Five years after the first reported cases of Sudden Oak Death, the disease that has infected millions of trees has reached epidemic levels in coastal California but has not significantly expanded its range, as feared.
University of California researchers reported Friday that the pathogen is firmly established in 14 counties - including Santa Clara, Santa Cruz, San Mateo and Alameda - and the number of infected trees within these counties is escalating.
They also announced that a lethal cousin of the pathogen, another foreign invader, is killing a rare and threatened native plant in the Gold Country.
"Sudden Oak Death is occupying more and more areas within the coastal region," Matteo Garbelotto of the University of California-Berkeley said Friday at a conference of The International Union of Forest Research Organizations in Monterey.
For instance, the Peninsula towns of Woodside and Portola Valley are reporting their first-ever cases, as are communities in western Sonoma County and western Marin County, near Point Reyes. Since last January, the San Mateo County Department of Agriculture has identified four new locations of the disease in Portola Valley and six new sites in Woodside, most up in the hills.
But the heartening news is that the disease does not appear to have spread into the Central Valley, Sierra Mountains or the vast forests of the Pacific Northwest.
Its spread is proven to be weather-related, said plant pathologist David Rizzo of the University of California-Davis.
New cases slowed this year, because it was a dry spring, said Rizzo. In contrast, infections accelerated during the wet springs of 2003, 2005 and 2006. Because it takes three to five years for an infected tree to succumb, many early-infected trees are dying this summer.
"With a wet year, it picks up right away," agreed Garbelotto.
Strict quarantines and other regulatory measures, combined with ecological factors, are also believed to have hampered the expansion of the Sudden Oak Death pathogen, called Phytophthora ramora, into new regions.
But the detection of a deadly relative of the pathogen in the Sierra foothills is creating new concerns among state biologists. Phytophthora cinnamomi, which has extinguished many western Australian forests, has been found between the Amador County towns of Ione and Buena Vista, in the Gold Country - where it has wiped out hillsides full of a unique type of manzanita.
"It has the potential to eliminate entire stands" of the rare plant, which only grows in the unusual, highly acidic soils of the central Sierra Nevada, said Garbelotto. Common in a variety of agricultural and horticultural crops, the pathogen is thought to have been introduced to the Sierras by Christmas tree farmers.
"What is disconcerting is its impact on native vegetation," Rizzo said. "It is something we should really keep our eye on."
The same warm-weather pathogen has also been found in the oak trees in San Diego County, said Garbelotto. It attacks fine roots, making the tree more vulnerable to drought.
There are more than 60 different species of versatile Phytophthora, whose name means "plant devourer." They attack everything from potatoes to soybeans to strawberries. One species caused the Irish potato famine in the 19th century.
Phytophthora pathogens are especially difficult to control because they come from an entirely different kingdom of life than most other pathogens and are impervious to most pesticides. One fungicide cannot cure the disease but has proven successful in protecting some individual trees, although it is not practical for widespread use in forests.
Prevention is the best strategy. California's quarantine on nursery plants from infected counties seem to be helping, scientists said. The pathogen spreads naturally through water, such as rain splashes and contaminated creeks, usually within a three mile range.
There is no evidence that the Sudden Oak Death pathogen continues to be introduced into the wild - rather, genetic tests suggest that there was an isolated instance, sometime in the past, said Garbelotto.
But now the disease is firmly rooted. First noticed on tanoaks in Marin County in 1995, the main distribution is from Big Sur to Southern Mendocino, with isolated pockets in Humboldt's Garberville area and southern Oregon.
"The wild population took off and is now totally independent. Introduction is not something that is still going on," said Garbelotto. As the pathogen evolves over time, he predicted that populations will develop different traits throughout its range.
"It is colonizing its new environment, and adapting," he said.San Jose Mercury News
