The cold weather is here, which means it’s wild mushroom season. As the days cool and the rain increases, foragers in gumboots head out to hunt among the yellowed leaves for their dinner.

But there’s one killer mushroom they don’t want to find. It lurks beneath oak trees, including in some of our most established suburbs in Melbourne, Canberra and Adelaide, and comes to life particularly after heavy rain. Amanita phalloides. The death cap.

The death cap is a paragon of truth in advertising. Even the smallest bite is packed with enough toxins to kill someone or, at the very least, maim survivors.

And it has a long history of victims.

The Roman emperor Claudius was thought to have been murdered with a plate of poison fungi in 54 AD. Another ruler, Charles VI of Austria, died in 1740 after apparently eating death caps, sparking an eight-year war over his succession. In his memoirs, Voltaire wrote that “this plate of champignons changed the destiny of Europe”.

But how do you spot a death cap? Where do they grow? And what happens if you accidentally eat some?

What do death caps look like?

Quite innocent, actually, when you think the death cap is responsible for 90 per cent of mushroom poisoning deaths, and especially when you compare them to seriously scary-looking mushrooms such as the bizarrely tentacled Octopus Stinkhorn.

“It’s quite an elegant-looking mushroom,” says Teresa Lebel, mycologist (fungi expert) at the Royal Botanic Gardens Victoria, of the death cap.

But looks can be deceiving.

There are a few clues to spotting a death cap mushroom. The first is a nearby oak tree. The death cap is a mycorrhizal fungus, which means it grows in a symbiotic association with the roots of a tree – in this case, oaks.

The other noticeable feature is the colour. The cap of the mushroom has a yellow-greenish sheen, which can also vary to an olive or brown hue.

“Other mushrooms have those colours but none grow under the oak tree,” says Dr Lebel.

Other key characteristics to look for are white gills underneath the cap, a small cup at the base of the mushroom and a skirt-like membrane around the stem.

The fully open caps are usually between 10 and 15 centimetres in diameter but it’s not uncommon to find them smaller than that.

They can be particularly dangerous for migrant Asian communities, due to the death cap's resemblance to the edible straw mushroom used in Asian cuisines.

Video: hunting for death caps (with an expert)

Where did they come from and where do they grow?

When oak seedlings were brought to Australia from the northern hemisphere, it is thought they brought the spores of death caps with them.

This means death caps are mostly found where oak trees were planted when Australia’s city were first established. In Melbourne, those areas are in the city’s leafy east, such as Camberwell, Kew and Malvern.

“Unfortunately, it was one of the key trees that they brought in early on, before quarantine,” says Dr Lebel.

Other suburbs in Melbourne where death caps have been spotted include Ashburton, Burwood, Canterbury, Clayton, Deepdene, Emerald, Heathmont, Heidelberg, Sandringham, South Yarra, Surrey Hills and Wheelers Hill. In regional Victoria, they can be found in Bendigo, Bright and Castlemaine.

Sydney, being a largely oak-tree-free zone, does not have a known problem with death caps. But the mushrooms have grown in Canberra since at least the 1960s and have also been found more recently in Adelaide and northern Tasmania.

And it’s not just Australia where the death cap has travelled: they grow on every continent except Antarctica.

What time of year do they appear?

Now. “From around Easter or Anzac Day we start getting an increase in calls from people who have swallowed a mushroom,” says Jeff Robinson, manager of the Victorian Poisons Information Centre. “The worst-case scenario is someone who has ingested the death cap,” he says.

A key point to remember is oak trees have large root systems, meaning the mushrooms can grow far from the trunks. The offending oak could be in your neighbour’s yard or on a nature strip.

Typically, the mushroom sprouts underneath the edge of the oak’s canopy, where the rain catches on the leaves and drips, right up to the trunk. It can take three days for the death cap to fruit. Even the Royal Botanic Gardens in Melbourne has a problem with them. Dr Lebel and her team of pickers find up to a kilogram of death caps a week lurking under the garden’s oak trees in peak season.

What happens if you eat one?

Once consumed, the symptoms can take a long time to show themselves, sometimes days. Violent nausea then follows, including abdominal cramps, vomiting and diarrhoea. By this stage, however, it is usually too late as the toxins have started to cause cell death. Liver failure isn’t far away – anyone who survives will likely need an urgent transplant.

As Robert Wasson, the first Westerner to popularise the consumption of hallucinogenic mushrooms, wrote for Harvard University in 1972, “The symptoms of poisoning by the deadly amanitas are distinctive, dramatic and terrifying. To begin with, the lethal amanitas taste good – on this the abundant testimony of victims shows no dissenting voice. Nothing arouses suspicion as the greedy diner consumes his fateful dish; nor does he suspect anything for many hours after.

“Indeed, the distinctive mark of this poison is the period of absolute quiescence that follows the ingestion of the mushrooms, a period that never lasts less than six hours and usually 10 or 12, sometimes 20 or even 40 or more.

“The victim goes about his affairs blissfully unaware that the fingers of death are entwining him.”

Can you survive?

Yes, and people do. But survival means moving fast. People who think they’ve eaten a death cap should call the Poisons Information Line on 13 11 26. Keeping a piece of the mushroom can help with identifying the right course of action. When cooking wild mushrooms, Dr Lebel says, “leave half for the coroner”.

Treatment includes“gastric decontamination” – getting the mushrooms out of your system – as well as vigorous fluid replacement to protect the liver and kidneys. Patients will also be given activated charcoal. An antidote called Silibinin, which is an extract of milk thistle seeds, has also been recently introduced.

The lethal dose for humans is the size of a 20-cent piece.

Some survivors with kidney damage may have to undergo dialysis as a result of ingestion and even have a kidney transplant. A patient may be placed on an emergency liver transplant list, according to South Australia Health, “but a compatible organ may not be available on such short notice. If too much of the mushroom has been ingested, the damage to multiple body organs may mean that a person has become inoperable and even a liver transplant won’t save their life.”

Cruelly, there can be “recovery” phase after death cap poisoning where a patient will feel better for two or three days. However, this only masks the onset of jaundice, seizures, coma and death, which occurs between one and two weeks after eating the death cap.

The lethal dose for humans is estimated at about 0.1 milligram for every kilogram of body weight, so about 8 milligrams for an 80-kilogram person – a piece the “size of a 20-cent coin,” says Dr Lebel.

Australians consume 2.9 kilograms of edible mushrooms, on average, every year.

This is scary. Should we go foraging at all?

A high-profile example of foraging gone terribly wrong was when the English author of The Horse Whisperer, Nicholas Evans, and his wife were hospitalised after eating Cortinarius speciosissimus mushrooms, which they had picked in the Scottish Highlands. He ended up having a kidney transplant from his daughter in 2011.

But it’s up to you whether you forage – just remember it comes with risks. “It’s not just the death cap that is dangerous,” says Robinson.“One of the most common causes of poisoning is the yellow-staining mushroom, which looks similar to a standard field mushroom.”

As for death caps, there is nothing you can do to make them OK to eat. Cooking, peeling, drying, freezing or soaking the death cap does not make the mushroom edible.

Some myths about avoiding poisonous mushrooms include the following:

• It's simply not true that if you can peel the cap, it becomes edible.
• It's not true that if you put a silver spoon in cooking liquid with your mushrooms, the spoon will discolour and this will tell you whether poison is present – this does not work.
• It's incorrect that if there are teeth marks in the cap, it is edible. The death cap is poisonous to mammals but not to slugs and snails.

‘I want to see the mushrooms before they’re cooked and put in the risotto.’

Go foraging with someone who knows what they’re doing. When in doubt, check with an expert before eating mushrooms.

Damian Pike has been foraging mushrooms and selling them at his Prahran Market stall in the inner-eastern suburbs of Melbourne for more than 30 years. He says people can confuse death caps with different, edible mushrooms they have seen in other parts of the world, including Europe.

“This is the problem – the identification is very, very, very, wrong. People just don't know what they are,” he says.

“If you can’t identify it then you should not eat it.”

Even seeing “wild mushrooms” on a restaurant’s menu causes the Royal Botanic Gardens' Dr Lebel to check exactly what varieties are being used.

“I’m not going to play Russian roulette with my life,” she says. “I want to see the mushrooms before they're cooked and put in the risotto.”

– with Paul Sakkal

You can phone the Poisons Information Centre 24 hours a day on 13 11 26