For Paul Stamets, the phrase “mushroom hunt” does not denote a leisurely stroll with a napkin-lined basket. This morning, a half-dozen of us are struggling to keep up with the mycologist as he charges through a fir-and-alder forest on Cortes Island, British Columbia. It’s raining steadily, and the moss beneath our feet is slick, but Stamets, 57, barrels across it like a grizzly bear heading for a stump full of honey. He vaults over fallen trees, scrambles up muddy ravines, plows through shin-deep puddles in his rubber boots. He never slows down, but he halts abruptly whenever a specimen demands his attention.

This outing is part of a workshop on the fungi commonly known as mushrooms — a class of organisms whose cell walls are stiffened by a molecule called chitin instead of the cellulose found in plants, and whose most ardent scientific evangelist is the man ahead of us. Stamets is trying to find a patch of chanterelles, a variety known for its exquisite flavor. But the species that stop him in his tracks, and bring a look of bliss to his bushy-bearded face, possess qualities far beyond the culinary.

He points to a clutch of plump oyster mushrooms halfway up an alder trunk. “These could clean up oil spills all over the planet,” he says. He ducks beneath a rotting log, where a rare, beehive-like Agarikon dangles. “This could provide a defense against weaponized smallpox.” He plucks a tiny, gray Mycena alcalina from the soil and holds it under our noses. “Smell that? It seems to be outgassing chlorine.” To Stamets, that suggests it can break down toxic chlorine-based polychlorinated biphenyls, or PCBs.

Most Americans think of mushrooms as ingredients in soup or intruders on a well-tended lawn. Stamets, however, cherishes a grander vision, one trumpeted in the subtitle of his 2005 book, Mycelium Running: How Mushrooms Can Help Save the World. Mushroom-producing fungi, he believes, can serve as game changers in fields as disparate as medicine, forestry, pesticides and pollution control. He has spent the past quarter-century preaching that gospel to anyone who will listen.