Yellow-Staining Mushroom

Toxic

Scientific name	Agaricus xanthodermus
Family	Agaricaceae
Portuguese name	Cogumelo-amarelecente
English name	Yellow-staining mushroom
Season	September, October, November
Habitat	Meadow, Pasture, Mixed forest
Look-alikes	Field Mushroom, Parasol Mushroom

Description

Agaricus xanthodermus (yellow-staining mushroom) is a poisonous species of Agaricus, responsible for the greatest number of mushroom poisonings in Western Europe. It resembles edible Agaricus species but is distinguished by the bright chrome-yellow staining of its flesh when damaged and its unpleasant phenolic (inky) smell.

Described by Joseph-Henri Léveillé in 1855. The species name xanthodermus comes from Greek ξανθός (yellow) + δέρμα (skin).

Cap

• Diameter: 5–15 cm
• Shape: hemispherical, then convex, often with an angular profile (trapezoid in cross-section)
• Colour: white, sometimes with a greyish-brown tinge in the centre
• Key feature: when damaged or pressed, the flesh turns bright chrome-yellow

Gills

• Young: pink
• Mature: dark brown to chocolate
• Attachment: free

Stem

• Height: 5–15 cm
• Width: 1–2.5 cm
• Colour: white
• Ring: double, persistent
• Key feature: when cut, the flesh at the stem base instantly turns chrome-yellow

Flesh

• Colour: white; when damaged — bright chrome-yellow (especially at the stem base and cap surface)
• Smell: unpleasant, phenolic (inky, “carbolic”, like printer’s ink) — intensifies when cooked
• Taste: unpleasant

Spore print

Dark brown.

Where and when

Season in Portugal

• Main season: September–November
• One of the most common Agaricus species in Portugal

Habitats in Portugal

• Meadows and pastures — alongside edible field mushrooms
• Parks, gardens, lawns — an urban saprotroph
• Forest edges — on organic-rich soils
• Mixed forests — less typical but encountered
• Saprotroph — does not form mycorrhiza

Look-alikes

Comparison with edible species

Testing rules

1. Press your fingernail into the cap — the yellow-staining mushroom instantly develops a chrome-yellow spot
2. Cut the stem base — the flesh turns bright yellow
3. Smell the cut — the smell is phenolic, unpleasant
4. If any test is positive — do not collect the mushroom

Poisoning

Symptoms

• Nausea, vomiting, diarrhoea
• Abdominal cramps
• Sweating
• Symptoms appear within 1–3 hours of consumption
• Duration: 24–48 hours

First aid

1. Call 112 or CIAV (808 250 143)
2. Do not induce vomiting
3. Save remaining mushrooms for identification
4. Drink water in small sips

Prognosis

Poisoning is not fatal but extremely unpleasant. Recovery is usually complete.

See Also

Safety:

• Safety Rules
• Common Look-alikes
• First Aid for Mushroom Poisoning

Related articles:

• Identification Basics

Sources

1. Léveillé J.H. — description of Agaricus xanthodermus (1855)
2. Beug M.W. et al. — Mushroom Poisoning in the Pacific Northwest (2006)
3. BioDiversity4All / GBIF Portugal — occurrence records
4. CIAV (Centro de Informação Antivenenos) — mushroom poisoning statistics
5. First-Nature — Agaricus xanthodermus, Yellow-staining Mushroom

Disclaimer: Identifying mushrooms from descriptions and photographs on the internet is not a substitute for consulting an experienced mycologist. The authors assume no responsibility for the consequences of collecting and consuming mushrooms. If you suspect mushroom poisoning, call 112 immediately.