Author: Stefan Blaser
During fieldwork in October 2025, Stefan Blaser found Septobasidium cf orbiculare on hazel and Septobasidium quercinum on oak near Fully in Valais. These recent findings prompted us to take a closer look at this genus.
The little-known genus Septobasidium contains species that colonize the bark of living trees. The tough, usually thin and inconspicuous fruiting bodies resemble lichens that grow on the bark. However, the color is typically brown and never greenish, even when moist, as is usually the case with lichens. The fruiting bodies are often sterile (without sporulation). The best time to find fertile fruiting bodies and thus spores and basidia is probably spring. However, regardless of the time of year, you can also moisten the sterile fruiting bodies and store them in a tin for a few days. With a bit of luck, basidia and spores will develop. The basidia are transversely septate (hence the genus name), the spores are usually long cylindrical, often curved and also septate. Septobasidium belongs systematically to the class of Pucciniomycetes, i.e. to the family of rust fungi, which are widely represented worldwide as plant parasites.
The fungi have a very special ecology. They feed on scale insects that suck plant sap. The fungus penetrates the insects with "sucking hyphae" (haustoria). It is therefore a type of parasitism. However, not all insects in a colony are attacked, and the others are well protected from predators and the weather under the fruiting body of the fungus. All in all, it is therefore likely to be a beneficial relationship for both organisms - a symbiosis. The aphids can occur without the fungus, but the fungus is strictly tied to the presence of the insects. Often several Septobasidium species can be found on the same stems in locations with an abundance of suitable scale insects(see here).
There is surprisingly little information on the genus in the literature. In particular, more recent scientific treatments of the genus seem to be lacking. However, numerous new species have been described, particularly from China. The most detailed information on the species can probably still be found in the old standard work by Couch 1938 or perhaps also in the somewhat more recent work by Azéma 1975. Neither book was available when this article was written. A lot of information and also portraits of some species can be found on the homepage of Elia Martini, an Aphyllophorales specialist from Ticino.
In the following, the five (or six?) species recorded in Switzerland so far will be briefly introduced and their known distribution discussed:
Septobasidium mariani has so far only been found in Ticino on living stems of hazel (Corylus avellana), where the species is probably not rare. According to GBIF, the species occurs in the south-east of the USA and in Europe, mainly in the Mediterranean region. The cross-section of the fruiting body can be divided into three distinct zones. Below the closed, tough upper layer is an intermediate layer consisting of loosely standing columns and below this is a denser, complex layer, which also forms the chambers in which the scale insects reside. The fruiting bodies of other species also have a similar three-layer structure.
Septobasidium orbiculare has so far been found on hazel( Corylus avellana) in Ticino and (if the identification proves to be correct) in the Follatères in Valais. According to GBIF, the species occurs mainly in Europe in the Mediterranean region.
has so far only been found at one location on hazel (Corylus avellana) in Ticino. According to GBIF, there are only two georeferenced findings in Switzerland and Austria.
Septobasidium carestianum was found scattered in northern Switzerland on the bark of young ash trees (Fraxinus excelsior) (Ticino findings: see note under S. alni); The sites of Beatrice Senn (pers. comm. Senn (pers. comm.) were humid (floodplain forest), but sunny (forest edge); the latest findings by B. Senn (2011) near Reutigen (Kander floodplain forest) and Uttigen (Aare floodplain forest) could not be confirmed by her since then (pers. comm.). According to GBIF, the species is distributed worldwide and occurs scattered in Europe in the Alps, Norway and Estonia.
Septobasidium quercinum grows together with mosses and lichens in bark cracks of oaks, both on young and older trees. In Fully, the species was found together with an abundant occurrence of the oak flour disc (Aleurocystidiellum disciforme). Further finds are known from the Lake Geneva region and from Ticino. The species is also conceivable in other dry and warm locations with oaks in Switzerland (e.g. at the southern foot of the Jura). According to GBIF, the species has been recorded on the Atlantic coast of Spain, in the Mediterranean region, in France and Switzerland. A portrait of the species can be found in Wilhelm 2011.
Septobasidium alni: Beatrice Senn has informed me that Elia considers her find of S. carestianum from Ticino to be S. alni. This means that S. alni is also represented in Switzerland. According to GBIF, the species is distributed worldwide and occurs in Europe in Italy, Portugal and Germany.
Septobasidium galzinii: The type of this species was found on broom heather (Calluna). The only reported find of this species in Switzerland was on oak. This find is considered doubtful.
For those who are now eager to go out and find a Septobasidium species, it should be noted that the species are easier to observe from winter to spring (without disturbing foliage). In the findings in Valais, it has been noticed that Septobasidium cf orbiculare has a more contrasting bark color when wet (in the rain) than when dry. With the exception of S. carestianum, suitable sites are also most likely to be found in warm locations (preferably in southern / south-western Switzerland).
Literature:
Azéma R.C. (1975). "Le genre Septobasidium.” Documents Mycologiques, 6(21) : 1-24.
Couch J.N. (1938). “The genus Septobasidium.” Chapel Hill, 480 p.
Piepenbring, M. (2022). “Schildläuse züchtende Rostpilzverwandte (Septobasidiales).” In: Mykologie. Springer Spektrum, Berlin, Heidelberg, 163-165.
Wilhelm M. (2011). “Zwei interessante Arten mit querseptierten Basidien: Phleogena faginea und Septobasidium quercinum.” Schweizerische Zeitschrift für Pilzkunde, 89(3): 108-109.