If a tree falls in the forest

The “sudden oak death” pathogen reaches the Sierra foothills

The “sudden oak death” pathogen caused weeping bark that makes trees vulnerable to fatal infestations of bark beetles (see photo below).

The “sudden oak death” pathogen caused weeping bark that makes trees vulnerable to fatal infestations of bark beetles (see photo below).

Photo courtesy of danr.ucop.edu

California without its oaks? It’s not a pleasant thought, but already sudden oak death has killed tens of thousands of oak trees along the coastline from Monterey to Sonoma counties. And now there are reports that the pathogen responsible for the disease is in the Sierra foothills.

What appears to be unique about so-called sudden oak death—actually a misnomer since it may take months for a tree to succumb—is that it does not restrict its attack to oaks. Madrones, bay laurels, big leaf maples, manzanitas, California honeysuckle, toyons, coffeeberries, buckeyes—even redwoods—have all been found to be hosts, raising the prospect of much of California becoming barren grassland devoid of trees.

First detected in 1995
Sudden oak death was first observed in 1995 by a hiker who found that the leaves of several clusters of tanoaks near Mount Tamalpais had all died.

Following up on anecdotal reports, a University of California researcher began investigating, and soon found that tanoaks were dying all along creek beds, hillsides and ridge tops in Marin County. Not long afterward, the disease was found in Santa Cruz, Monterey and Sonoma counties. By 2001, it had been detected in at least six additional California counties (Alameda, Mendocino, Napa, San Mateo, Santa Clara and Solano) and one county in Oregon.

Besides tanoaks, coast live oaks and California black oaks also exhibit the signs of infestation, which include cankers, leaf spots and twig dieback. In the later stages of the disease, the trees may suffer massive attack by bark beetles.

The cause of the disease, not identified until 2000, turned out to be a previously unknown microbe that was dubbed Phytophthora ramorum (literally “infector of twigs”). Researchers were particularly troubled to find that the pathogen produces several types of spores—including some that can swim as well as others that can survive hot, dry conditions.

No one is sure where P. ramorum came from. One theory is that it arrived in Marin County with a shipment of infected rhododendrons from Germany, where the pathogen has also been found. Another idea is that it is related to one that has been attacking Port Orford cedars in Oregon.

An ecosystem problem
Oak woodlands make up approximately 10 percent of California’s land area. Besides preventing soil erosion, landslides and floods, the woodlands are home to at least 300 species of amphibians, reptiles, birds and mammals; 1,000 native plant species; and 5,000 kinds of insects and spiders.

Explains ecologist Steve Zack of the Wildlife Conservation Society: “Oaks bear acorns, and are at the base of a very integrated food chain that involves a huge roster of animals—from black bears to rodents, crows and jays, even titmice. If the trees die, the acorns shut down locally, and that’s a huge issue.”

Bark beetles

Photo courtesy of danr.ucop.edu

Although Zack feels it is too early to predict the consequences of the epidemic, he notes that ecosystem effects such as fire hazards, soil erosion and flooding are all intertwined. “The coastal forests are under siege. It’s all new and uncertain where it is heading,” he says.

Other potential costs are less difficult to fathom. Many whose livelihood derives from agriculture or forestry—from plant nurserymen to composters to loggers—stand to be affected. Plant pathologists are currently investigating whether grapes, avocados or any of California’s other cash crops are likely to be the pathogen’s next targets.

Although California oaks are not harvested for timber, the logging industry is worried. P. ramorum has been reported to infect redwood shoots, and the pathogen has now reached the Sierra foothills—raising concerns that that region’s timber industry may also be shut down.

Halting the rampage
Although sudden oak death has been known for over five years, it was not until May 2001 that the California Department of Food and Agriculture placed quarantines on host plants in designated infestation zones. Then in February 2002, the U.S. Department of Agriculture announced a superceding federal quarantine. Whereas the state restricted the movement of plant materials within the stricken counties, the federal regulations places limits on what leaves those counties.

Although the federal rules have been in place since mid-February, they are not yet final. At a public hearing in Petaluma late last month, the quarantine came under heavy attack by operators of plant nurseries, waste haulers, and state officials who fear devastating economic consequences.

Part of the delay in ordering sudden oak death quarantines can be traced to fears of economic hardship and to a lack of good scientific data about the disease. But Janet Santos Cobb of the California Oak Foundation is not happy. “I believe these quarantines are late in coming—five years late to be exact. The state has been remiss in not taking this deadly disease seriously. This has become a national issue now. At least the U.S. Forest Service understands the impact this disease could have on the hardwood industry of the South. We are talking perhaps $30 billion,” she says.

Washington-state plant pathologist Olaf Ribeiro, who has written a book about Phytophthora strains, agrees. “They’ve known about this problem since 1999. So they’ve done a pretty good job of spreading it around by now,” he says.

The outlook
Although a federal quarantine is now in place, no one knows whether it will prove effective given how little is actually known about how the pathogen spreads. Says Ribeiro, “Quarantines don’t stop people from moving things around. You can regulate nurseries but there’s no way to regulate the movement of firewood. And the pathogen can move on people’s shoes.”

One bit of good news is that some oak species seem to be immune to the disease. The so-called white oaks—notably blue oak and valley oak—so far have not gotten sick. Moreover, not all of the susceptible oaks in infested areas have succumbed, and some infected ones have even managed to recover.

Given that a good portion of the oaks in the Sierra foothills are blue oaks, Douglas McCreary of the Sierra Foothill Research and Extension Center is hopeful that the region will escape a major onslaught. But if the oaks do succumb, McCreary is prepared for that too. “Nature seems to have a way of filling in voids that are created and repairing itself, so in the long term, new environmental conditions will be established and a new equilibrium created,” he says.

But Steve Zack feels it is too soon for any summary statements. No one knows for sure which species will end up being infected or whether some species that currently appear to be immune will succumb. Says Zack, “There is nothing in the fossil record to show that we are within the normal range of effects of pathogens on ecology.”