As the rainy season begins, the Bay Area's most dangerous mushrooms are about to come up. Health officials are hoping not to see a repeat of last winter, when California recorded its largest outbreak of mushroom poisonings on record.

A record outbreak

In a May report in the CDC's Morbidity and Mortality Weekly Report, physicians from UCSF and the California Poison Control System described 39 suspected cases of amatoxin poisoning in Northern California between Nov. 18, 2025, and March 17, 2026. Four people died and three needed liver transplants. The poison control system normally sees fewer than five such cases a year.

The authors wrote that it was the largest outbreak of mushroom-related liver poisoning ever reported in California and the largest in the United States in decades. Above-average rain in late November 2025 set off what regional mycologists called "superblooms" of Amanita mushrooms in Bay Area parks and wildlands.

Many patients had foraged mushrooms that looked like edible ones they knew from other countries. Together, patients spoke at least six languages other than English, including Spanish, Mixteco, Mam, Ukrainian, Russian and Mandarin. One family of four, including children ages 6 and 8, was poisoned by Amanita ocreata picked in a regional park. All four recovered.

Know the two killers

The East Bay Regional Park District says the Bay Area is home to two of the world's most toxic mushrooms. The death cap (Amanita phalloides) usually has a greenish-gray cap, white gills, a white ring on the stem and a large white sac at the base. It fruits early in the fall, often right after the first rains, mainly near oaks. It was accidentally brought to North America on the roots of European cork oaks.

The western destroying angel (Amanita ocreata) is a native species with a creamy white cap. It fruits from late winter into spring and grows only with oaks. The CDC report says the combined peak season for the two runs from October through April.

Why cooking doesn't help

San Francisco's health department warns that cooking, boiling, drying or freezing does not destroy the toxin. Symptoms often start 6 to 24 hours after eating: vomiting, severe cramps and watery diarrhea. Then comes a deceptive phase where people feel better while liver damage is building, followed by liver and kidney failure in severe cases. In one fatal case in the CDC report, a 29-year-old was sent home from an emergency room with a gastroenteritis diagnosis and died days later.

What to do

Health officials' advice is simple: don't pick or eat wild mushrooms, including ones picked by friends or family. Mushroom collecting isn't allowed anywhere in the East Bay Regional Park District. Watch dogs closely on walks, since several dog deaths were reported last winter. If someone may have eaten a wild mushroom, seek care immediately and call California Poison Control at 1-800-222-1222.