usual "non-mushroom" shape, and their usual growth on wood as wood decomposers. You probably know that most boletes fruit on the ground as mycorrhizal fungi, having mutualistic relationship with the roots of trees and other plants.. In addition, the pore layer of boletes can usually be easily peeled off from the flesh (context). A related group, the crust-like corticioid fungi (family Corticiaceae and similar fungi), are also wood-decay basidiomycetes, but they are typically non-poroid and may have a wide variety of hymenophore (spore bearing surface) configurations; most of them are "flat" without any recognizable topology, although some of them are toothed, folded and even poroid. 

The polypores and corticioid fungi are important in natural ecosystems as decomposers of wood, recycling the nutrients and minerals in the wood and releasing them over a long period of time--- sometimes several hundred years from a single large down tree--- where they can be used by other forest organisms. Many species can also act as mild to severe pathogens of living forest trees. In addition to their scientific and ecological interest, some of the species are highly regarded by mycophagists (e.g. Laetiporus sulphureus, the sulfur shelf or chicken of the woods, and Grifola frondosa, hen of the woods, sheepshead or maitake). Many polypores can be used as natural dyes for wool (e.g. Phaeolus schweinitzii and Hapalopilus nidulans). Several polypores are used in oriental herbal medicine, mostly in making tea-like extracts, including Ganoderma lucidum (reishi), Polyporus umbellatus, and Grifola frondosa (maitake). The polypore use that holds the most potential benefit for people is probably in biotechnology. In addition, some of these fungi are highly valued by biotechnologists because of their wood-degrading (and especially lignin-degrading) abilities. More on this later. 

It is important to be able to distinguish the genera and species because proper identification and knowledge of relationships between taxa is the key to further study of the ecological, pathological, genetic, physiological, and biotechnological aspects of these fungi. For example, if you know a particular species is valuable for biotechnology, you might want to check out a closely related species for further usefulness 

Polyporus was once a catch-all genus for "non-mushroom-shaped" fungi with pores, but now there are more than 100 genera of polypore fungi that have been described and are now accepted. Most of these belong in the family Polyporaceae, but 5-7 other families (such as Ganodermataceae, Albatrelllaceae, Bondarzewiaceae, Fistulinaceae, and Hymenochaetaceae) are also now represented. This paper will focus on the types of macroscopic and microscopic characters that may be used to identify polypores to genus and to species, the ecological niches occupied by these interesting fungi, and how they can be exploited for human use. 

Early mycologists based their species and generic delimitations mostly on gross features of fruiting structures. This was the system developed by Linneaus for plants and later by Elias Magnus Fries (1794-1878) for fungi. Mycologists often refer to Friesian characteristics or Friesian families; these are based on macromorphological characters such as those used by Fries, who, for example, classified every gill-bearing fungus into the genus Agaricus and every pored fungus into Boletus (then later into Boletus or Polyporus, based largely on the shape and hardness of the fruiting body). This certainly made genus identification easy, but this was a gross oversimplification. Moreover, the names were not particularly informative about anything but a single character of the fungus. Modern generic classification should convey a multitude of information about a fungus. These two single-character genera, Boletus or Polyporus, are now often accepted at the order level of classification as the Boletales and the Polyporales. 

As described below, modern genera of polypores are largely distinguished on the basis of microscopic characteristics. I highly recommend Gilbertson and Ryvarden’s (1986, 1987) two volume set called North American Polypores. These volumes contain excellent keys and thorough introductory materials. I will summarize some important features of polypore systematics in this paper. 

Figure 2. Brown rot (upper picture) and white rot (lower). See text for details. 

Characters important in the delimitation of polypore genera 

Nutritional niche. An important character at the genus level is the nutritional niche occupied by the fungus. Most polypores are wood decay fungi. There are two fundamentally different ways in which wood can be rotted. Wood is composed mostly of two substances: cellulose (white) and lignin (brown). Cellulose forms the primary wall of all plant cells. Many plants add a second wall of lignin inside the primary wall, especially in wood. Brown rot fungi can degrade only the white cellulose and leave the brown lignin behind. In their simplest form, white rot fungi degrade the lignin and leave the white cellulose behind. Things get more complicated with the so-called simultaneous white rotters—these fungi can degrade both cellulose and lignin, albeit at different rates. In any case, the lignin is used up first and the white color of the cellulose can be seen. Even if you’re color blind, you can feel the wood to understand the differences. Brown rot fungi degrade the primary walls and leave the secondary lignin walls behind. Thus brown rotted wood crumbles to dust between your fingers since there is no primary wall structure. White rot fungi leave the stringy cellulose of the primary walls behind. There are often "sister" genera in the polypores, with seemingly identical characters, except that one causes a white rot and one causes a brown rot. A good example of this is Tyromyces, which causes a white rot and Oligoporus, which causes a brown rot. This distinction is also used in the Agaricales, where, for example, Pleurotus causes a white rot and the closely related Hypsizygus causes a brown rot. 

If you’re thinking ahead you realize there are a couple potential biotechnology uses for these white rot fungi. 

·Biopulping: One of the biggest energy expenditures in paper-making comes from removal of the brown lignin from the wood so that only the white cellulose is left to make paper. Usually this is done with chemical bleaches that are often contaminated with dioxins. There are ecological problems with disposal of these chemical. What if paper companies could use the enzymes of a white rot fungus to remove the lignin? This could result in a savings of both energy and time and avoid pollutive wastes being dumped out of the mills. The ideal fungus for this endeavor would be fast growing, able to tolerate the high temperatures of composting, and leave the cellulose virtually untouched. This ideal fungus would have the exact characteristics of Phanerochaete chrysosporium, a corticioid fungus, or Ceriporiopsis subvermispora, a resupinate polypore. The fungus works very well on the laboratory bench, but, as with many industrial bioprocesses, there have been problems with scaling up the process to an industrial level. Compare this to using a recipe for making chipped beef on toast at home to feeding the troops with the same recipe in battle; it just doesn’t work as well. 

·Bioremediation: Some of the lignin-degrading enzymes of white rot fungi will also degrade some toxic wastes that have the same general chemical configuration, such as PCB's, PCP's and TNT. There is enormous potential to use these fungi to clean up even Superfund sites. Again, this works very well on a small scale, but there are many of the same problems in scaling up the process 

Although most polypores cause wood decay, several genera have members that are mycorrhizal, forming mutualistically beneficial relationships with the root of trees. This might include Bondarzewia, which is probably not very closely related to the other polypores, and almost certainly belongs in the Russulales with Russula and Lactarius. Bondarzewia species have ornamented amyloid spores and sphaerocysts just like Russula and Lactarius, and when young even have lacticifers that produce a milky latex, as does Lactarius. Another mycorrhizal genus is Albatrellus. One must be careful not to ascribe mycorrhizal status to any fungus fruiting on the ground. Many of these ground polypores are root rot fungi (such as Inonotus tomentosus, Laetiporus cincinnatus, and Grifola frondosa), and many others typically grow from buried pieces of wood (e.g. Polyporus radicatus and P. melanopus). 

Even within the general nutritional categories, many polypores are restricted in their host range. This character is usually more important at the species level rather than at the genus level The largest dichotomy lies in hardwood vs. conifer hosts. However, some are even more specific, especially Phellinus species, where the species are almost all host-specific—so it would be nearly impossible to determine which Phellinus species without knowing the host. Fomes fomentarius and Piptoporus betulinus are found almost exclusively on birch trees (Betula spp.). Bridgeoporus nobilissimus (properly pronounced bridge-uh-PORE-us, since it’s named after William "Bridge" Cooke who first described the species) is known only from noble fir (Abies procera) and pacific silver fir (Abies amabilis), both of which are restricted to the Pacific Northwest in the U.S.A. It is important to note the host tree when collecting. This can be difficult, especially when the bark has already fallen off the tree. On a practical level all you can do sometimes is note which other trees are in the area; chances are pretty good the host tree will be one of those. Note that some geographic restriction of a polypore may be a consequence of the geographic restriction of the host tree. 

Table 1. Summary of characteristics of some common or important genera of polypores 
 

Genus	Nutritional niche			Hyphal system (-mitic)			Clamps
	White rot	Brown rot	Mycorrhizal	Mono-	Di-	Tri-
Albatrellus			X	X			Y/N
Bjerkandera	X			X			Y
Bondarzewia	X				X		N
Bridgeoporus		X			X		N
Ceriporia		X		X			Y/N
Ceriporiopsis	X			X			Y
Daedalea		X				X	Y
Daedaleopsis	X					X	Y
Ganoderma	X					X	Y
Grifola	X					X	Y
Inonotus	X			X			N
Laetiporus		X			X		N
Oligoporus		X		X			Y
Oxyporus	X				X		N
Phellinus	X				X		N
Polyporus	X				X		Y (1N)
Pycnoporellus		X		X			N
Pycnoporus	X					X	Y
Rigidoporus	X			X	X		N
Trametes	X				X		Y
Trichaptum	X				X		Y
Tyromyces	X			X	X		Y

Form of the fruiting body. Polypores can take various forms. They may be pileate, having a pileus or distinguishable cap. Some may be stipitate, having a stalk. Or they may be resupinate (effused), lying flat on the substrate. Some may be effused-reflexed, which mean they lie flat on a flat (i.e. parallel to the ground) substrate, but form shelves where the substrate surface is not parallel to the ground Some genera are consistent, with all its species having one of these forms. More often there are mixed forms within a single genus. This character is more important at the species level, although sometimes even a single species may not be consistent. 

Form of the hymenophore (spore bearing surface). Since many fungi can grow only in a narrow ecological niche, they must produce enormous numbers of spores so that by chance some of their wind-dispersed spores will land on the right substrate and survive. Not surprisingly, most polypores do actually have pores, small holes on the underside of the fruiting body that increase the surface area for bearing basidia with their spores. However some genera have enlarged pores that may be mazelike or gill-like. Some even become hydnoid, with downward pointing teeth or spines. Some genera are consistent within these groups (mostly with pores), but here are many genera that have two or three of these hymenophore configurations. This character is more important at the species level, but again, there are some species that are quite variable depending on genetics and on ecological conditions. The form may even change depending on which side of the substrate the fungus is fruiting, especially if the substrate suddenly changes to be perpendicular to the ground.