Lecanora expallens

9 May 2025

TAXON:	Lecanora expallens Ach. (1810)
RECENT SYNONYMS:	–
FAMILY:	Lecanoraceae
GROWTH FORM:	Crustose, heteromerous
SUBSTRATES:	Bark; lignum and worked wood; siliceous rock and worked stone
PHOTOBIONT:	Trebouxia alga
REPRODUCTION:	Soredia; apothecia occasional
ASCUS:	Broadly clavate; apex Lecanora-type; 8 spores
SPORES:	Ascospores if present ellipsoid, aseptate, 10-16(-17) × 4-7(-7.5) µm
NOTABLE FEATURES:	Thallus pale yellow-green, effuse or rimose-areolate, typically completely obscured by sorediate crust; soralia initially small but quickly spreading and becoming confluent; soredia yellow-green, farinose; apothecia if present lecanorine, with sorediate margins; disc yellow, pink or brown
CHEMICAL TESTS:	Thallus C+ orange, K+ yellow, K/UV(wet)+ green, K/UV(dry)+ yellow, Pd/UV(dry)+ yellow, UV+ orange (zeorin, thiophanic acid, usnic acid, ±arthothelin, ‘expallens unknown’)
HABITAT:	Dry and well-lit situations where suitable substrate available
DISTRIBUTION:	Widespread and common in Britain
CONSERVATION STATUS:	Least Concern
LICHENICOLOUS FUNGI:	Athelia arachnoidea, Diplolaeviopsis ranula, Erythricium aurantiacum, Lichenodiplis lecanorae, Paranectria oropensis
IDENTIFICATION DIFFICULTY:	Green 2: Field identification possible with care
CONFUSION SPECIES:	Botryolepraria lesdainii, Lecanora barkmaniana, L. compallens, L. confusa, L. orosthea, Lepraria finkii, Lepraria incana, Lepraria membranacea, Lepraria vouauxii, Psilolechia leprosa, Psilolechia lucida, Pyrrhospora quernea

FIELD NOTES

Lecanora expallens is one of the ‘dreaded’ green sorediate crusts (GSCs) that are widespread in Britain on bark and stone. They’re dreaded, especially by beginner field lichenologists, because they look very much alike in their sterile forms. Although some produce apothecia, they don’t normally do so, and it can be even more confusing to find them fertile.

Many GSC species, including L. expallens, are not only widespread in Britain, but also common. More often than not, you’ll have to consider alternative identifications when you encounter a GSC. Fortunately, L. expallens is one of the easiest to identify, because among all GSCs only L. expallens and Pyrrhospora quernea react C+ orange from thiophanic acid.

If you encounter a ‘dreaded’ GSC, then, test it first with C. If your specimen reacts C+ orange, you know you have L. expallens or P. quernea. Look carefully: the reaction from L. expallens can sometimes be faint, or even nearly undetectable from specimens in shaded situations. In that case, the reliable K/UV(wet)+ green test for xanthones can help, but try to observe it fairly quickly. Once the spot dries, it turns a bright yellow reminiscent of the K/UV(dry)+ yellow test for atranorin, which L. expallens lacks.

After you’ve established you have either L. expallens or P. quernea, you can then distinguish between them in multiple ways, including the following:

L. expallens has a yellow tinge from usnic acid, whereas P. quernea is brownish
L. expallens lacks the dark prothallus normally found with P. quernea
The soredia of L. expallens tend to be farinose, whereas those of P. quernea tend to be granular
The C-reaction of L. expallens tends towards a bright orange, whereas that of P. quernea tends towards a dull or even slightly reddish orange
When fertile, the apothecia of L. expallens have yellow, pink or brown discs, whereas those of P. quernea are purplish to red-brown

It’s also worth noting that fertile L. expallens can resemble L. confusa. This resemblance can be problematic, as the two species also have similar spot-test reactions, and their ascospores overlap in shape and size. But although the thallus of L. confusa is granular, it’s not truly sorediate, and its apothecia don’t have the sorediate thalline margins that characterise those of L. expallens.

L. expallens may be one of the ‘dreaded’ GSCs, then, but it’s of lesser dread. If your GSC fails to react C+ orange, or K/UV(wet)+ green, you’ll have more work to do to identify it. Field notes for other GSCs will appear on this website from time to time, but in the interim this attempt at a tabular key may be useful.

TAXON	SUB-STRATES (MAIN IN BOLD)(1)	FORM	TINT	C-TEST	K-TEST	KC-TEST	PD-TEST	UV-TESTS(2)	FERTILE?
Botryo-lepraria lesdainii	Ca, Mu	leprose	vivid or bluish	–	–	–	–	–	–
Lecanora bark-maniana	C	rimose to verrucu-lose	dull yellowish	–	K+ yellow	–	Pd± yellow	UV+ pale orange; K/UV(dry)+ yellow	rarely
Lecanora com-pallens	C, Ca, L, Si	con-tinuous	yellowish, minty	–	K+ yellow to yellow-brown (soredia)	KC+ weak yellow	–	UV± pale orange	rarely but unknown in Britain
Lecanora expallens	C, L, Si	granular to rimose	yellowish	C+ orange	K+ yellow	KC+ orange	–	UV+ orange; K/UV(wet)+ green; K/UV(dry)+ yellow; Pd/UV(dry)+ yellow	occasion-ally
Lecanora orosthea	C, L, Si	con-tinuous	yellowish	–	K+ yellow turning brown	KC+ yellow	–	UV+ dull orange	occasion-ally
Lepraria finkii	C, Ca, L, Mu, Si	leprose	bluish, greyish	–	K+ yellow	–	Pd+ orange	UV+ reddish brown; K/UV(dry)+ yellow	–
Lepraria incana	C, L, Mu, Si	leprose	bluish, greyish	–	K± red-purple (patches)	–	–	UV+ blue-white; UV± pale orange (patches)	–
Lepraria membra-nacea	C, Mu, Si	leprose	creamy, yellowish	C+ yellow	K+ yellow-orange	–	Pd+ orange-red	–	–
Lepraria vouauxii	C, Ca, L, Mu, Si	leprose	bluish, greyish	C+ dirty yellow	K+ dirty yellow-orange	–	Pd+ yellow-orange	–	–
Psilolechia leprosa	Hard, metal-rich substrates	granular to leprose	greyish	C+ red	–	–	–	UV+ whitish	occasion-ally
Psilolechia lucida	C, L ,Si	granular to leprose	sulphur-green	–	–	–	–	UV+ orange	rarely
Pyrrho-spora quernea	C, L, Si	granular	brownish	C+ orange	–	KC+ orange	Pd± weak yellow	UV+ dark orange	occasion-ally

C = bark (corticolous), Ca = calcareous hard substrate, L = lignum, Mu = moss (muscicolous), Si = siliceous hard substrate
X/UV-tests only if personally observed

LAB NOTES

Lecanora expallens can normally be identified without microscopy.

SPECIMENS

Lecanora expallens : England : VC26 West Suffolk : TM0540 : May 2025 : On Fraxinus trunk