Whatever was attacking the trees in California wasn’t subtle. It began when apparently healthy tanoaks suddenly began oozing black or reddish-brown sap from tiny holes in the bark. Within months, the trees were dead. The disease was dubbed “sudden oak death,” a name that indicated both official alarm and scientific bafflement.
That was in 1995. By 1999, the mysterious disease was spreading like a slow forest fire, not only killing true oaks and tanoaks (an oak relative), but infecting other woodland plants like madrone and huckleberry, leaving ugly brown lesions on their leaves and twigs.
Any new plant disease is interesting to a plant pathologist such as Jennifer Parke. But a disease that kills some of its victims and only wounds others—that’s fascinating. “What really got me,” said Parke, an Agricultural Experiment Station researcher with OSU’s Department of Crop and Soil Science, “was the breadth of the pathogen’s host range. I started to wonder how many other species might be susceptible.”
Madrone and huckleberry are members of a large family of plants that includes rhododendrons, azaleas, blueberries, cranberries, heather and pieris. These plants are the backbone of Oregon’s nursery operations and small-fruit farms. And Parke realized, these are plants we see out our windows and in people’s yards everywhere. “Here’s where I said to myself, ‘Oh, my goodness. We may have something serious here.’”
Meanwhile, Everett Hansen, another plant pathologist at OSU, was puzzling about what kind of pathogen could be behind these mysterious deaths and infections. The pathogen’s California sleuths, Susan Frankel of the U.S. Forest Service and David Rizzo of the University of California, Davis, were calling on colleagues across the country who were as intrigued and baffled as they were. They called Hansen.
Finding the cause of a plant disease can be tricky. Often other organisms move in behind the initial attack to take advantage of the weakened plant, and this can muddy the diagnostic waters. Frankel and Rizzo had investigated many possible causes, but all had proved to be secondary—these organisms had come to feast on the dying trees, but they hadn’t caused them to die.
Hansen is an expert on an organism known as Phytophthora (fy-TOF-thor-a), which attacks many plants and kills several important ones. He’s particularly familiar with a nasty species of Phytophthora that kills Port-Orford-cedar trees by rotting their roots.
In June of 2000, Hansen took a field trip with Frankel and Rizzo to look at the dead and dying trees for himself. “It didn’t really look like Phytophthora,” he said, “but there was something about the bark, the cankers, that rang a bell.” The scientists collected samples and took them back to Rizzo’s lab, where they cultured the samples in a special medium. Bingo! Phytophthora grew.
But it wasn’t a Phytophthora they had ever seen before. “It looked like a brand-new thing,” Hansen said. The California team ran DNA tests and found that the organism was indeed a brand new species of Phytophthora.
In the meantime, Parke was testing an assortment of blueberry varieties for susceptibility to the yet-unnamed pathogen. She found that some were alarmingly susceptible.
Parke joined forces with her colleague Bob Linderman of the USDA Agricultural Research Service’s horticultural research lab in Corvallis. Together they cast the net wider and began testing other plants. Linderman has a long acquaintance with Phytophthora, having worked extensively in the early 1970s on P. syringae which causes lesions on the leaves of rhododendrons. “In fact,” he said, “I thought this one was P. syringae, because its symptoms in rhododendron were just about identical.”
For the past several years, plant pathologists in Europe had noticed something similar to the sudden-oak-death pathogen scarring the leaves of rhododendrons and viburnums. A German pathologist, Sabine Werres, identified it as a Phytophthora and gave it the species name “ramorum,” meaning “in the branches.” But several Phytophthoras can cause trouble in rhododendrons, and nobody got too excited about this new one in Europe.
But as the California team was closing in on the sudden-oak-death culprit, a British colleague of Hansen’s, Clive Brasier, flew to California to take a look at the strange new pathogen. Brasier peered into a microscope and realized that what he saw looked remarkably like what he’d seen in Werres’s lab. In fact, it was the same thing. The Phytophthora that was disfiguring leaves on rhododendrons in Europe was the very one that was killing oak trees in California.
There are many Phytophthora species, and some are notorious—the name derives from a Greek root meaning “plant destroyer.” It was a Phytophthora that caused the Irish potato blight in the mid-nineteenth century, wiping out the entire crop and triggering widespread starvation and mass emigration. (“Phytophthora infestans is why I’m in America and not Ireland,” Parke said with a smile.) From the human perspective, this new species seems to be right up there with the most notorious. Not only is it indiscriminate in its victims, but, according to Linderman, the plants it doesn’t kill become carriers of the disease. Infected leaves act as reservoirs of spores that spread through the air via wind-blown rain.
Phytophthoras are shape-shifters, with several kinds of spores. Some thick-walled spores help the organism survive through periods of drought. Other spores are like little sacs that are carried through water or splashed and blown around by wind and rain. If these land on a susceptible plant, they release their contents on the wet leaf surface. Then tiny, tailed zoospores spread through the water film until they find a place to settle. They lose their tails and become cysts that puncture the host’s cell walls. They grow long strands that invade the plant’s tissues, killing cells as they go.
As more trees died in California, the Oregon departments of forestry and agriculture were watching the situation, preparing to jump on the disease if it popped up in Oregon. In the summer of 2001, a forestry department survey airplane spotted 30 clumps of brown foliage in the forested ridges east of Brookings. The clumps were tanoak, and nine trees tested positive for the sudden-oak-death pathogen. The agriculture department imposed a quarantine, and forestry crews cut and burned the diseased trees and neighboring plants.
By this time, Parke and Linderman were testing every plant they could think of that might be susceptible. “We started with rhododendrons, blueberries and their relatives, which is a big family,” Parke said. “And then we started saying, ‘What about lilac? What about viburnum?’” Between them they tested about 100 species, and found that three-fourths of them—including lilac and viburnum—showed some susceptibility.Their immediate worry was the damage sudden oak death could do to Oregon’s nurseries. The nursery industry is Oregon’s leading agricultural enterprise, with more than 2,000 businesses grossing around $727 million yearly. About three-fourths of Oregon’s nursery stock is sold out-of-state, and an uncontrolled outbreak of a serious plant disease could trigger an economically devastating quarantine.
In May of 2003, confirming the scientists’ fears, P. ramorum was discovered on rhododendrons in a wholesale nursery in Clackamas County. In June it was found at a retail affiliate of the Oregon nursery in Washington state, also on rhododendrons.
The Oregon Department of Agriculture acted fast. Working with the nursery’s owners, they promptly destroyed the infected plants. They tested adjacent plant blocks and surveyed neighboring properties and found no more incidences of the pathogen. Then they traced the origin of the shipment and found that it had come from a nursery in Canada.
DNA fingerprinting conducted in Hansen’s lab has shown that the European strain of P. ramorum is genetically slightly different from the one found in California and Oregon. The pathogen found in the Clackamas nursery was similar to the European strain and may have come from Europe via Canada, or from yet another, unknown place. According to Hansen, a promiscuous pathogen like P. ramorum is hard to trace.
The existence of two mating types of P. ramorum has raised the fear that the two will meet in some nursery, cross, and produce a super-pathogen. So far this has not happened. The state agriculture department continues rigorous surveillance in all Oregon nurseries. As for P. ramorum in the Oregon woods, a quarantine remains in place around the patch of Curry County forest where it was first discovered. The state forestry department conducts aerial and ground surveys several times a year and destroys any infected plants it finds.
California is having a harder time containing P. ramorum. Since it first appeared in 1995, the pathogen spread widely throughout the counties surrounding San Francisco Bay and along the coast, infecting and killing tens of thousands of oaks and tanoaks. In March, 2004, P. ramorum was found on camellias in California’s largest nursery—but not before thousands of the plants had been shipped out of state. The pathogen has now been detected in several states on nursery stock from California, potentially carrying the disease to gardens, parks and forests throughout the U.S., according to Parke. A federal quarantine prevents further shipments of infested nursery stock from California.
“What we all feared might happen, did happen,” Parke said. “It underscores the importance of preventing nursery infections here in Oregon.”
In April, 2004, Parke was called to help organize a multi-state Rapid Response Project to coordinate research and extension activities among several of the nation’s universities and agencies. The USDA-sponsored project focuses on ways to contain further spread of the pathogen across the country and prevent its movement from nurseries to native vegetation. Hansen and Linderman will join her on the project.
The work of Parke, Hansen and Linderman has been crucial in helping Oregon’s agriculture authorities detect and contain sudden oak death before it had a chance to infect nurseries and small farms. “It’s critically important that our regulatory agencies be able to quickly identify plant disease, so we can quickly assure our marketplace that we don’t have the problem,” said John Aguirre, executive director of the Oregon Association of Nurseries.
ODA’s Nancy Osterbauer agreed. “We’ve cooperated quite closely with OSU, the Oregon Department of Forestry and the Agricultural Research Service since the beginning,” she said. “Jennifer and Bob are helping us answer fundamental questions that pertain to our regulatory mission.”
Forestry officials are similarly appreciative. “We can’t do our own testing, because as soon as we find something in the forest, we destroy it,” said Alan Kanaskie of the ODF’s monitoring and eradication team. “We’re counting on Jennifer’s and Bob’s work to help us with our piece of this puzzle.”
The pathogen, P. ramorum, that causes sudden oak death infects more than oaks and does not always cause death. The list of plants that are host to the pathogen includes many garden favorites, and their symptoms may include leaf spots, defoliation, stem cankers and withered shoots. Unfortunatly, these symptoms are similar to those caused by other plant diseases, and accurate diagnosis requires careful laboratory analysis. The photos above show infections of ramorum leaf blight in camellias (top left), viburnum (top right), and rhododendrons (bottom left). The last photo shows two kinds of ramorum spores on an infected Oregon myrtlewood leaf. The fungus-like pathogen produces several types of spores to increase its chances of survival.