You don’t have to wander very far off the road this time of year before stumbling upon Amanita phalloides, the deadliest mushroom in California.

On a recent foray in Roy’s Redwoods Open Space Preserve in Marin County, David Campbell, who has been hunting mushrooms in the county for 40 years, immediately spotted specimens of that toxic fungus, more commonly known as the death cap, growing near an entrance to the preserve.

• 
• 
• 
• 
• 

Local conditions are ripe for fungus feasting gone wrong, according to Campbell, who has helped the San Francisco division of the California Poison Control System identify mushrooms when a poisoning has been suspected.

The death cap, which is native to Europe, has been flourishing in the Bay Area, in a symbiotic relationship with local oak trees.

This year, after the fall rains, the mushrooms are “out in large numbers, and early,” said J.R. Blair, a lecturer in the biology department at San Francisco State University.

The most serious illnesses occur when fungus enthusiasts pick and eat death cap or other poisonous mushrooms.

According to Campbell, “The chance of us having an incident any day now is very high, because people who wouldn’t normally notice mushrooms are seeing them.”

“Plenty of people have this mushroom in their backyard,” he said.

From 2009 to 2010, 271 people in the Bay Area required treatment at a health care facility after ingesting a mushroom, according to the California Poison Control System. Among them were 136 children age 5 or younger.

“We’ve never had a case of serious poisoning from an accidental mushroom poisoning where a child ate something in the backyard,” said Dr. Kent Olson, medical director of the San Francisco division of the California Poison Control System. “It’s really people preparing the mushroom thinking that it was edible.”

Statewide, two people died and 10 others suffered a major health problem, like kidney or liver failure, after eating wild mushrooms from 2009 to 2010, according to the most recent data. The deaths and the most serious illnesses have been linked to Amanita phalloides and its cousin Amanita ocreata, better known as the destroying angel. Both cause liver damage.

While the two species are equally deadly, Amanita phalloides is more frequently collected and eaten, experts say.

The death cap is often a dull green color, but it can be tinged with brown or gray, or even be white. It is also reputed to be delicious. “One guy that died said it was the best mushroom he’d ever eaten,” Olson said.

State health officials advise against picking and eating wild mushrooms unless a mushroom expert evaluates the spoils first.

“It is very difficult to distinguish which mushrooms are dangerous and which are safe to eat,” Dr. Ron Chapman, director of the California Department of Public Health, said in a recent statement warning about the hazards of consuming the wrong fungus.

Health officials suggest consulting a local mycological society, like the Mycological Society of San Francisco, which is hosting its 42nd annual Fungus Fair this weekend at Lawrence Hall of Science in Berkeley.

Amanita phalloides was introduced on the West Coast in California, according to Anne Pringle, associate professor of organismic and evolutionary biology at Harvard University, who has studied the fungus’ DNA to determine its origin.

The earliest confirmed collection of the species on the West Coast was in Monterey County in the 1930s on the grounds of what was then the Del Monte Hotel, a venue famous for its gardens full of exotic species.

In 1945, the mushroom was gathered on the campus of the University of California, Berkeley, as well. Since then, it has spread, coming to thrive in the oak woodlands and mixed evergreen forests around San Francisco Bay and San Pablo Bay.

The death caps found in California are much larger than the same species back in Europe. “They’re massive,” Pringle said.

“In California, they’re also found more abundantly than in Europe,” she added.

William Freedman, chairman of the toxicology committee for the Mycology Society of San Francisco, has experienced this bounty. “In many places where I go looking for mushrooms this time of year,” he said, “they are the most common to be found.”

This article also appears in the Bay Area edition of The New York Times.