The Mighty Are Falling

By Jeff Hudson

A species of the organism that caused the devastating Irish potato famine is now killing the California oaks.

Something is killing the oak trees—the handsome, long-lived, slope-dwelling trees that are the visual signature on many of Marin County’s beautiful and expensive hillsides.

Arborists started noticing dying tanoaks in 1994. By 1996, people were noticing that coast live oaks were dying as well. And so were the black oaks.

The foliage would turn brown, and sap would ooze from the bark on the trunk. Seemingly healthy trees could become dead trees within a matter of weeks.

The newspapers soon gave the phenomenon a name: “Sudden Oak Death.” But no one was sure what was causing the trees to die, though a number of entomologists expressed the hunch that a bug might be found at the bottom of it all. Others thought it might be a change in climate, perhaps even a symptom of global warming.

But in June 2000, two UC researchers—Dave Rizzo, associate professor of plant pathology at UC Davis, and Matteo Garbelotto, forest pathology extension specialist and adjunct professor at UC Berkeley—made a surprising finding from evidence collected on their second trip into the field to study the problem. They found that the oak trees were being killed by a new species of fungus-like organism from the genus known as Phytophthora (pronounced “fy-TOFF-thoruh”).

Working from their background in plant pathology, Rizzo and Garbelotto had suspected—even before they went into the field—that Sudden Oak Death was being caused by some sort of Phytophthora, because the organism has been behind several large-scale die-offs among plants in other parts of the world. In fact, on occasion, Phytophthora has proven disastrous.

In the 1840s, a kind of Phytophthora caused the devastating Irish potato famine. Potatoes originated in South America and were transported to North America in the 1600s by European settlers and to the British Isles in the 1700s. By the 1800s they had become a staple of the Irish diet, but in the middle of that century, the blight took hold, causing field after field of potatoes to rot. Scientists now understand that it was caused by a species known as Phytophthora infestans, which multiplied quickly in the damp Irish soil. Hundreds of thousands of people starved, and many thousands more died of scurvy and other malnutrition-related diseases, prompting the hasty emigration of over a million more, mostly to the United States. The population of Ireland was reduced from 8.5 million to 6.5 million in just six years. The mass starvation and exodus not only left an indelible dark streak through Irish poetry and drama but also shaped a young America, eventually producing such leaders as Presidents Kennedy and Reagan.

But there are some 60 species of Phytophthora, Rizzo noted. “And some of those others are also important.” One species introduced in Australia in the 1920s converted over 100,000 acres of eucalyptus forestland into grassland. Another is attacking the roots of Port Orford cedars in the Pacific Northwest.

Phytophthora has even turned up on the UC Davis campus. During the 1970s, when eager landscape gardeners watered the Quad from edge to edge to produce a lush green lawn, the long-established cork trees on the Quad’s perimeter began to die back—a development that was ultimately traced to Phytophthora. When the overwatering was stopped, Phytophthora declined, and the cork trees returned to health.

Rizzo, who joined the UC Davis faculty in 1995, began his research into Sudden Oak Death with the idea that some sort of Phytophthora might be a likely cause. “But it was in an odd place: on slopes,” Rizzo said. Sunny hillsides with well-drained soil are not the usual habitat for moisture-loving Phytophthora.

Initial lab work on Bay Area samples indicated that Phytophthora was present. But it wasn’t a species that Rizzo or Garbelotto recognized. So Rizzo called on John Mircetich, a retired UC Davis lecturer and former research leader for fruit and nut tree diseases under a USDA program on campus. Mircetich had studied Phytophthora for decades. “But he told me, ‘This doesn’t look like anything I’ve been seeing here for the last 30 years,’ ” Rizzo said. “This is something new.”

New to California, but not—as it turned out—unique. In September 2000, Rizzo met with Clive Brasier, a visiting scientist from England who was on vacation in Oregon. The British scientist came south to California and looked at some dead trees and Garbelotto’s cultures. What had been planned as a short conversation turned into an extended session, which led to a further exchange of data with a Dutch researcher.

It turned out that the same Phytophthora that Rizzo and Garbelotto were studying in California had been isolated in the early 1990s in ornamental rhododendrons in Germany and the Netherlands. The species would come to be called Phytophthora ramorum.

No one will ever know for sure exactly how it got to California, but Garbelotto can manage a guess: “People don’t generally export oak trees around the world. But they export rhododendrons.” And those rhododendrons get planted in people’s hillside gardens.

Rizzo is careful about describing Phytophthora ramorum. Although it’s often described as “fungus-like,” it’s not a true fungus. “It walks like a fungus, and it looks like a fungus, but it’s closer to brown algae or diatoms” (single-cell plants found in fresh water, salt water and moist soil). True fungi are more closely related to animals than to Phytophthora ramorum.

Phytophthora ramorum kills trees by attacking the phloem and cambium—the food-conducting tissue that carries nutrients—stopping the transfer of food, “though how it does this, whether it’s toxins or what, isn’t exactly clear yet,” said Rizzo.

After learning about the European research, Rizzo, along with Santa Cruz County farm advisor Steve Tjosvold, made a trip to a nursery in Santa Cruz County and collected samples of landscape and commercial rhododendrons that were near previously sampled oak trees. Sure enough, Phytophthora ramorum was present in the rhododendrons.

“So we started looking at related things,” Rizzo said—other species of plants that grew in the same environment as the oak trees. And soon, using a molecular probe that Garbelotto had developed, the two labs were identifying Phytophthora in huckleberry, madrone, California buckeye, bigleaf maple, toyon, honeysuckle and other plant species. Phytophthora ramorum doesn’t necessarily kill those species the way it kills oaks, but it can be spread through them. Rizzo and Garbelotto are now looking into whether Sudden Oak Death may also infect grape vines. Oaks and grape vines are shoulder to shoulder in much of Sonoma, Napa and Mendocino counties, where wine grapes are far and away the most important agricultural crop.

“One day we went out in the woods, and I grabbed a sick-looking leaf off a California bay tree as a negative control. And that bay leaf had more Phytophthora than anything I’d looked at,” Rizzo said. “Phytophthora ramorum has a very wide host range.”

And they have found it has a wide geographic range. Rizzo and Garbelotto, along with a number of graduate and postgraduate assistants, have identified Phytophthora ramorum in Big Sur, in the Santa Cruz mountains, up through San Mateo County and in the hills of Sonoma, Napa and Mendocino counties, as well as in Solano and Alameda counties. In October 2001, it was found way up north on the southern coast of Oregon, in Curry County. The researchers are now investigating whether it occurs in the Sierra as well.

The research has taken Rizzo to forests and fields throughout coastal California, from state and national parks to privately owned property like the oak-studded hillside belonging to Phil Lesh, a former member of the Grateful Dead.

Currently, the research involves dozens of trips each month into the field—amounting to quite a bit of travel time for the 12 members of the team who work from UC Davis. “We end up driving two hours to reach many of the sites,” said Jenny Davidson, who earned a Ph.D. from UC Davis in population biology in 2000. Davidson often makes the trip twice a week, driving alongside commuters on busy Interstate 80, Highway 37 and Highway 101 before turning off into a state park, onto water district land or open space reserves.

They target tanoak-redwood and coast live oak forests to determine which plants are infected with the disease. “I go walking around, looking for symptoms, putting out plots [marking out areas] to help guide that sampling effort,” said Davidson. “In addition to vegetation, I collect soil, rainwater, stream water, because we really want to trace the spread of the spores of the pathogen.”

Davidson has studied Phytophthora before, as a doctoral student in Panama. “The big difference is that, in Panama, the Phytophthora species was probably a native species. It was actually a very important component of the forest, regulating plant populations, helping to maintain a diversity of tree species. Whereas here in California, we’re dealing with a species of Phytophthora that is apparently new to the area. It’s a virulent pathogen that is doing a great deal of damage to these forests and is probably exotic and should not be here.”

Another member of Rizzo’s team is Shannon Murphy, who is working on her master’s degree at UC Davis. She works mostly in state parks, including Henry Cowell Redwood and Big Basin parks in Santa Cruz County and Armstrong Redwoods in Sonoma County—scenic locations that many Californians regard as a prime territory for summer vacations.

Murphy’s work typically involves camping out for three or four nights at a time. But she’s not there to enjoy the scenery. “I do elevational transects, starting at valley or river bottoms, then hike straight up the hills. And they’re steep hills, some with very thick bushes and lots and lots of poison oak.”

The team now has a much better understanding of the problem posed by Phytophthora. In a series of experiments conducted with Monterey County farm advisor Steve Koike, the researchers infected healthy plants and recreated the symptoms. “We proved cause and effect, not just association,” said Rizzo.

And, as they found more and more host species, they developed a better picture of how the disease can spread. “Now we think it may go from a bay tree to an oak tree, but not necessarily from an oak tree to an oak tree,” Rizzo said.

Another finding: “It seems to move through the air, especially through rain splash. That’s rare for a forest species of Phytophthora. That’s made it more interesting and complex.” They also know it can be spread through transfer of infected soil, wood and green matter.

But the movement of the disease isn’t entirely clear. “At this point, we can’t really predict where it’s going to jump. Even in an area like Marin County that’s pretty hard hit, it’s still somewhat patchy. We are researching how it’s spreading, what the role of humans is, and what’s happening naturally,” Rizzo said.

The role of humans may be particularly important. Though much of the landscape impacted by Sudden Oak Death looks wild at first blush, humans have definitely been present. In the parks, fire has been suppressed for decades, and human amenities—like restrooms—added. “And on private property, there’s been everything from grade changes to the introduction of drip irrigation,” Rizzo said.

Added Murphy, “It seems like some of the parks that have the most visitors have a lot of infection. I would not be surprised if human dispersal is having something to do with how it’s being spread. But it varies a lot, even within parks.”

Rizzo and Garbelotto have also learned how the disease progresses once an oak tree is infected. As it turns out, Sudden Oak Death isn’t all that sudden. “From the point of infection to death may take a year or more,” Rizzo said. “The name ‘sudden’ came from the fact that, to bystanders, it seemed the tree’s foliage would turn brown in a matter of weeks. But really, the tree might have been dead for a lot longer”—from the time that Phytophthora shut down the tree’s phloem and cambium. “If you cut a Christmas tree, it can stay green for weeks,” said Rizzo, in comparison. “But it ain’t going to come back to life.”

As Rizzo and Garbelotto get a better understanding of the species involved and the territory infected, they are looking for ways to minimize Phytophthora’s impact on the landscape. “There are some hints in preliminary experiments on coast live oaks that there may be some level of genetic resistance,” said Garbelotto, “and we’re looking for ways to use that to our advantage.”

But, for now, the best way to deal with Sudden Oak Disease is through control. “We don’t want people moving sick plants from areas that are affected,” Rizzo said. “One of the main ways to control it is through public education. In that sense, Sudden Oak Death is like AIDS.”

To prevent the spread, first a state and then a federal quarantine have been imposed. Sudden Oak Death strikes close to home for Sen. Barbara Boxer—a one-time Marin County supervisor who was instrumental in drafting the Federal Sudden Oak Death Quarantine, announced on Feb. 13. Superseding an earlier state quarantine, the federal domestic quarantine regulates movement of nursery stock, forest stock (including bark chips and mulch), firewood and lumber, soil and wreaths (along with garlands and greenery).

In an interview for this article, Boxer said, “Sudden Oak Death has emerged as one of the most alarming and aggressive plant diseases of the last generation. And the groundbreaking research being conducted by UC scientists Dave Rizzo and Matteo Garbelotto is laying the foundation for a better understanding of this scourge.

“This disease endangers California’s timber and nursery industries and threatens to forever change the beautiful landscape that defines our state,” she said. “The work being done at UC Davis is so very important and underscores the ongoing need for university-based research on Sudden Oak Death and a host of other subjects.”

The quarantine—especially if it is broadly observed by the public—could buy valuable time for researchers to find ways to control the spread of the disease. “We have good evidence from different lines of research that green material is the most dangerous to move around,” Garbelotto said, but added that it’s a challenge to get people to realize they shouldn’t move green clippings from not only oaks, but bay trees, huckleberry and honeysuckle, for example.

The researchers themselves are careful to minimize any possibility of spreading the disease, using precautions similar to those used by veterinarians in the United Kingdom last year working to bring foot-and-mouth disease under control.

“We all use a second pair of shoes when we’re in the field, and we try to remove all the mud,” Rizzo said. And the researchers follow the standard procedure of using an autoclave, an apparatus that generates superheated steam, to sterilize lab equipment and related items.

Compounding the problem is the question of what to do with the trees that have died—without moving them from the area. As Rizzo put it, “What do you do with 100,000 dead trees? And the clippings from [diseased] bay trees?” Converting diseased greenwaste and dead wood into mulch could spread the disease.

The researchers are optimistic that the public will be helpful. Davidson believes the enthusiasm and protective feelings that Californians have for the landscape could be a great ally in efforts to control the disease. “Californians love their oak trees, and many people have put their homes in oak woodlands or on the edge of open space districts. It’s devastating for them to see those oak trees dying.”

One indication of the importance of the trees has been the attention that the news media has paid to the problem.

Stories about Sudden Oak Death have triggered front-page headlines again and again in the Bay Area, Sacramento and elsewhere over the past few years. “Oaks definitely have an appeal to editors,” Rizzo acknowledged. “After the redwoods, the oaks define Northern California’s coastal landscape.”

But if the quarantine isn’t successful, then what?

Rizzo spends a fair amount of time dealing with reporters’ questions about a “worst case scenario” for Sudden Oak Death, though he doesn’t regard that outcome as likely. The worst case might mean a catastrophic die-off of one or more species of oak, akin to the damage done by Dutch elm disease, which killed untold thousands of shade trees across North America over the last few decades.

When asked, Rizzo will acknowledge that the worst case can’t be ruled out. “But then [the reporters] don’t write the 20-minute speech about why the worst case may be unlikely.” Taking the long view, history shows that the Phytophthora-caused blight that resulted in the Irish potato famine—dreadful as it was—was something that people eventually learned to live with. People are still eating potatoes, in Ireland and elsewhere.

Rizzo offers this more likely scenario: “The odds are that, within California, it’s going to spread up and down the coast—certainly up north where there’s cool, wet weather. But if we can slow it down with the quarantine, maybe we can learn how to live with it, just like people learned to live with potato leaf blight.”

Jeff Hudson, former Marin and Santa Cruz County resident and avid hiker, is familiar with many of the locations impacted by Sudden Oak Death. Since 1995, Hudson has made his home in Davis. He is heard regularly on Capital Public Radio and writes for the Sacramento News & Review, Comstock’s Business, The Davis Enterprise and other publications.