One of the world's largest and most diverse collections of amanitas—the group of fungi that includes death caps, destroying angels and the polka-dotted mushrooms of Super Mario renown—is kept in a converted garage in Roosevelt, N.J. The stockpile is maintained by Rodham E. Tulloss, aged 70, who has documented species so rare they have been seen only once or twice in the past 50 years. His climate-controlled Herbarium Rooseveltensis Amanitarum may contain more distinct species than any university or museum. “I've never counted,” he says. “I can tell you I have well over 6,000 collections of Amanita alone.”

Tulloss, a retired electronics engineer and Bell Labs Fellow, is a passionate amateur who has collaborated with professionals. He has worked with evolutionary biologists at Harvard University and co-authored a paper with them in PLOS ONE that showed how amanitas lost genes associated with breaking down cellulose as they evolved—in effect, moving from free-living organisms into a long-term, symbiotic relationship with trees. He is also an honorary research associate at the New York Botanical Garden in the Bronx and has worked with mycologists at the Chinese Academy of Sciences's Kunming Institute of Botany and many others to reliably identify and describe new species.

Of the estimated 1.5 million fungi species worldwide, only a small percentage have been categorized. One hurdle is the biodiversity magnitude; another problem is that the fruiting bodies, the things we call mushrooms, can be inconspicuous and fleeting. Thomas Bruns, a microbiologist at the University of California, Berkeley, says, “If you had to identify all of the plants on earth by their fruit alone, it'd be a pretty tough job, and you'd probably make a lot of mistakes at it. That's kind of what we've got here.” Two years ago, when Bruns convened a meeting of the North American Mycoflora Project, an ambitious attempt to catalogue and map the distribution of species, he looked to Tulloss's garage. “He has a supervaluable collection,” Bruns says.

Genetic sequencing has revealed many misclassifications in the fungi world in recent years. Tulloss's late mentor, Dutch mycologist Cornelis Bas, called him a bear because of his persistence in sorting out the conflicting labels. He took the description to heart and calls himself the Amanita Bear. Motto: “Only you can prevent taxonomic and nomenclatural confusion!”

Tulloss's obsession does not extend to all mushrooms. In August he was walking in a cemetery near Steuben, Maine, when he ducked into the woods and spotted an edible fungus, Hypomyces lactifluorum, which resembles a cooked crustacean. “Lobsters!” He shouted. While his companions bent to collect them for dinner, Tulloss walked on in search of tall, white fungi with a ring around the stem: amanitas. “I don't know how much time I have left,” he says, “so I want to give it my all.”