Sudden Oak Death

Carol Greenberger

Tens of thousands of oak trees in Northern and Central California are dead or dying, stricken by a disease discovered less than a decade ago. Scientists fear that the disease could spread to threaten oaks in the east as well. Sudden Oak Death is caused by the fungus Phytophthora ramorum, which kills tanoaks and other species of oaks native to California, and infects many other plants. While it may take years for an infected tree to die, only a few weeks separate the first visible symptom of sap bleeding from the trunk to the trees’ leaves turning brown, giving the appearance of sudden death.

The disease was first identified in Muir Woods and the nearby Marin County town of Mill Valley in 1995. Tanoaks were the first trees affected, an evergreen species of the beech – oak family, native in the United States to California and southern Oregon. Tanoaks were once used to generate tannin for processing leather but are now considered a trash tree by foresters because they hold little commercial value. American Indians in the region utilized tanoaks by grounding the acorns into flour after leaching the tannin from them. The trees provide habitat for more than 5000 types of insects and over 300 species of animals.

Since its appearance, Sudden Oak Death has killed tens of thousands of tanoak, coast live oak and black oak along California’s coast and in southwestern Oregon. A Wildlife Conservation Society biologist surveyed tanoaks along a 350-mile stretch of California’s coast in the summer of 2000, and found that infection ranged from 15 to 80 percent of the trees in the area. There are an estimated 10 to 11 million acres of oak trees along 1500 miles of the California and Oregon coast, and they are an integral part of the landscape. Preliminary tests show that California’s majestic redwoods may also be susceptible to the disease. Scientists discovered dead sprouts from the trunks of redwood trees at a state park. When tested, spores of Phytophthora were detected. However, the recognized pattern of mortality has not been found in redwoods, and testing has not yet been completed to determine if redwoods are a host of the Sudden Oak Death member of the Phytophthora family. Redwoods belong to the conifer family, unlike the other hosts of the disease. Scientists are concerned about this threat to the state’s dwindling old-growth forests as well as the timber industry, but caution jumping to dire conclusions before all the necessary lab work and field observations are completed.

Sudden Oak Death has also infected the leaves and branches of rhododendron, big leaf maple, California bay laurel, Shreve’s oak, western azalea and evergreen huckleberry. The fungus causes leaf spots and some twig dieback on these host plants and may produce spores that transfer to the oaks, which are killed by the disease. The highly contagious disease, first found in European rhododendrons in 1993, has not been seen in European oaks. There is concern however that the disease will spread to forests in areas of Europe with climate conditions similar to California. Scientists are uncertain whether the disease was transmitted from Europe to California, or vice versa, or whether it came from a different unknown location. The disease moved from California into southwestern Oregon.

Sudden Oak Death poses a serious threat to urban and rural forests, as well as California’s nursery industry. Oregon has imposed a quarantine on nursery stock, lumber, firewood and bark chips from affected types of oaks from all of California, not just the known infected counties. Canada imposed severe import restrictions on containerized nursery plants from California and a federal quarantine is being finalized. The disease threatens the forests as the fallen dead trees create fire hazards. Live oaks are fire resistant and actually slow the spread of wildfires under normal circumstances. But the numerous brown-leafed, dead trees have been described by an ecologist as a “wildfire waiting to happen.” The forest ecosystem is affected as wildlife habitats are destroyed and animals’ food supplies are disrupted. Wildlife Conservation Society researchers are concerned about the effects of the disease on acorn, leaf and flower production which provide food to wildlife ranging from bees to mountain lions. Black bears, deer, squirrels and many birds rely on acorns as a crucial food source. Fire hazards, erosion and possible landslides also threaten suburbs as the surrounding woods deteriorate.

The genus Phytophtora contains 60 known species that have affected the world in significant ways. A member of this species, Phytophtora infestans, caused the 1845 Irish potato blight. The blight of the potato crop left acre upon acre of Irish farmland covered with black rot. The devastation of the potato crop led to the Irish famine that took as many as one million lives. The social and cultural structure of Ireland was changed as a result of the famine, and new waves of immigration to America and England were spurred. Phytophtora cinnamoni is a danger to chestnuts and pines. Other members of the genus are linked to the deaths of eucalyptus trees in Australia and oaks in Mexico, Spain and Portugal. Phytophtora leteralis has been responsible for the deaths of thousands of Port Orford cedars in Oregon, an ongoing problem since 1938. The Sudden Oak Death microbe is most closely related to this last fungus. This discovery may mean that the microbe changed its genetic makeup and moved from one kind of tree to a completely different species. Scientists know that Phytophthora hybridizes very easily. One theory holds that the fungus may have mutated, evolved over time or mated with a different species to cause the new Sudden Oak Death microbe.

The fungus attacks trees by destroying the nutrient and water conducting tissues. Tiny, lemon-shaped spores are produced that are activated by water and cool temperatures. The spores germinate and the fungus secretes enzymes that digest bark and food conducting cells. This causes the first symptom, the bleeding of a dark reddish-brown sap typically on the lower portion of the trees. Young shoot tips of new branches wilt, and leaves and twigs die. The sap of the weakened trees attracts three species of beetles, the western oak bark beetle, the oak ambrosia beetle and the minor oak ambrosia beetle. These small beetles bore into trees and the young beetles mature to adult within the trees’ tissues. Sawdust created by the burrowing beetles may cause infection in other trees when it is spread by the wind. Mature beetles fly off to mate and infest another oak. The beetles are prolific and can produce up to two generations per year. The beetle infestation ultimately causes death by completely choking off food and water. Researchers believe that the fungal infection alone would kill the oaks, but the beetles speed up the process. The disease can take anywhere from months to years to kill a tree, but the quickly dying foliage gives the appearance of a sudden death.

Sudden Oak Death has not been found outside of 10 counties in California and Oregon to date. But scientists warn that the disease could pose a threat in the east, as northern red oaks and pin oaks seem to be susceptible to the disease. Red and pin oaks have a combined range that spreads from northeastern Texas to Nova Scotia and are dominant in those forests. Research done by Dr. David Rizzo of the University of California at Davis has shown that seedlings of red and pin oaks can succumb to the disease. Dr. Rizzo said, “That doesn’t mean that if the disease makes it to the Eastern forests, it will spread in the same way. The trick is to avoid spreading it, and because it has multiple pathways, that’s not easy to do.” While scientists are uncertain exactly how the fungus moves from place to place, they warn that the disease may be spread by dirt on car tires, hiking boots and pets’ paws, and even firewood. Harvesting foliage for the interstate floral trade may spread the fungus. In 2000, more than 177 tons of foliage, including tanoak and huckleberry branches, was taken from California and Oregon forests. Shane Sela, a forestry specialist with the Canadian Food Inspection Agency expressed concern over the lack of U.S. Department of Agriculture regulatory controls restricting movement of infested materials across state lines. “We’d like to see some interstate controls,” he said.

Emergency funds of $200,000 from the University of California Division of Agriculture and Natural Resources and the U.S. Forest Service created the Oak Research Team. The 30-member team is composed of botanists, entomologists, foresters, ecologists, geographers and pathologists. Scientists continue to work to discover the pathogen’s origin and methods to prevent the spread of the disease. Aerial surveys of California are ongoing to monitor Sudden Oak Death’s progress across the state. Experts fear that if the microbe continues to spread at its current rate it could render California’s oak trees virtually extinct. Matteo Garbelotto, a forest pathologist at the University of California at Berkeley said, “The effect to the ecosystem is major. If it’s really a new disease and the trees don’t have resistance, the same thing could happen here that happened to the chestnut trees on the East Coast. You’d be wiping out the main host as well as all the organisms that rely on those trees”.

The California Oak Mortality Task Force (COMTF) was then formed to coordinate the research team’s work with public agencies, non-profit organizations and private interests, working together to implement a unified and comprehensive approach for research, education and public policy. A $1 million grant from a San Francisco based foundation was awarded to scientists on COMTF. Researchers at the University of California Berkeley and Davis will use the funds to try to determine how the pathogen moves, what conditions are necessary for its survival, where it is, and what plants are susceptible.

Senator Boxer of California introduced an emergency supplemental bill that passed in 2001, awarding $1.4 million to help fight the disease during the wildfire season. The United States Forest Service committed to an additional $1 million that will be used to implement fire prevention, monitoring, research and treatment measures. Another $400,000 for further research is in the Agriculture Appropriations Bill for fiscal year 2002. To date about $ 4.5 million in federal funds have been delivered to fight Sudden Oak Death. Another $70 million in funding to be used by local, state and federal agencies over the next five years was approved by the Senate and is now being considered by a joint House and Senate Conference Committee.

A search of “Sudden Oak Death” on the Internet yields over 50,000 hits with everything from tips for homeowners on how to identify and treat infected trees to scientific abstracts. Californians are cautioned to inspect their rhododendrons, refrain from transporting wood products from affected areas and to clean dirt from their vehicles and shoes after visiting Sudden Oak Death areas. Foresters, biologists, researchers, nurserymen and homeowners all continue to keep a close watch on this devastating disease.