After a string of poisonings from “death cap” mushrooms — one of them fatal — California health officials are urging residents not to eat any foraged mushrooms unless they are trained experts.

Doctors in the San Francisco Bay Area have blamed the wild mushroom, also called Amanita phalloides, for 23 poisoning cases reported to the California Poison Control System since Nov. 18, according to Dr. Craig Smollin, medical director for the system’s San Francisco division.

“All of these patients were involved with independently foraging the mushrooms from the wild,” Smollin, who is a professor of emergency medicine at the University of California, San Francisco, said at a news conference Tuesday. “They all developed symptoms within the first 24 hours, including nausea, vomiting, diarrhea and abdominal pain.”

Smollin said some of the patients were parts of cohorts that had consumed the same batch of foraged mushrooms. The largest group was about seven people, he said.

All of the patients were hospitalized, at least briefly. One died. Five remain in hospital care. One has received a liver transplant, and another is on a donation list awaiting a transplant, Smollin said. The patients are 1½ to 56 years old.

Mushroom collectors said death cap mushrooms are more prevalent in parts of California this season than in years past, which could be driving the increase in poisonings.

“Any mushroom has years that it’s prolific and years that it is not. … It’s having a very good season,” said Mike McCurdy, president of the Mycological Society of San Francisco. He added that the death cap was one of the top two species he identified during an organized group hunt for fungi last week, called a foray.

In a news release, Dr. Erica Pan, California’s state public health officer, warned that “because the death cap can easily be mistaken for edible safe mushrooms, we advise the public not to forage for wild mushrooms at all during this high-risk season.”

Dr. Cyrus Rangan, a pediatrician and medical toxicologist with the California Poison Control System, said the “blanket warning” is needed because most people do not have the expertise to identify which mushrooms are safe to eat.

Still, he said, “it’s rare to see a case series like this.”

The California Poison Control System said in a news release that some of the affected patients speak Spanish and might be relying on foraging practices honed outside the United States. Death cap mushrooms look similar to other species in the Amanita genus that are commonly eaten in Central American countries, according to Heather Hallen-Adams, the toxicology chair of the North American Mycological Association. Because death caps are not often found in that region, foragers might not realize the potential risk of lookalikes in California, she said.

Anne Pringle, a professor of mycology at the University of Wisconsin-Madison, said there is a litany of poisoning cases in which people misidentify something because their experience is not relevant to a new region: “That’s a story that comes up over and over again.”

Over the past 10 years, mushroom foraging has boomed in the Bay Area and other parts of the country. At the same time, information resources about mushroom toxicity — reliable and otherwise — have proliferated, as well, including on social media, phone apps and artificial intelligence platforms. Experts said those sources should be viewed with skepticism.

Longtime mushroom hunters maintain that the practice can be done safely. McCurdy, who has collected and identified mushrooms since the 1970s, said he bristled at the broad discouragement of foraging.

“No, that’s ridiculous. … After an incident like this, their first instinct is to say don’t forage,” he said. “Experienced mushroom collectors won’t pay any attention to that.”

But McCurdy suggested that people seek expertise from local mycological societies, which are common in California, and think critically about the sources of information their lives may be relying on.

Pringle and McCurdy both said they have seen phone apps and social media forums misidentify mushrooms.

“I have seen AI-generated guidebooks that are dangerous,” Pringle said.

The death cap is an invasive species that originated in Europe and came to California in the 1930s, most likely with imported nursery trees. The mushroom is usually a few inches tall with white gills, a pale yellow or green cap and often a ring around the base of its stalk.

The species is found across the West Coast and the Eastern Seaboard, as well as in Florida and Texas, according to Hallen-Adams, who is also an associate professor of food science at the University of Nebraska-Lincoln.

In California, it typically grows near oak trees, though occasionally pines, too. The mushroom’s body is typically connected to tree roots and grows in a symbiotic relationship with them.

The toxin in death cap mushrooms, called amatoxin, can damage the kidneys, liver and gastrointestinal tract if it is ingested. It disrupts the transcription of genetic code and the production of proteins, which can lead to cell death.

Hallen-Adams said the U.S. Poison Centers average about 52 calls involving amatoxin each year, but “a lot of things don’t get called into poison centers — take that with a grain of salt.”

Amatoxin poisoning is not the most common type from mushrooms, but it is the most dangerous, she added: “90% of lethal poisonings worldwide are going to be amatoxin.”

It takes remarkably little to sicken a person.

“One cubic centimeter of a mushroom ingested could be a fatal dose,” Hallen-Adams said.

Symptoms of amatoxin poisoning often develop within several hours, then improve before they worsen. There is no standard set of medical interventions that doctors rely on.

“It’s a very difficult mushroom to test for,” Rangan said, and “also very difficult to treat.”

One drug that doctors have leaned on to treat some of the California patients — called silibinin — is still experimental and difficult to obtain.

“All of our silibinin comes from Europe,” Hallen-Adams said.

Death cap mushrooms have continued to grow abundantly since their introduction, and Pringle’s research has shown that the species can reproduce bisexually and unisexually — with a mate or by itself, alone — which gives it an evolutionary advantage.

“If Eve can make more of herself, she doesn’t need Adam,” Pringle said. “One of the things I’m really interested in is how you might stop the invasion, how you might cure a habitat of its death caps. And I have no solutions to offer you at the moment.”