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Images of British Lichenicolous Fungi

Athelia arachnoidea (Berk.) Jülich

Basidimycota, Atheliales: as understood here, an aggressive pathogen of lichens and free-living, chlorococcoid algae, forming white to cream, ring-like growths, the fruiting body thin and arachnoid (like cob-webs), often barren but frequently producing small, brownish sclerotia and, less often, mostly two-spored basidia from its upper surface, basidia without basal clamps. Widespread, common especially in urban areas throughout Britain, often seen as whitish rings on tree trunks and damp stonework.

Refs: Diederich in Nash et al. (2004) 633; Jülich & Stalpers (1980), 47; Hawksworth (1983), 34; Jülich (1984), 136; Wirth (1995), 1: 470 (photo); Vesterholt in Hansen & Knudsen (1997), 145 (may be broader species concept); Eriksson & Ryvarden (1973), 102 (line drawing), 103 (may be broader species concept).

This fungus is sometimes described as a parasite of Lecanora conizaeoides, which it indeed can be, but it has remained common in urban areas despite the virtual disappearance of L. conizaeoides following reduction of sulphur dioxide pollution. Probably Physcia tenella is the most common lichen host, by virtue of it having become the most likely lichen to be clothing the trunks of urban trees.

This is the only Athelia species accepted as lichenicolous by Hawksworth (1983, 2003), though in the latter publication he also mentions A. epiphylla as a bark saprobe that sometimes spreads onto adjacent lichens. Legon et al. (2005) list A. arachnoidea more as a general pathogen and saprotroph on decaying wood and leaves, giving little emphasis to this fungus on algae and none to its conspicuous occurrence as an urban species, though they state, "known to parasitise various species of lichen." They also list A. epiphylla and state, "Also reported as a parasite on lichens such as Lecanora and Xanthoria spp." Eriksson & Ryvarden (op. cit.) describe A. arachnoidea as being, "Predominantly on deciduous wood but also as a parasite on lichens." They also describe A. epiphylla as occuring on dead lichens as well as on other substrates. Breitenbach and Kränzlin (1986) describe this species as being on branches of hardwoods and while their description and line drawings indicate 2-spored basidia, their photograph is of a more solid, non-arachnoid fruitbody that could easily be taken to be A. epiphylla and is on wood. On the other hand, descriptions in Hawksworth (1983) and Diedrich (op. cit.) unequivocably refer to an aggressively lichenicolous fungus. The species of the A. epiphylla complex have 4-spored basidia and A. arachnoidea is the only 2-spored Athelia species accepted as British.
My own field experience has not yet led me to make a connection between the aggressive pathogen and Athelia as seen on dead wood. Evidently 2-spored Athelia occurs on wood, but whether it is actually the same species seems open to question. This same point was discussed by Diedrich and he stated, "Non-lichenicolous Athelia-specimens growing directly over wood are often attributed to the same species, but molecular studies proving their conspecificity are still missing." At the time of writing, I believe this still to be the case. This would also explain reports of the asexual stage of "A. arachnoidea" having been isolated from stored carrots!
It should be noted that past records of A. arachnoidea are known to have been made as "A. epiphylla" and it will be past such records and misidentifications that most likely lead to repeated mentions of A. epiphylla being authentically lichenicolous. It is likely that not only A. epiphylla, but also other Athelia species, will occur on moribund lichens.


Athelia arachnoidea on Physcia tenella
Athelia arachnoidea on Physcia tenella
On Physcia tenella on Norway Maple (Acer platanoides) in town centre, Paisley, Renfrewshire, May 2009.
Athelia arachnoidea on Physcia tenella
Athelia arachnoidea on Physcia tenella
Athelia arachnoidea on Physcia tenella
Athelia arachnoidea on Physcia tenella, showing sclerotia
On Physcia tenella on Sycamore (Acer pseudoplatanus), Edinburgh, September 2011. Lowest photograph shows developing sclerotia.

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Uploaded March 2013, last modified or updated April 2014