BAY AREA — The next time you go for a hike, keep a close eye on your dogs and kids. Growing beneath oak trees and possibly other hardwoods is a lethal mushroom that will cause severe gastrointestinal distress about 12 hours after consumption, with liver and kidney failure following if treatment is not sought right away. The name says it all: Death Cap.

Death Cap mushrooms typically pop up after the first fall rains. “Both the Death Cap and Western Destroying Angel grow near oak trees,” said East Bay Regional Park District Naturalist Trent Pearce, who is based at Tilden Regional Park in Orinda and documents the fungi in East Bay Regional Parks. “They can be lethal to both humans and pets if consumed.”

The Death Cap (Amanita phalloides) and Western Destroying Angel (Amanita ocreata) are two of the world’s most toxic mushrooms. The Death Cap is a medium-to-large mushroom that typically has a greenish-gray cap, white gills, a white ring around the stem, and a large white sac at the base of the stem, according to the parks district. Though the Death Cap is mainly associated with oak trees, it has been found growing with other hardwoods. It was accidentally introduced to North America on the roots of European cork oaks and is now slowly colonizing the West Coast.

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The Western Destroying Angel is a medium-to-large mushroom that usually has a creamy white cap, white gills, a white ring around the stem that disappears with age, and a thin white sac at the base. It is most common from late winter into spring. It is associated exclusively with oaks. Unlike the Death Cap, it is a native California mushroom.

“Dog owners should keep a close watch on their dogs during the winter months,” said Mason. “Pet owners should contact a veterinarian immediately if they suspect their pet may have eaten a toxic mushroom.”

Death Cap and Western Destroying Angel mushrooms are responsible for most cases of mushroom poisonings in California. However, two other mushroom species found in the Bay Area are also poisonous, the Galerina and Lepiota mushrooms.

If you’d like to learn more, the East Bay Regional Parks will hold its annual Tilden Fungus Fair on Sunday, Feb. 3, 2019, at Tilden Nature Area’s Environmental Education Center.