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SAN DIEGO – Six years after identifying Sudden Oak Death syndrome, scientists are still struggling to understand the disease that’s killing thousands of trees in California. 

Since its discovery in Marin County in 1995, the pathogen Phytophthora ramorum has been found attacking oaks in 10 California counties as well as in southwestern Oregon, Germany and Holland. 

It quickly kills certain trees and, even more insidiously, lingers in other plant species that spread the disease while barely showing symptoms themselves. 

“A large part of California could be infected and we just don’t know about it,” Matteo Garbelotto, a forest pathologist at the University of California, Berkeley, said Wednesday at a symposium in San Diego. 

Scientists admitted there is much to learn about Sudden Oak Death. 

Where it came from, how it spreads and how it can be contained remain uncertain, said Mark Stanley, chairman of the California Oak Mortality Task Force. 

The disease is caused by a fungus-like pathogen related to the one that caused the Irish potato famine of the mid-1800s. 

It causes bleeding or oozing of a dark sap in the coast live oak, the black oak and the Shreve oak. In the tanoak, which is not a true oak, it causes drooping in new growth. Weakened trees then become vulnerable to attacks by insects and wood-decaying fungi. 

Recent evidence suggests the disease was brought into the state, possibly through the trade of ornamental plants, Garbelotto said. 

Birds, humans trading plants and even spores wafting on the breeze may spread the disease. 

The state of California has imposed regulations for transporting host plants and material in the 10 counties where Sudden Oak Death has been documented: Alameda, Marin, Mendocino, Monterey, Napa, San Mateo, Santa Clara, Santa Cruz, Solano and Sonoma. 

But infested host plants — such as bay laurel, rhododendron, huckleberry and many others — could easily go undetected, Garbelotto said. 

“The symptoms are so small and minute that only the very trained eye can see them,” he said. “The problem is this is a microorganism. So if it doesn’t cause a huge tree to die, how do you see it? How do you see a microorganism that is minuscule in a bunch of soil?” 

Experts at the symposium called for a strengthened effort to diagnose and monitor the disease. The state and federal governments have allocated $7.6 million to study and help contain Sudden Oak Death, Stanley said. 

In the meantime, researchers urged Californians to be cautious about moving possibly infected material: Firewood should not be moved from one place to another; cyclists should clean soil from their tires. 

Property owners should dispose of leaves and other debris in proscribed burns or in compost piles capable of reaching a 131-degree temperature. If someone wants to take material to a designated compost facility, they should check with local authorities to see if a permit is needed. If they do move it, they should properly seal the transport container. 

“The worst thing that you could do is to put it in the back of your pickup truck and then drive all across the county with the leaves flying out of the truck,” Garbelotto said. 

ease, people should not move infected leaves, wood or soil. Wood that already has been moved should be burned. Visitors to coastal forests should clean their tires, shoes and animals’ feet thoroughly. Construction workers should wash equipment. Ornamental plants that could be carriers of the disease should not be moved from infected counties.