One scorching June day two years ago, plant pathologist David Rizzo paid a visit to China Camp State Park. China Camp is a sanctuary of rolling golden hills and dense oak forest that hugs the shores of San Pablo Bay, California. It's a popular spot for hikers and campers, with a gentle wildness that makes it seem much more than 20 miles from San Francisco. But Rizzo was there on a serious mission, hot on the trail of a mysterious disease that was destroying the park's prized coast live oaks. Local rangers had spent years tearing out foreign interlopers like eucalyptus to make room for native trees. Now their work was unraveling. Month by month, the majestic, gnarled oaks were dying. "It was like losing old friends," ranger Patrick Robards says. "I kept saying: 'What's happening to my old friends?'" Rizzo and three colleagues circled through one of the park's campgrounds, grateful for the shade cast by tall bay trees and broad oaks. There were plenty of ailing trees, marked by dark cankers, rivulets of sap, and trails of frass, telltale sawdust left by burrowing beetles. Rizzo pulled out his ax and chopped away a piece of the discolored bark. He could see a dark stain spreading underneath the healthy wood, its edges outlined by a crisp black line that marked the advancing infection. Rizzo slipped a sliver into a Baggie and moved on to the next tree. The plague had been smoldering in the woods of Marin County for years. The first victims had been tanoaks, a distant relative of the stately trees we consider true oaks. Foresters called tanoaks a "trash species" and were almost happy to see them go. When county officials began investigating, the first experts on the case were entomologists. Because the dying trees were riddled with ambrosia and bark beetles—"there was a rain of sawdust coming down," one observer recalls—some entomologists assumed the trees were being killed by a beetle outbreak. The trees were sampled for pathogens, but the results came up negative, which only reinforced the beetle theory. Then the problem began to spread. During the next few years, reports of dying trees came in from all over Marin County. "I had clients calling, saying, 'Oh my God, what's happening? I've lost three trees and more are bleeding,'" says Ken Bovero, an arborist in Mill Valley. "I've lived here all my life, and I was seeing something way out of context of anything I'd seen before." By the late 1990s, tens of thousands of trees were dead in four Bay Area counties, and their dry remains were a potential fire hazard. Sudden oak death, as the catastrophe came to be called, is one of the most virulent forest epidemics ever to hit the United States. Its name is something of a misnomer: The disease is neither sudden nor limited to oaks nor invariably fatal. It now threatens forests in the East and South as well as California, and scientists confirmed in September that it has infected Douglas firs and redwoods—among our most valuable and beloved trees. Pathologists are investigating the disease, using both centuries-old techniques and DNA technology. They have conducted aerial surveys, worked on chemical cures, and found evidence of the disease as far away as Europe. In a modern world where any pathogen can hitch a ride across any ocean, their efforts may well write the book on how to cope with plant epidemics. Still, on that June day in China Camp, Rizzo was dubious. Short and balding with an unassuming manner and a wry sense of humor, he isn't given to hysteria. He knew that oaks everywhere go through periodic die-offs, and he figured some familiar culprit was at work. So it came as a surprise when he walked into his lab at the University of California at Davis two days later and found his petri dishes teeming with tiny lemon-shaped spore sacs. It looked a bit like Phytophthora—a genus of funguslike organisms responsible for the Irish potato famine. But it wasn't like any phytophthora Rizzo had ever seen.
One of thousands of coast live oaks killed by a forest plague in China Camp State Park. "The biggest lesson for me," a Forest Service pathologist says, "is that an incredible number of trees can die before most people take notice."
