A mountain biker flew down Patrick Ridge in Marin County’s China Camp State Park. He was focusing on the rocky fire trail and an upcoming sharp left turn, so it’s unlikely he noticed the dying trees that line the trail. These coast live oaks are victims of sudden oak death (SOD), a disease caused by the pathogen Phytophthora ramorum. While SOD may not be getting as much press as it once did, it is no less virulent in its spread. Bay Area oaks that are susceptible to the pathogen are coast live, California black, Shreve’s, and canyon live, not to mention tanoaks, which, while not true oaks, are extremely vulnerable.
According to Michael Swezy, natural resource specialist for the Marin Municipal Water District, there is probably no stand of tanoaks in Marin that remains untouched, with some suffering a 50 to 90 percent die-off. In Solano County’s Green Valley, where only a few SOD-infected trees were found in 2001, hundreds of diseased or dying trees have been reported in a wide swath emanating from the original infected trees.
“It’s worse than it’s ever been,” says Katie Palmieri, public information officer for the California Oak Mortality Task Force. Researchers believe the spike in infection is due to the wet, warm springs of 2005 and 2006. The pathogen is spread primarily in wet years, says plant pathologist Ted Swiecki of Phytosphere Research in Vacaville.
There’s more troubling news for our woodlands. Researchers have recently discovered another Phytophthora pathogen in the hills above Miwok Meadows in China Camp. Phytophthora cinnamoni is known as the cinnamon fungus in Australia, where it has caused extensive damage to forests. This aggressive pathogen has been named one of the world’s worst invaders by the Global Invasive Species Database. It is common in California nursery plants and agriculture, says Swiecki, but until recently, it hadn’t caused damage to native plant communities in wild areas.
Swiecki believes there are two reasons these pathogens are spreading into our wild areas. “First, global travel and commerce increase the opportunities for these organisms to hitchhike out of their native ecosystems into new areas,” he says. “Second, both development in and near wildland areas and increased recreational use of wildlands provide more opportunity for these exotic organisms to be introduced into native plant communities.” Even the incidental movement of soils and plant materials can cause pathogens to spread. “Phytophthora seems to take advantage of human activity more than most plant pathogens,” Swiecki says.
If you use woodland trails in moist conditions, the California Oak Mortality Task Force recommends that you clean your boots, bike tires, horse’s hooves, and dog’s paws before leaving the area. Experts suggest keeping an old screwdriver, stiff brush, and a towel for this purpose in your car.
“People need to get attuned to the fact that they are vectors of pathogens, insects, and weed seeds,” says Swiecki. “Exercising caution should be a part of the new wilderness ethic.”
To learn more about SOD and how to prevent its spread, go to The California Oak Mortality Task Force (COMTF) website.