Seeing a bunch of mushrooms sprout on your lawn might make you think you've hit the free-mushroom-meal jackpot, especially if the mushrooms are white. A common myth says all white mushrooms are edible, but unfortunately, this is not the case -- some white mushrooms are highly toxic. You must be able to identify a mushroom as an edible, nontoxic variety without a doubt, even if it is white.
This mushroom was formerly known as Lepiota, and it is poisonous. Its common names include parasol mushroom and fairy ring mushroom. The fairy ring name derives from the ring-like patterns that appear on lawns; in folklore, the rings mark areas where fairies have been dancing. The mushroom has a large cap that is white to light tan, with reddish scales. The spores in the gills leave a greenish-gray imprint. Wash your hands immediately if you have handled one of these.
Many of the mushrooms in the Amanita genus have well-deserved names such as death angel and death cap. At least one variety is actually edible, but the risk from the other species means you're better off admiring Amanita mushrooms from a distance. Species such as Amanita bisporigera and Amanita virosa have a bright white cap and a fuzzy or warty stem. Amanita phalloides, or the death cap, has a light yellowish-tan cap, but the stem is surrounded by a white sheath. David Fischer notes on the website AmericanMushrooms.com that the death cap Amanita is common in, but not native to, the San Francisco Bay Area. It looks remarkably like an edible Asian mushroom called the paddy-straw, and Fischer says this resemblance has led to several cases of fatal poisoning among immigrants from Southeast Asia.
The giant puffball, Calvatia gigantia, is a spherical, puffy white mushroom that insects like to infest. It is edible at a certain stage but toxic at other points. Student David Landers writes on the Cornell Mushroom Blog that the "button" stage of a gilled mushroom can sometimes make the mushroom look like a puffball, so you have to cut the puffball in half to look for gills before preparing it.
Also known as the meadow mushroom, Agaricus campestris is a common lawn mushroom. It has a white cap with a pink to brown gills. Michael W. Beug notes on the Evergreen State College website that the mushroom can have an almond-like odor that should be pleasant. If it isn't, he warns, the mushroom could cause stomach problems.
Never go by a vague description or myth -- always have a mushroom positively identified by an agricultural expert. If you have eaten a mushroom that you did not identify or that you think might be poisonous, contact a doctor immediately. Save the rest of the mushroom to show to the doctor. Larry Grand at North Carolina State University says that if you face a delay of more than 20 minutes, "induce vomiting."