SAN FRANCISCO — Researchers have found sudden oak death in two more Northern California counties and have discovered that the fungus causing the disease afflicts two more plants than originally believed.
The discoveries deepen the mystery of how the disease is transmitted and how widespread it is throughout California.
The fungus was originally thought to affect three types of oak trees along the California coast. Earlier this year, researchers found the fungus, Phytophthora ramorum, in rhododendrons, a common garden plant, along with California bay laurel trees, Shreve's oak, madrones and wild huckleberries.
Earlier this week, the California Oak Mortality Task Force said the fungus had also struck big-leaf maple and manzanita trees. These other plants are believed to be hosts that may help spread the fungus.
The fungus "uses some of these less susceptible hosts as a springboard to the oaks, which it actually kills off," said David Rizzo, associate professor of plant pathology at UC Davis and a member of the oak task force.
"The fact that you go to a forest and every single woody plant is a host to these things is rather significant," Rizzo said. "And it's changed our views on how this spreads."
The fungus was recently discovered in Solano and Alameda counties; the fungus had earlier been found in eight California counties and in southern Oregon.
Currently, southern Mendocino County is the farthest north that sudden oak death is found in California before it jumps to Oregon, more than 200 miles away.
"What's going on is a tough call to make," Rizzo said. "The site in southern Mendocino County looked like a fairly new site. Did it skip Mendocino, Humboldt and Del Norte counties and jump into Oregon? It seems the more we learn the more questions we get."
As researchers, based largely at Davis and UC Berkeley, look for answers, agriculture officials in the afflicted counties struggle with containing the disease, which has caused acres of once majestic oaks to whither.
"What we have to do now is do further surveys in the county to see if our nurseries are within a quarter mile of a known site," said Ronnie Easton, Alameda County deputy agricultural commissioner.