| TAXON: | Phlyctis argena (Spreng.) Flot. (1850) |
| RECENT SYNONYMS: | – |
| FAMILY: | Phlyctidaceae |
| GROWTH FORM: | Crustose, heteromerous |
| SUBSTRATES: | Bark of deciduous trees; rarely on bark of conifers, basic rock, or overgrowing moss |
| PHOTOBIONT: | Symbiochloris alga |
| REPRODUCTION: | Soredia; apothecia, pycnidia rare |
| ASCUS: | If present, broadly clavate; apex with I+ weakly blue tholus and K/I+ blue thin outer wall; 1 spore |
| SPORES: | Ascospores if present elongate to subglobose, densely muriform, (75-)90-130-(140) × (25-)30–50 µm; conidia if present narrowly ellipsoidal, sometimes curved |
| NOTABLE FEATURES: | Thallus chalky-white to greenish-white, often forming more-or-less circular patches; prothallus white, conspicuous, often cottony; soralia white, irregular, vaguely reminiscent of blisters, sometimes becoming confluent |
| CHEMICAL TESTS: | Thallus K+ yellow turning red; KC+ red; Pd+ yellow-orange (norstictic acid) |
| HABITAT: | Well-lit woodlands and waysides |
| DISTRIBUTION: | Widespread and locally common in Britain |
| CONSERVATION STATUS: | Least Concern |
| LICHENICOLOUS FUNGI: | Lichenostigma alpinum, Phacographa zwackhii, Spirographa lichenicola, Taeniolella sp. |
| IDENTIFICATION DIFFICULTY: | Green 2: Field identification possible with care |
| CONFUSION SPECIES: | Haematomma ochroleucum var. porphyrium, Pertusaria lactescens, Phlyctis agelaea |
FIELD NOTES
Phlyctis argena is hard to miss. It forms striking chalky-white patches on trees, which are usually large or abundant enough to be obvious from a distance. It’s one of only two Phlyctis species in Britain, but it’s common, widespread and pollution-tolerant, and you’re fairly sure to find it on a tree near you.
If you do find Phlyctis, you should take a moment to confirm its identity. Both of the two British species, P. argena and P. agelaea, can look very similar, especially at first glance. They also both test K+ yellow turning blood-red and Pd+ yellow-orange from norstictic acid.
P. agelaea produces apothecia rather than soralia, but its dark-grey discs are hidden by a copious white pruina that can make them resemble soralia. And while P. argena only very rarely produces apothecia, its young soralia can be mistaken for apothecia because they often take an annular form that appears like a thalline margin. See the specimen galleries below for examples.
Once you look closely, however, it will become clear that you have one or the other species. P. agelaea is by far the rarer and more geographically limited of the two, so it’s likely that your lichen will be P. argena. But it’s always worth checking, particularly if you’re lichenising in southwest England, southwest Wales, or other areas where P. agelaea could potentially be encountered.
On rare occasions, P. argena can be found on basic rocks. In that case, it can be confused with similar-looking Haematomma ochroleucum var. porphyrium or Pertusaria lactescens. But the former doesn’t react K+ yellow turning red, and the latter is isidiate rather than sorediate. And while isidia can resemble soredia if they’re well-rubbed or eroded, close examination should enable you to tell them apart.
LAB NOTES
Phlyctis argena can normally be identified without microscopy.
SPECIMENS
Phlyctis argena : England : VC28 West Norfolk : TF8842 : October 2024 : On Fagus trunk
Phlyctis argena : England : VC28 West Norfolk : TF9428 : October 2024 : On Crategus trunk
Phlyctis argena : England : VC26 West Suffolk : TL8184 : April 2024 : On Fagus trunk
Phlyctis argena : England : VC27 East Norfolk : TM0780 : February 2023 : On Acer trunk
