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Arthonia helvola (Nyl.) Nyl.

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Honningflekklav.
Creative Commons Attribution Share-alike | Andreas Frisch

Arthonia helvola growing in a cavity of an old Betula sp. at Bergsåne in Vang (O-L-227183).

Arthonia helvola is a rare species in the shady interior of humid old-growth forests. It is known from a single locality in Vang municipality, Innlandet. Arthonia helvola is easily identified by the dark orange to rusty red, maculate apothecia on a thin to indistinct thallus with trentepohlioid photobiont, and colorless, 2-septate spores without an enlarged apical cell.

Description

Thallus

The thin thallus is inapparent, olive-grey or brownish, and immersed in the substrate. The margin is not determinate. The photobiont is a species of the family Trentepohliaceae.

Fruitbodies

The apotheci are maculate, orange to rusty red, and without pruina. They are flat to slightly convex, immersed in the substrate to raised, irregularly rounded to elongated, and partly confluent. They are 0.2–0.8 mm in size and 50–90 μm tall.

The epithecium is indistinct.

The hymenium is orange to orange-brown and 40–60 μm tall.

The hypothecium is indistinct.

The paraphysoids are 1–1.5 μm wide and not enlarged in the tips.

The asci are clavate, with stipe, 25–42 × 8–11 µm, and 8-spored.

The spores are colorless, narrowly obovoid, at least incised at the primary septum, 8–13 × 2–4 μm in size, and divided by 2 transverse septa.

Anamorph

Pycnidia are not reported for the species.

Chemistry

The thallus does not react with C, K, KC, Pd, or UV (C–, K–, KC–, Pd–, UV–). Traces of skyrin have been detected by TLC.

The hymenium reacts I– (negative) and KI+ pale blue. A KI+ blue ring structure in the asci has not been observed.

The orange-red pigment in the apothecia changes to reddish violet in K solution.

Honningflekklav.
Creative Commons Attribution Share-alike | Andreas Frisch

Arthonia helvola growing in a cavity of an old Betula sp. at Bergsåne in Vang (O-L-227183).

Ecology

Arthonia helvola is a rare species in the shady interior of humid old-growth forests. It usually occurs on smooth to slightly fissured bark of deciduous and coniferous trees where it often grows towards the bases. In Norway, Arthonia helvola was found in a tall-herb deciduous forest in a river gorge. It grew on wood at the base of an old, hollow-stemmed downy birch (Betula pubescens).

Distribution in Norway and the Nordic countries

Arthonia helvola is in Norway currently only known from Vang municipality in Innlandet. In the Nordic countries, it is further known from southern Finland and Sweden.

Global distribution

Outside the Nordic Countries, A. helvola is widely distributed in temperate to southern boreal climates in central and eastern Europe and in North America. The species is seldom collected and appears to be extremely rare in most of its distribution area.

Similar species

Arthonia helvola is easily identified by the dark orange to rusty red, maculate apothecia on a thin to indistinct thallus. Arthonia vinosa, growing in similar habitats, differs in the darker, orange-brown to brown-black apothecia on a grey to pale fawn thallus with mostly distinct patches of an orange-yellow pigment. The spores have only 1 transverse septum and are brown and warted when old.

Arthonia incarnata has paler, orange to orange-brown apothecia without K+ reddish violet pigments, and spores with 2–3 transverse septa. This species mainly occurs at the base of old goat willow (Salix caprea) and Norway spruce (Picea abies) in humid spruce forests. It is known from Finland and Sweden but not yet reported from Norway.

Vinflekklav.
Arthonia vinosa Leighton

Literature

Klepsland JT (2020). Thirty lichens and lichenicolous fungi new to Norway. Graphis Scripta 32(7): 120–143.

Sundin R (1999). Phylogenetic and taxonomic studies within Arthonia Ach. (Ascomycetes, Arthoniales). Botaniska institutionen, Stockholms universitet, Stockholm.

Wirth V, Hauck M and Schultz M (2013). Die Flechten Deutschlands, vol. 1+2. Ulmer, Stuttgart. 1244s.