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BIODIVERSITY REFERENCE
 
   Russula emetica (Schaeff.) Pers.   
 
'Sickener'


Kingdom: Fungi
Phylum: Basidiomycota
Subphylum: Agaricomycotina
Order: Russulales – russules, milk-caps and former 'Stereales'


British distribution: Widespread and common where suitable habitat exists, but much over-recorded, at least in the past.
World distribution: Europe, North America, elsewhere?.


Russula emetica
Russula emetica under Scots Pine (Pinus sylvestris), Auchentyre Moss, Perthshire, 1980.


Russula: an overview
Russula is a conspicuous, easily recognised and well known genus, featuring on most autumn field meeting ("foray") lists and with a number of species given in the popular guides. However it is not an easy genus and it allows a somewhat cynical classification of field mycologists:

  • relative beginners — those able to name very few species of Russula in the field;
  • experienced field mycologists — those confidently able to put names to a substantial number of Russula species in the field;
  • expert field mycologists — those able to name very few species of Russula in the field.
Russula species, in many cases, vary widely in colour, with quite possibly yellows, greens, reds and blues all possible in the one species. Gill and stem characters can also be variable or unreliable, and vital identification features include the precise colour of the spores in a thick spore print, the details of the ornamentation of the spores under the high-power of the microscope, and the nature and chemical staining properties of the hyphal terminations and dermatocystidia in the cap cuticle. These microscopic characters are well described in Rayner (1985), a valuable guide to British species though now significantly outdated. With 141 British species listed by Legon et al. (2005), many rare, but some of these common in some years, Russula is a difficult genus, requiring specialist (i.e. expensive!) literature for identification of the majority of species.

Species of the genus Russula are morphologically similar to the "true toadstools", the Agaricales, but belong to a different taxonomic order, the Russulales. Similarity can be regarded as a result of convergent evolution. They notably differ from the Agaricales in having fragile flesh that readily breaks in any direction, crumbling like Cheddar cheese. This feature is a consequence of the presence in the flesh of abundant rounded cells, sphaerocysts.

Russula is closely allied to Lactarius, the milk-caps, the two genera being reasonably distinct in Europe, but said to be less so in Africa. Recent molecular work has shown that Stereum, Peniophora and a few other corticioid (encrusting) genera are also closely related, and so current classification puts them all together in the Russulales.

Probably all species of Russula are mycorrhizal, forming mutually beneficial partnerships with roots of trees and certain herbaceous plants. Most species show a degree of host specialisation, though niche overlap between species appears substantial, and the high number of distinct species is perhaps surprising.


Russula emetica
In contrast to many species, R. emetica is relatively constant in its field characters. It is a medium to rather stocky species with a scarlet cap (sometimes with paler areas), somewhat knobbly-sulcate at the margin, white or slightly yellowing stem, pale cream gills, a cap cuticle that peels half way or more to the centre, revealing pink flesh beneath the cap, a very hot, peppery taste (once used as an emetic), spore print white, spore ornamentation (requires staining with Melzer's iodine) with large warts to 1.12 µm high (Romagnesi, 1985) or to 1.25 µm in named variants and about 1 µm across at base, joined by raised connective lines to form a ± complete mesh on the spore surface.

It is common in damp ground (often with thin Sphagnum moss carpets) under pine (Pinus) and, according to the literature, with Spruce (Picea). Reports of R. emetica with deciduous trees probably always relate to other species, some of which were formerly included in a broader concept of R. emetica.


Related species
There are many other species of Russula that have, or can have, red caps (and note that the pigment can be partly lost in heavy rain). Of those species that have a pure white spore print and a hot taste, the following can be thought of as belonging to the R. emetica group:

  • R. nobilis — cap rather hard, more a pinkish red, often with paler areas tinged yellowish, gills white with a slight greenish tint, smell faintly of coconut. Common under Beech (Fagus). Formerly known as R. mairei.
    R. fageticola is considered separate by some continental authors and is said in Courtecuisse & Duhem (1995) to be characteristic of neutral to acid soils and more common than R. nobilis, but the distinction seems scarcely tenable. Legon et al. (2005) treat it as a synonym, probably rightly so.
  • R. sylvestris — Similar to R. emetica but generally somewhat smaller and more fragile and cap not such a deep scarlet, flesh under the peeling cuticle often white, smell faintly of coconut, dimensions of spores, spore ornamentation and dermatocystidia somewhat smaller (see Romagnesi, 1985; Rayner, 1985; as emetica var. "silvestris"). Under deciduous trees, especially Sessile Oak, Quercus petraea, where it is a characteristic species of the ground flora dominated by the mosses Leucobryum glaucum and Polytrichastrum formosum, reportedly also under pine, but Rayner (loc. cit.) considered that pinewood forms were better regarded as true R. emetica. Formerly known in Britain as R. emeticella. This is a common species of the Scottish oakwoods but has seemed, at least in the past, to be less well known to mycologists in the south.
    [As a species, or as a variety of R. emetica, the epithet is often spelled as "silvestris" rather than "sylvestris", but this appears to be an invalid orthographic correction of the original spelling.]
  • R. nana — a small, neat, attractive species with a bright crimson to cherry red cap, flesh said to be white but discolouring pale greyish. In montane turf with dwarf willows (in Britain with Dwarf Willow, Salix herbacea), Mountain Avens (Dryas octopetala) and Alpine Bistort (Persicaria vivipara). Some doubt has attached to P. vivipara as a host, it being a herb, not a dwarf shrub, but in my limited experience on Scottish mountains it seems to be the commonest evident host, often well away from Salix herbacea beds (and note that P. vivipara commonly grows within S. herbacea beds), and additionally R. nana also fruits in evident association with Alpine Lady's-mantle (Alchemilla alpina).


References
•   Courtecuisse, R., & Duhem, B., (1995). Mushrooms and toadstools of Britain and Europe, HarperCollins, London.
•   Legon, N.W., Henrici, A., Roberts, P.J., Spooner, B.M., & Watling, R., (2005). Checklist of the British & Irish Basidiomycota, Royal Botanic Gardens, Kew.
•   Rayner, R.W., (1985). Keys to the British species of Russula, 3rd ed., British Mycological Society, [London].
•   Romagnesi, H., (1985). Les russules d'Europe et d'Afrique du Nord, 2nd ed., J. Cramer, Vaduz.


Useful Russula Web-link
•   Russulales News
    http://www2.muse.it/russulales-news
    Reincarnation of what was a superb information site, including an interactive key to American taxa, and a very large and good-quality photo-gallery.
 



© A.J. Silverside
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