Red ring rot is a hugely important stem-decay disease, certainly the most important one on conifers in the northern hemisphere. In preparing the page on red ring rot, I found that investigating the current knowledge and ideas of the pathogens involved was complex. Here are the details, including my conclusions (opinions) based on the literature. But certainly more work is needed and the conclusions could change.
The most thorough analysis relevant to North America is by Brazee & Lindner , and I rely on that heavily. Using 41 isolates and four loci, they compared neotype isolates of Porodaedalea pini and P. chrysoloma with isolates from Europe and North America. Most (28) of the non-type isolates fell into a fairly uniform group they called the “Holarctic group”.
- The pathogens causing red ring rot belong in the genus Porodaedalea.
- The two species that have long been recognized in North America (NA), P. pini and P. chrysoloma, apparently do not occur in NA.
- There are two undescribed phylogenetic species in southeastern and southwestern NA.
- Of the two most recently described species in NA, P. cancriformans is probably good but needs more work; P. gilbertsonii is doubtful.
- A large group of collections in NA and northern Europe is likely one species. In Eurasia, it has been lumped with P. chrysoloma but more recently called P. laricis. It appears to be conspecific with P. piceina, which has priority as a name.
- Porodaedalea piceina is the most common and widespread species in NA and also occurs in northern Europe (often called P. laricis there).
The old species
Porodaedalea pini (Brot. 1804) Murrill 1905
The genus Porodaedalea was erected by Murrill in 1905 for P. pini, the traditionally recognized pathogen of red ring rot. It is really a complex of species. Taxonomy of the group is difficult because of confusing, variable morphology, and it is still unsettled. This is surprising, considering it is the most important stem-decay pathogen in the world. Murrill’s genus was widely ignored until additional species were recognized. Now there are about 15 species and additional undescribed phylogenetic species .
Since a new type specimen of P. pini from its original location (Portugal) was collected and isolated, it is possible to compare American isolates to P. pini sensu stricto (in the strict sense) in molecular studies. Although it is the name traditionally used in North America, now it is well accepted that P. pini s.s. does not occur in North America (NA) [3–6, 14], or if it does, it hides itself pretty well.
Porodaedalea chrysoloma (Fr. 1861) Fiasson & Niemelä 1984
This species was described by Fries as Polyporus chrysoloma in 1861. Molecular studies with European specimens have shown it to be a good species, reasonably distinct from P. pini [3, 15]. However, a lot of what was identified as P. chrysoloma in Europe was not, mostly it was what Tomšovský et al. called P. laricis . In America, the entity that has been called P. chrysoloma (or Fomes pini var. abietis; see next) is particularly common on Picea but often kills sapwood and causes cankers on Abies spp . But like P. pini, P. chrysoloma s.s. has never been found in NA[3–6, 14].
∴ Just to recap, the two European species that were almost universally used for the complex in NA do not occur there. If you don’t like “name changes”, get ready for trouble!
Porodaedalea abietis (P. Karst. 1882) Bernicchia & Gorjón 2020
Overholts  treated a form in NA as Fomes pini var. abietis P. Karst. 1882 (although he gave himself credit for the combination). For reasons that are unclear to me, Karsten apparently described it also as a species in the same year, F. abietis P. Karst. 1882. They are now considered synonyms . Relative to P. pini, the conks are thin, grow for only a few years, and tend to have a more tomentose pileus .
Gilbertson & Ryvarden  considered this to be a synonym of P. chrysoloma. This synonymy is generally followed in Europe as well [1, 13], but apparently not by all, as the species was recently transferred to Porodaedalea .
∴ If a synonym of P. chrysoloma, that species does not occur in NA. If not, there is no evidence P. abietis occurs in NA.
Porodaedalea piceina (Peck 1889) Niemelä 1985
This was described by Peck based on a specimen from Picea sp. in New York. Today, it too is usually considered a synonym of P. chrysoloma , if it is considered at all. However, now that it is clear that P. chrysoloma doesn’t occur in NA, and since P. piceina was described in NA, it can’t very well be considered a synonym anymore.
Working in Quebec, Niemelä considered it a unique American species . “The closest relative is P. chrysoloma (Fr.) Fiass. & Niemelä, which is its North European counterpart in many respects.” He said P. piceina is smaller, has smaller pores that are less labyrinthine, the hymenial setae are shorter and more obtuse, and the embedded setae are more common and darker. He collected it from both Picea spp. and Larix laricina. His description is probably better than the original:
Niemelä’s description of P. piceina in North America
Perennial; pileate or effused-reflexed, imbricate, caps semi-circulate or elongated along fallen trunk, hard, 1.5-5 cm wide, projecting 1-3.5 cm. Surface brown, zonate, some zones with short stiff hairs; margin acute, golden brown. Pore surface concave, cinnamon or golden brown; pores round or angular, regular, (2-)3-5(-6) per mm. Dimitic; generative hyphae simple-septate and hyaline; skeletal hyphae brown; embedded setae lanceolate, 35-65 × 4.5-10 µm, scattered in dissepiment trama; hymenial setae stout, with strong and blunt apex, (29-)33-41(-50) x (5.5-)8-10 µm, common; spores ellipsoid, with slightly thickened walls, (4.5-)5-5.5(-6) x (3.6-)4-4.7 µm, IKI-, CB+.
∴ Porodaedalea piceina must be considered separate from P. pini and P. chrysoloma, and present in NA.
Porodaedalea laricis (Jacz. ex Pilát 1933) Niemelä 2005
This was described by Pilát from Larix sibirica in Siberia. Although it was largely ignored for a long time, more recently it was widely accepted in Eurasia. It has been found on Larix, Picea and Pinus spp. . Doing a phylogenetic analysis with ITS sequences, this species was clearly isolated from P. pini and P. chrysoloma in Europe . However, using another gene (tefa) broke it into two unrelated clades.
Using two isolates from Fennoscandia purported to be P. laricis based on morphology in their multilocus analysis, Brazee & Lindner could not distinguish it from the large Holarctic group . They then added the ITS sequences of P. laricis generated by Tomšovský et al.  and analyzed with their ITS sequences, and those too fell into the Holarctic abyss.
Similarly, Szewczyk et al. found that sequences of the North American Holarctic group were in a clade with those of P. laricis from Europe . On that basis, they suggested that P. laricis occurs in NA. I believe they are correct, but used the wrong name for it (see conclusions below).
So it seems this species is present in NA, but we need to establish the correct name. ∴ Porodaedalea laricis = ??
Porodaedalea cancriformans (M.J. Larsen, Lombard & Aho 1979) T. Wagner & M. Fisch. (2002)
This species was described from Abies spp. in northern California and southwestern Oregon, first as P. pini var. cancriformans in 1979 , then raised to species rank . It was said to differ from “P. pini” by causing cankers (the specific epithet means “canker-forming”), forming gregarious and imbricate conchate basidiocarps, and white to pale cream cultures having slow growth. Conversely, basidiocarps of the typical form of P. pini are usually ungulate and solitary, not associated with cankers, and with yellow- to tan-colored cultures having faster growth rates . However, quantitative morphological characters overlap with other species  and morphology in general is highly variable in the species of this genus .
In their 2-volume treatise, Gilbertson & Ryvarden did not mention P. cancriformans, even as a synonym . Presumably they considered it to be P. chrysoloma, which has similar features. They did make the point that P. chrysoloma commonly causes cankers on Abies spp.
Based on a multilocus phylogenetic study, bootstrap support was weak for separating P. cancriformans from the majority of isolates from around the northern hemisphere (the Holarctic group) . However, estimates of gene flow between the two groups were very low. Brazee & Lindner preferred to maintain the species because it is restricted to Abies spp., as Larsen claimed . However, aside from the lack of evidence of host specificity, using host as a criterion for defining species is problematic. It is circular: the species is defined as being only on this host; you can identify it by being only on this host. If it occurs on another host, by definition it can not be P. cancriformans.
Primarily considering only ITS sequences, some studies have supported the species. Three novel isolates formed a tight cluster, fairly distinct from American isolates then identified as “P. pini” and “P. chrysoloma” . In that study, DNA fingerprinting of more isolates also supported the distinction of the species. A later study using only ITS segregated the species (using Brazee & Lindner’s sequences of two isolates) ). In a later 2-locus study using the same sequences, the abstract seems to suggest all the species were supported, but there is no mention of it in the two paragraphs of results and there is no discussion . ∴ Porodaedalea cancriformans may be a distinct species, but more study is needed.
Porodaedalea gilbertsonii (M.J. Larsen 2000) V. Papp 2018
This was described from Pseudotsuga menziesii in northern California. Larsen intended it as a name to replace the invalid Daedalea vorax .
Larsen said one of the chief characters for recognizing it is its restriction to Pseudotsuga menziesii var. menziesii. Additional distinguishing characters are absence of tramal and contextual setae, urniform basidia, perennial basidiomata with a sulcate pilear surface with radially oriented grooves, and pores that become coarsely daedaloid.
Regarding the characters that distinguish it, host specificity is very weak. No evidence was provided to show it being host-specific. Moreoever, using host as a primary character for defining species is problematic (see P. cancriformans above). Regarding tramal setae, that is a variable character that is unreliable taxonomically in Porodaedalea . We’re left with nothing more than some macroscopic characters that are notoriously variable.
Finally, the multilocus phylogenetic analysis by Brazee & Lindner including two isolates of P. gilbertsonii failed to separate them from the “Holarctic group” . Similarly, a two-locus study including one isolate of P. gilbertsonii was ambiguous . Although I love the idea of honoring Dr. Gilbertson this way, ∴
What species do we have?
It is important to remember that Porodaedalea pini and P. chrysoloma, the only species recognized in North America until recently, were described from Europe. Now:
- Not to put too fine a point on it, but neither P. pini nor P. chrysoloma appears to occur in North America, based on multilocus phylogenetic analysis, intersterility studies, and more [3–6, 14]. This is perhaps the only thing all studies agree on. American species that we have used those names for are not the same as the European species.
- Porodaedalea gilbertsonii is indistinct from the common, widespread Porodaedalea sp. in NA.
- In the southwestern and southeastern United States, there are two undescribed phylogenetic taxa .
- P. cancriformans may be a good NA species, but the evidence for that is mixed. More study is needed.
- The majority of isolates from NA and northern Europe (called by Brazee & Lindner the Holarctic group) appear to be a single species, but the name of it has not been carefully considered. Here is some consideration:
The widespread and common Holarctic group has a wide host range, is widespread geographically, and is morphologically variable . Candidate names for this group include Porodaedalea gilbertsonii, P. laricis, and P. piceina. Although in Europe it been referred to as P. laricis, this is no longer tenable if the group includes P. piceina from North America. Of these names, P. piceina clearly has priority, having been described in 1889. (Daedalea vorax Harkness 1879 would have priority, but it is an invalid name because there was no substantive description.)
Although it is not settled that this is a single species, I will use the name Porodaedalea piceina for lack of a better one. The fungus causing cankers on Abies spp. in northern California and southern Oregon may be distinguished as P. cancriformans.
After spending the better part of a week researching this, I found this very authoritative paper that reached the same conclusion . Bingo!
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