After half a century of patiently waiting in the dark and biding their time, the deadliest mushroom in the world has arrived in Idaho.
For decades, Amanita phalloides — commonly known as the death cap mushroom — has found its home in the forested areas of the east and west coasts of the United States. The large fungi, responsible for about 90% of the world’s mushroom-related fatalities, primarily grow at the base of trees along the coasts of California, Oregon, New Jersey and other coastal states during the moisture-heavy months of late summer through early winter.
But in the fall of 2021, for the first time on record, death caps were found growing in the Treasure Valley.
“It seems to be associated with oak trees right now in Idaho,” Krista Willmorth, president of the Southern Idaho Mycological Association, told the Idaho Statesman. “In other places, it has moved to other species of tree, but it seems to be associated with mature oak trees here in Idaho so far.”
Don’t worry; the Treasure Valley isn’t about to be swarmed by a plague of killer mushrooms. But the highly-poisonous fungi pose an issue for four-legged friends who aren’t as educated as humans when it comes to eating random mushrooms growing at the base of trees.
Especially during late fall and through December, when the mushrooms tend to grow at their most voraciously.
“It is a deadly mushroom, to humans and dogs for sure,” Willmorth said. “It is one of the most toxic mushrooms out there.”
How toxic are death cap mushrooms?
Death cap mushroom caps grow to about six inches in diameter at their largest. Still, ingesting even half of that amount can be fatal to humans, the Idaho Department of Health and Welfare warned Idahoans in April. An even smaller portion is enough to kill a dog.
The progression of death cap poisoning is just as sneaky as the fungi itself. The early symptoms of poisoning include persistent and violent vomiting, abdominal pain and watery diarrhea for up to 24 hours following ingestion, according to IDHW.
An apparent recovery with no symptoms will follow that initial 24-hour period before sudden jaundice, loss of strength, coma, and ultimately death due to liver and kidney failure will follow a few days later.
The IDHW recommends calling the poison control center at 800-222-1222 if you or someone you know digests an unknown mushroom. If your pet has eaten a wild mushroom, the department recommends calling the ASPCA Animal Poison Control Center’s 24-hour hotline at 888-426-4435 or a veterinary emergency clinic.
The Southern Idaho Mycological Association offers a yearly mushroom education course to educate people on the different types of mushrooms found in Idaho.
“One of the first things we do, rather than going right into ‘these are the mushrooms you want to look for to eat,’ is we talk about the relatively few mushrooms that are either very toxic or deadly toxic,” Willmorth said. “Many of them happen to fall into the Amanita genus.”
How did death cap mushrooms end up in Idaho?
Members of SIMA have known that this has been coming for a while. In fact, for half a century.
Death cap mushrooms first found their way to the United States in the early 1900s, according to previous Statesman reporting, hitching a ride on imported Scotch pines.
And just like how they arrived via imported trees on the coasts, that’s how they came to Idaho: Oak trees imported from California.
But why has it taken so long for the mushrooms to appear?
“It could be 40 to 60 years before an oak tree is mature enough,” Willmorth said. “The tree, up until that point, is putting so much energy into growing and maturing that it doesn’t have a lot of sugar or food to spare, and that’s what it’s sharing with the mushrooms.”
Robert Chehey, one of the founders of SIMA, had been predicting that Amanita phalloides would sprout up in Idaho one day.
“He had mentioned a number of times that we don’t have Amanita phalloides in our area, but he said it’s just that we don’t have it yet,” Willmorth said. “Because the conditions are there, we have trees from nurseries in California, so it’s certainly plausible that it could show up at some point.”
That came to fruition last fall when Susan Stacy, a member of SIMA, brought a death cap mushroom into a monthly SIMA meeting.
Stacy now heads up a project, working alongside the Boise city forester, to find more death cap mushrooms on trees known to be over 50 years old and imported from California. So far, the project has found multiple more death cap mushrooms in the North End.
Identifying death caps and what to do with them
One of the most noticeable features of any mushroom in the Amanita genus is the universal veil, a membranous tissue from which the fungi emerge.
Death cap mushrooms will look like tiny eggs at the base of trees before popping out, Willmorth said. A small part of the universal veil will remain on top of the cap while the rest will sit at the bottom of the mushroom like a white cup.
“They will have white gills that will stay white, and they have white spore prints, and then a greenish to brownish cap that’s almost metallic when fresh,” Willmorth said. “Then it’ll broaden out and flatten and then become maybe browner or a little more dull as it gets older.”
Death cap mushrooms will most likely be found in older neighborhoods due to the imported oak trees in the area being old enough to support fungi.
If you come across Amanita phalloides, Willmorth recommends pulling them out of the ground and throwing them away — simply touching them won’t cause any adverse effects. Wash your hands afterward, though.
Pulling the mushrooms out of the ground also doesn’t harm the organism — it’s similar to picking an apple from a tree — so they will grow back, but it’s better than using fungicides or pesticides, she said.
“Putting fungicides on or trying to poison it or eradicate it is really just going to be putting poisons out there that you also don’t want around kids and dogs,” Willmorth said. “And it’s actually not going to eradicate the mushroom.”