The St. George's mushroom (Calocybe gambosa) is a popular edible mushroom, usually picked in May. Unfortunately, however, it also carries a risk as it can easily be confused with the young deadly fiber cap, which can potentially lead to fatal poisoning. The reason for this is the mushroom toxin muscarine, which is present in high concentrations in the deadly fiber cap. Muscarine is best known from the fly agaric, in which it was first discovered.