Ebba Peterson was driving home from work along Highway 101 in Oregon this April when something caught her eye—a flash of red along the side of the road. Peterson, a plant epidemiologist, recognized it as the foliage of sick trees and pulled over.
After bushwhacking to reach the site, Peterson was dismayed by what she saw: two trees, seemingly in the throes of a disease called sudden oak death. They had flaring brown-red canopies and blackening twigs. “I’m looking out the window, I see these dead crowns, I think: ‘Shit!” Peterson recalls.
She clipped some samples and took them back to her lab for analysis. “The second time I cursed was when I looked at those petri plates,” she remembers. The culture tested positive: It was sudden oak death.
The disease primarily affects tanoaks, which grow along the coast of California and southern Oregon. It’s caused by a pathogen called Phytophthora ramorum. It’s a peculiar organism: It appears similar to a fungus but is actually a water mold, more closely related to kelp than fungi. A related species, Phytophthora infestans, was responsible for the 19th-century Irish potato famine, during which about a million people died and millions more fled the country. Phytophthora ramorum is causing a natural disaster on a similar scale in western forests: It has killed over 30 million trees in California and Oregon in just 20 years.
The spores start in the canopy, reproducing in the leaves and fine twigs. They move around—from tree to tree, or from upper canopy to bark—thanks to wind, rain, and fog. Once they reach the bark, the infection forms cankers: dead spots and breaches. The foliage of a sick tree can turn brown in weeks, but, contrary to the disease’s name, actual death usually comes after years of infection. Spores in the soil form long-resting structures that don’t spread as easily, but they can survive and spread from soil or leaf litter.
Many kinds of vegetation can be infected, but tanoaks are the most susceptible to illness and death. They play a critical ecosystem role; for animals, they are often the most important nut-producing tree in forests dominated by pines (whose nuts are smaller and harder to access) and redwoods. An infestation would have massive economic implications for Oregon, where forestry and wood products are the state’s third-largest industry, worth over $8 billion.
In neighboring California, forestry officials have waged a 20-year battle against the pathogen, with no success to speak of. The state is plagued by millions of dying and dead tanoaks. Now their colleagues in Oregon are faced with an unwelcome choice: To stop the spread of the pathogen, they’ll have to kill trees.
“It is the most drastic prescription that we have,” says Sarah Navarro, a sudden oak death pathologist for the Forest Service in Oregon. But, she says, these are “the necessary steps we need to take in Oregon to mitigate the risk of disease spread.”
Phytophthora ramorum was likely introduced to American forests around 1995, when large numbers of dying tanoaks were observed in the counties of Santa Cruz and Marin. The cause was a mystery for years, until scientists identified Phytophthora ramorum as the culprit. Imported nursery plants were believed to be the source, but the pathogen wasn’t found in a nursery until it was isolated from rhododendrons in Santa Cruz in 2001.
In California, “we were always too late from the get-go," says Susan Frankel, a plant pathologist with the Forest Service Pacific Southwest Research Station and the former head of California’s Oak Mortality Task Force. When tanoaks started to die, they weren’t a highly prized commercial species, so “most people ignored it,” Frankel says.
No one understood how serious it was until it was too late. “In California, there were no statewide or region-wide attempts to contain the disease,” Frankel says. And in those early days, there were no diagnostic tools, either. “A lot of what we were doing in the beginning was developing the tools, developing the methodology,” Frankel says.
Today, citizen scientists participate in annual “SOD Blitzes” in which people collect samples to quantify how many trees in California are affected. The Forest Service and other organizations mount education efforts. But no one is seriously trying to stop it. “California's attitude is sort of learning to live with the disease,” Frankel says.
The story in Oregon is different. Peterson's roadside quarry was disturbing because sudden oak death was not supposed to be there. In Oregon, the disease was found in wild tanoaks in 2001, just seven months after the nursery detection in California, and a year after Phytophthora ramorum was identified. But this delay made all the difference—scientists and forest managers had already been discussing the disease and planning which actions they’d take if it turned up in Oregon.
With a fast response, they were able to keep it restricted to the "generally infested area," an 89-square-mile tract of land where sudden oak death runs rampant. No one is allowed to move tanoaks in or out of this area. Conifer logs coming from the infested zone need to be cleaned of all debris and soil. Government reports have predicted that if the infested zone were to spread, Asian markets could sanction exported Oregon lumber. Officials regularly monitor Oregon nurseries for Phytophthora ramorum. Up until Peterson’s discovery in April, the pathogen was believed to be successfully contained.
Peterson’s find was over 20 miles from the quarantine border, too far for spores to travel on their own. Analysis of the samples confirmed that the trees were infected with Phytophthora ramorum, but that wasn't the only bad news: It was a new variant of the pathogen, “North American Two,” or NA2 (named for the continent where it was first isolated in a lab), that had previously only been detected in nurseries. That meant it was a new introduction—likely also from an imported nursery plant, though it hasn’t been confirmed—and a new, unpredictable version of the organism.
To Peterson, this was shocking and, most of all, disappointing. “The nursery industry, especially, has invested a lot of energy into monitoring for Phytophthora ramorum and trying to prevent these outbreaks,” she says. But plant pathogens, like human viruses, are tiny, wily, and hard to defend against. The disease is “living in the soil, and there's so much movement of plant material and soil between one location and another that this sort of stuff happens,” Peterson says. “It's not that surprising that it eventually did.”
No one knows yet how NA2 will behave in a forest, though in some laboratory studies it appears more aggressive than NA1, the strain that’s now widespread. In a worst-case scenario, a more contagious strain could spread beyond tanoak to other species, possibly even the Douglas fir and other commercially important logging trees.
Dealing with this new battle front fell largely to Sarah Navarro, the sudden oak death pathologist. She and her team surveyed the infected area, finding that over 146 of the 186 tanoaks and wild rhododendrons they sampled tested positive. The NA2 infestation was much larger than they had originally thought.
They could have tried to clear and then compost the tanoaks, but that’s a slow, complicated process that leaves behind piles of logs that can be mistaken for firewood, creating opportunities for unwitting campers to accidentally transport infected logs. Navarro felt she had only one real option: cut and burn. "It’s not the business I got into," Navarro says. But it's the best tool she has to try to slow the spread.
Successful containment requires a buffer zone of 600 feet between an infected tree and any susceptible tanoaks, so the treatment will ultimately cover 521 acres. That’s over 390 football fields’ worth of forest. The entire area won’t be destroyed; insead every tanoak will be cut down and individually sent up in flames in a controlled burn. The process has just begun, with Forest Service staff applying a herbicide to the tanoaks to prevent the infected trees from resprouting. They likely won’t complete the treatment until next spring. Officials have estimated it will end up costing about $1.7 million.
In the past, dealing with landowners whose properties had infected trees could be contentious and even end in court. This time it’s been exceptionally smooth—and Navarro thinks that’s in part because Covid-19 has made epidemiological terminology go mainstream. People now understand the value of containing aggressive variants. “It's easier to talk about susceptibility, hosts, how things spread, and things like that,” Navarro says. “People are more aware after the last year and a half of the messaging and the science that has come out with Covid.” The Oregon Department of Forestry has even set up a sudden oak death dashboard, virtually identical to the Johns Hopkins Covid-19 data display.
Yet there are no vaccinations against sudden oak death, and so no way to create the herd immunity that will keep the disease from spreading. Even with the burn, they think the disease can continue to spread. “Our treatments aren't 100 percent effective. We don't have a silver bullet,” Navarro says. Navarro is a self-described optimist, but even she speaks in terms of "delay," not "defeat." The NA2 introduction was a harsh reminder that Phytopthora ramorum, or another pathogen, could spill over from imported nursery plants at any time. “That's something that always has the potential to torpedo our efforts,” she says.
“We know that eradication is no longer possible, given the resources and the staff and the amount of disease on the landscape,” Navarro continues. “But we do know that we are making a difference to slow the spread of this disease, lessen the impact for as long as we can.”
Update 9-27-2021 1:37 pm: This story was updated to correct details of the herbicide application to infected tanoaks and of the activities involved in the "SOD Blitzes."