Death Cap Mushroom (Amanita phalloides). This species is deadly poisonous! It is responsible for most of the mushroom-related deaths in California. This view clearly shows the white sac or volva at the base of the stalk that is a characteristic of deadly Amanitas. (It may not always be so obvious, however.) Henry Cowell Redwoods State Park. Felton, Santa Cruz Co., Calif.