Didymium tussilaginis (Berk. & Broome) Massee

Sessile, solitary or grouped sporocarps or plasmodiocarps, groups up to 10-15, hemispherical to cushion-shaped with ± constricted base, ca. 0.5-1 mm Ø, plasmodiocarps up to ca. 3 mm long, lead-grey, mostly ± iridescent and with scattered lime crystals.

Inconspicuous, ± membraneous, as a narrow rim around individual sporocarps or continuous under a group, mostly almost colourless or pale yellowish.

Thin, membraneous, rather fragile, hyaline with scattered very pale brownish spots in TL, almost colourless by RL, on the outside with scattered, angular lime crystals up to ca 30 µm broad


Of ± parallel, thin threads, 1-2.5 µm broad, sparsely branched ± dichotomously, sometimes with expansions apparently contaning crystalline or amorphous granules, radiating from base to peridium, hyaline to pale brownish by TL.

Globose to slightly irregularly shaped, dark brown by RL, medium greyish brown by TL, occasionally slightly paler on one side, with medium dense, ± regularly distributed short spines, up to ca 0.5 µm high, 10.5-12.7-15 µm Ø, almost all spores with 1-5, large refractive inclusions (oil drops?).

Not seen.

On underside of leaves of living Petasites hybridus.

Distribution in Denmark
Although the species in recent times have only been recognized for a few years, it does seem to be fairly common in Denmark. It has been found in aproximate one third of the Petasites stands searched. At present it is known from more than 20 localities in Zealand, Fuen and Jutland.
Of the only old Danish report ( Raunkiær, C. 1888, as D. Cookei (Rfski.) Raunkiær) no collection seems to be preserved.

Global distribution
Kuhnt, Baumann and Nowotny mentions finds from Austria, Germany and France. Further it is known from The Netherlands (Hooff, pers. com.) and Denmark. It is to be expected througout the European range of Petasites hybridus.

Didymium tussilaginis (Berk. & Broome) Massee dates back to 1892, but had been forgotten until Kuhnt, Andreas; Baumann, Karlheinz & Wolfgang Nowotny brought it back from oblivion in 2014, and reported it from three European countries. Meanwhile it has been considered a synonym of Didymium squamulosum (Alb. & Schwein.) Fr., and many previous finds may be hidden under that name.

D. tussilaginis is closely related to D. vernum Kuhnt, K. Baumann & Nowotny and shares with this species the very special habitat. Macroscopically they differ by the grey sporocarps of the former versus brown sporocarps of the latter. Microscopically D. tussilaginis has clearly larger spores than D. vernum.

The description is based on four Danish collections: hfg 2419, hfg 2424, hfg 2426, and hfg 2505.


Abbreviations: TL : transmitted light; RL : reflected light; Ø : diameter; TBU : Danish Botanical Topographical District.

Colour-codes refer to: Kornerup, A. & J. H. Wanscher 1974: Farver i farver. 5th reprint. Politikens Forlag, Copenhagen.

When not otherwise indicated spore measurements are given as: min-mean-max or (extreme-)min-mean-max(-extreme). Spore measurements are exclusive of ornamentation.

Line drawings are free hand sketches.

Distribution maps: Maps of Danish distributions are based on material seen by the author. World distributions are based on reports extracted from literature.