The 90-year-old man died on Saturday, state Department of Social Services spokesman Oscar Ramirez told The Associated Press. The Placer County Sheriff's Office identified him as Frank Warren Blodgett.
Ramirez said an official with Gold Age Villa in Loomis called Monday to report the third death. State officials are continuing to investigate the incident that sheriff's investigators have described as a terrible accident.
Three other people were sickened when they ate the soup made from the poisonous mushrooms on Nov. 8, including the caretaker who made it. Their conditions have not been made public.
The caretaker at the six-bed care facility did not know the mushrooms were poisonous, investigators said. The following day, those who ate the soup were ill.
Vomiting and diarrhea associated with mushroom poisoning can take 12 hours or longer to develop, which often makes it difficult to diagnose, said Dr. Kent R. Olson, medical director of the San Francisco division of the California Poison Control System.
"People don't necessarily make a connection right away," unlike food poisoning, which comes on much more quickly, Olson said.
The biggest problem with mushroom poisoning is liver toxicity. Olson said the organ stops producing normal proteins, and it eventually shuts down and dies.
"Once that happens the liver can't regenerate, so they go into a liver failure," he said. "It's quite dramatic."
Treatment includes massive amounts of IV fluids to prevent kidney failure, and activated charcoal to absorb the poison. In the past, the mortality rate was as high as 90 percent worldwide. But with the supportive care, Olson said it has declined in recent years to about 15 percent.
In Northern California, it's the season for wild chanterelle mushrooms a highly sought-after variety and for the amanita species of mushrooms that include what are known as "death cap" and "death angel" varieties.
Young poisonous North American amanitas often look like an edible version of a wild mushroom popular in Asia. Olson said they grow in large numbers in the San Francisco Bay area around Sacramento and in the Sierra foothills.
Investigators were quickly able to pinpoint the soup as the source of illnesses at the care home because the only person living there who did not eat dinner that night did not fall ill.
The two other people who died have been identified as Barbara Lopes, 86, and Teresa Olesniewicz, 73.
The California Department of Public Health periodically issues warnings about consumption of wild mushrooms, especially after someone eats a poisonous variety. The state recorded 1,700 cases of mushroom-related illnesses from 2009 to 2010, including two deaths.
State food regulations do not prohibit the use of foraged ingredients in food prepared at care facilities, though the rules do prevent the use of home-canned foods and unpasteurized milk.