Rusty Webcap

Cortinarius rufus is immediately recognisable as a webcap by its three defining genus-level traits:

1. 1a rusty-brown spore print
2. 2a cobwebby cortina visible in young specimens
3. 3the progressive transformation of pale young gills to cinnamon-rust as spores ripen

Cap

Within that framework, C. rufus is distinguished by its strongly rufous to rusty-red cap colouring - the epithet rufus means red or rusty in Latin. The colour is most vivid and saturated in fresh, moist young caps and fades to ochre-buff or tawny-brown as specimens age or dry.

Stem

The fibrillose-silky stem is concolorous or slightly paler than the cap, never dramatically differently coloured.

Cortina

The cortina - the single most important field feature to look for on any suspected webcap - appears as pale ochre cobweb threads spanning from the cap edge to the upper stem in young fresh specimens. On older specimens only the rust-brown spore-stained zone remains on the stem, but this ghost of the cortina is still diagnostic.

Flesh

Flesh is pallid ochre and does not bleed latex, change colour, or have a distinctive strong odour beyond a faint earthiness.

The danger of C. rufus lies in three compounding factors:

• taxonomic obscurity
• toxicological uncertainty
• a habitat shared with confirmed deadly webcaps

As the historically founding member of sect. Austroamericani, it is one of the more studied South American webcaps - yet even that relatively early description has not been followed by comprehensive toxin screening.

Patagonian Nothofagus forest is home to an incompletely described Cortinarius flora. The 2018 monographic revision of sect. Austroamericani revealed multiple new species in this clade, meaning that even an experienced regional mycologist cannot reliably identify all webcaps encountered.

Any rufous to tawny webcap found in Patagonian southern beech forest should be considered potentially deadly until proven otherwise by laboratory analysis, and that proof does not yet exist for this species.

The specific toxin content of Cortinarius rufus has not been confirmed by published analytical chemistry studies. It is classified as potentially poisonous on the basis of genus-level risk.

Cortinarius globally produces orellanine-class bipyridine N-oxide nephrotoxins (confirmed in C. orellanus, C. rubellus, and multiple Australasian species) and other uncharacterised toxic compounds in some sections. Sect. Austroamericani, the South American clade to which C. rufus belongs, has not been systematically screened.

However, the structural relatedness of Southern Hemisphere Nothofagus-associated Cortinarius species to confirmed toxic lineages, and the documented nephrotoxic incidents associated with Patagonian Cortinarius in general, justify a precautionary poisonous classification.

GI irritant compounds may also be present independently of any orellanine-type toxin.

If orellanine-class toxins are present - as the precautionary classification assumes is possible - symptoms would follow the distinctive orellanine pattern: a long and medically deceptive latent period of two days to three or more weeks before onset, followed by:

• intense thirst
• altered urination (increased or paradoxically reduced)
• nausea
• vomiting
• fatigue
• headache
• flank pain progressing to acute tubular necrosis and kidney failure

Even if C. rufus proves to contain only GI irritants rather than orellanine, symptoms from GI-irritant Cortinarius typically include nausea, vomiting, abdominal pain, and diarrhoea within one to four hours.

The uncertainty over which toxin profile applies makes any Cortinarius ingestion a medical emergency requiring evaluation regardless of the initial symptom picture.

Treat Cortinarius rufus as potentially toxic and do not eat it. The species has not been fully characterised toxicologically, and it belongs to a genus notorious for containing heat-stable nephrotoxins that are not destroyed by any preparation method.

Its membership in Cortinarius sect. Austroamericani, a clade embedded within a global genus known for orellanine-type kidney toxins, is sufficient grounds for the precautionary classification as poisonous.

If this or any other Cortinarius has been consumed - even days or weeks ago - seek emergency medical advice from a poisons centre without waiting for symptoms. Orellanine poisoning, if present, produces no early warning signs for two to three weeks, by which time significant kidney damage may be irreversible.

• In Chile contact CITUC (Centro de Información Toxicológica UC, +56 2 2635 3800).
• In Argentina contact CIAT (Centro de Información, Asesoramiento y Asistencia Toxicológica, Buenos Aires) or local emergency services.

Bring a photograph or the actual specimen to help clinicians.

This species is a strict ectomycorrhizal partner of South American southern beech (Nothofagus) trees, the dominant tree genus in cool-temperate Patagonian forests and montane Andean woodlands.

The Nothofagus ecosystem supports a largely endemic mycorrhizal flora that evolved in long isolation from the Northern Hemisphere. C. rufus is one of the founding members of the Austroamericani section - a lineage of webcaps that arose with, and remains bound to, these southern beeches.

It will not be found outside intact Nothofagus forest. None of the following will produce this species:

• exotic plantations
• gardens
• deciduous-tree parks
• grassland

Knowing that any rufous-capped Cortinarius in Patagonian beech forest belongs to this or a closely related section helps narrow the identification - but critically, it does not make the species safer, since the entire section remains toxicologically under-characterised.

Fruiting follows the Southern Hemisphere seasonal rhythm. The species emerges in late summer and peaks through autumn - broadly February to May, with the main flush in March and April. Falling temperatures and increasing autumn rainfall create the conditions Nothofagus-associated ectomycorrhizal fungi require for reproductive investment.

In Patagonia, the transition from summer to autumn is often abrupt; fruiting is linked to the first sustained period of cool, wet weather after the austral summer dry season.

Elevation plays a role. At higher Andean altitudes, the season can start earlier (late January) and finish earlier; in lower coastal Patagonian forest it may extend into May.

Like all Cortinarius species, C. rufus is obligately ectomycorrhizal and cannot reproduce without a living Nothofagus host.

The visible mushroom is simply the reproductive structure of a perennial mycelial network threaded through the soil around host tree roots, exchanging mineral nutrients for photosynthate in a relationship the tree depends on as much as the fungus.

Fruiting bodies emerge singly or in loose scattered groups along root corridors, occasionally two or three together; dense clusters are not characteristic.

The same sites may produce fruiting bodies in successive years under suitable conditions, because the underground partnership is long-lived. Fruiting bodies are ephemeral - typically lasting five to ten days before collapsing - but the organism itself persists for decades.

Southern Hemisphere autumn species fruiting February-May. Patagonian forests are persistently moist; fruiting is primarily governed by cooling soil temperatures falling below ~14°C after the austral summer, combined with sustained autumn rainfall. Flush lag is 7-21 days after triggering conditions are met, shorter at warmer lower-elevation sites, longer at high altitude or in cool, wet years.

The most significant dangerous confusions in Patagonian forest are with deadly orellanine-bearing webcaps in the same habitat - particularly Cortinarius magellanicus, C. patagoniensis, and the broader Cortinarius sect. Austroamericani complex, all of which share rusty-brown to tawny colouring, rust-brown spore prints, and Nothofagus ectomycorrhizal habitat.

Species-level separation within this genus requires microscopy and molecular tools; field identification alone is insufficient to distinguish safe from toxic webcaps.

Among superficially similar non-webcaps:

• Cantharellus (chanterelles) are the most commonly confused: golden-to-orange chanterelles grow in the same Nothofagus forest and are similarly coloured, but are distinguished absolutely by their forking, blunt, ridge-like false gills (decurrent onto the stem), absence of any cortina or rust-brown spore print, and white spore print.
• Ruddy boletes (Xerocomellus or Suillus species) might share the rufous cap colour but have tubular pores rather than gills.
• Gymnopilus species share a rusty spore print and cortina but tend toward more brilliant golden-orange tones and grow on wood.
• Dermocybe and some Phlegmacium webcaps in the same forest also have rusty spore prints and must be treated as equally suspect.