To the untrained eye, it's a plain-looking white mushroom, popping up in yards and boulevards of Vancouver, Victoria and the Fraser Valley.

But experts recognize Amanita phalloides — the so-called death cap mushroom that poisoned someone in Victoria this week — as the deadliest mushroom in the world, responsible for an estimated 90 per cent of fatal mushroom poisonings worldwide.

Twenty years ago, it hadn't been spotted in B.C.

But now, the death cap is growing in prevalence in urban areas of the South Coast, prompting a warning from health officials last month.

Among B.C.'s thousands of mushrooms, this is one that should be on your radar.

"It's worthwhile learning how to recognize this particular species because of the consequences if you accidentally eat it," said Andy MacKinnon, a biologist and forest ecologist in Metchosin who teaches field courses on mushroom identification.

Toxins in the death cap attack the liver, kidney and other organs and ingesting one cap is enough to kill.

Paul Kroeger holds up young death cap mushrooms, which can resemble puffballs or straw mushrooms. These were found near Main Street in Vancouver. (Tristan Le Rudulier/CBC)

Lurking underground for years

The first death cap mushroom was found in B.C. in 1997, though the fungus was likely lurking underground for years, if not for decades, before that.

It's believed to have hitched a ride to B.C. on the roots of imported trees, including European oak, chestnut and hornbeam, that were planted to beautify city streets, said MacKinnon.

The mushrooms emerged years later: in Mission in 1997, Victoria in 1998, and not until 2008 in Vancouver itself.

More are coming, said Paul Kroeger, a founding member of the Vancouver Mycological society and a mushroom consultant on poisonings in B.C.

"We're looking at many many locations now where the suitable host trees are just beginning to produce the death cap mushroom."

The list of suitable hosts could also be growing, said MacKinnon, with anecdotal evidence the death cap may be spreading from European oak to the native Garry oak.

"If the death cap mushroom can somehow figure out how to live with Garry oaks, then there's a much greater range for them to explore ... and that's a little worrisome."

Death cap mushrooms are now common in Vancouver, Victoria and the Fraser Valley. The fungus that sprouts these mushrooms is believed to have arrived on European oaks and other imported trees. (Paul Kroeger)

Looks like edible straw mushroom

Death caps can vary in size and colour, so the best way to learn how to spot them or any mushroom is to see them in the wild — or city yard — with an expert, said MacKinnon. Mycological societies in Vancouver and Victoria regularly hold mushroom walks.

"Absolutely the best way to learn mushrooms is to go out with other people who know their mushrooms."

That said, there are distinguishing features, said MacKinnon and Kroeger:

Fairly large when mature, up to 15 cm across.

White or yellowish stem, often with shades of green or brown in cap.

Cap rounded when young and flattens with age.

Bulbous base of stem, which may be buried in soil.

Skirt-like ring on the stem when mature.

It doesn't closely resemble any popular edible species that are native to B.C., but young death caps do look a lot like the paddy straw mushroom that grows in Asia, said MacKinnon.

"It's possible to confuse it with other mushrooms … if you're used to collecting the edible paddy straw mushroom in another part of the world."

In one of the B.C. poisonings, it was confused with a paddy straw mushroom, said Kroeger. In another, a very young death cap was confused with a puffball mushroom.

It's not known why the most recent poisoning victim ate the mushroom.

Death cap mushrooms can look like puffballs when they're young, but if you slice them open it's possible to see the death cap developing, said Kroeger. (Paul Kroeger)

3 poisoned in B.C.

No one has died from eating a death cap in B.C., according to the B.C. Centre for Disease Control, though this is the third significant poisoning.

"Fortunately so far we've had relatively few actual ingestions and poisonings from our death cap mushrooms here," said Kroeger, who helps identify mushroom species responsible for poisonings.

"But given they've only been known in the province for a couple of decades, we've still had three significant mushroom poisonings. Not fatal, but nearly so."

Amanita phalloides contains amatoxin, and someone poisoned may have no symptoms for anywhere from six hours to two days, according to the U.S. Food and Drug Administration.

Then comes sudden, severe abdominal pain, persistent vomiting and other gastrointestinal problems. The patient may appear to recover, before the toxins overwhelm the liver.

"Death occurs in 50 to 90 per cent of the cases," the U.S. FDA reports, following a coma and possible convulsions.

Last month, the B.C. CDC warned it was "especially concerned" about the death cap, and calls to the Drug and Poison Information Centre about foraged mushrooms had doubled this past July, compared to the year before.

The increase may have been due to unseasonably wet weather in the late spring and early summer, said Kroeger.

"And now unfortunately it looks like we've got another flush of mushrooms and exposures to them."

If you suspect you've eaten a poisonous mushroom, keep a sample of the mushroom, and call B.C.'s Drug and Poison Information Centre at 1-800-567-8911 or call 911 and seek medical attention.