Causing all the damage, Rizzo and partner (the nation’s leading experts on the subject) have discovered, is Phytophthora ramorum, a relative of the fungus thought to have caused the Irish potato famine in the 1840s.

Woodland Wizard

Discoverer of oak-killing scourge leads effort to stop its devastating ooze through the nation’s forests

Plant disease scientists from across the nation gather in a Santa Rosa, Calif., auditorium to hear one of their stars, University of California at Davis forest pathologist Dave Rizzo (’83), describe an oak-killing scourge he helped discover.

Knowing that the audience is anxious for news on whether sudden oak death afflicts California’s majestic redwoods or some other new host plant or tree, Rizzo, barely concealing a mischievous grin, flashes a photo of a tomato on a screen and announces that the juicy fruit is the latest host. Laughs follow all around. The audience gets the point: Even a devastating disease that has spread through coastal California and could potentially reach the East Coast can’t possibly harm every living plant or tree.

“It would be nice if this thing was only on oaks. Rhododendrons were a big shift. Then we realized it was on other hosts and another continent. When we hit bay, then we thought, oh my gosh, it can hit all sorts of families,” says Rizzo, a former biology major who found his life’s passion at JMU during backpacking and camping trips through woods and forests from the Blue Ridge Mountains to the Allegheny Plateau.

For almost 20 years, much of Rizzo’s work in forest pathology has been on oak-root fungus and wood-decay fungi in labs and forests far from the public eye. Rizzo’s life changed two years ago, however, when he and U.C.-Berkeley colleague Matteo Garbelotto discovered the microbe that causes sudden oak death. The forest pathologists find themselves in demand across the nation for conference presentations, town hall briefings and news media interviews. Each now commands six-figure research budgets and oversees sizable lab teams aimed at finding a way to save California’s landmark oaks and prevent sudden oak death from spreading.

They have been finding and announcing one new host after another, tallying a worrisome list of 17 plants and trees. Sudden oak death has been found across 12 coastal California counties, and regulators fear it could reach into the Sierra Nevada and even endanger East Coast forests, where two oak species — northern red oak and pin oak — might be susceptible. The Virginia of Rizzo’s college days is at high risk. European scientists have found the same organism that causes sudden oak death on rhododendrons in Germany and the Netherlands.

“It is a national priority now,” says Paul Tooley, who leads the U.S. Department of Agriculture’s studies into the threat sudden oak death might present to a variety of East Coast plants and trees. Tooley’s experiments are conducted in a former army lab at Fort Detrick, in Frederick, Md. He credits Rizzo, Garbelotto and their assistants with learning a lot in a relatively short time and helping Tooley’s staff to get going on similar research. “Science moves forward with people like Dave, who share their knowledge freely and welcome others in, getting together with colleagues and kicking things around,” Tooley explains. “That’s helped tremendously in advancing the science and prioritizing research.”

Norlyn Bodkin isn’t surprised his former biology student has excelled in the field. He says Rizzo was enthusiastic and sharp — one of those quick studies that stands out in class. “There are kids who you can tell are obviously with you every step of the way. Those are the kinds of people who keep you in the business, and now he’s doing it,” says Bodkin, who was a biology professor and director of the Edith J. Carrier Arboretum before retiring two years ago.

Rizzo, 40, grew up in suburban Philadelphia and has been a research scientist and professor at U.C. at Davis since 1995. He became one of the foremost research scientists in his field while working at four universities in the East and Midwest before arriving at Davis. The bookish Davis resident is a family man who savors time with his wife and two young daughters. In the field, he has experimented on pine and cedar diseases in Yosemite Valley and pine and fir diseases in the Lake Tahoe basin. He dropped a wood-decay fungi project in Yosemite to make time for sudden oak death but has kept experiments going in the Sierra Nevada on both oak-root fungus and the response of insects and diseases following control burns.

Rizzo was teaching in the spring of 2000, when a USDA Forest Service scientist called him to help with some fieldwork in Marin County, Calif. — ground zero for sudden oak death. Garbelotto, the U.C.-Berkeley scientist, had spotted a dark ooze on tree trunks that he suspected revealed the culprit that had caused the disease.

Days later, Rizzo, known for having a sharp eye for oak maladies, went into the same Marin County woods and took the first sample after just one look at the dark ooze — with the first whack of his axe. “Two days later it grew out, and my life changed, that’s fair to say,” he says. “Now I sit in front of my computer and talk to reporters while my students are getting out into the woods.”

Rizzo grew it in petri dishes, and Garbelotto identified its genetic code through sophisticated lab methods. They determined that an exotic, fungus-like organism never before seen in California causes sudden oak death. Sudden oak death is caused by Phytophthora ramorum. It is genetically distant from most of the other 60 species within the genus Phytophthora. The pathogen is related to the fungus thought to have caused the Irish potato famine in the 19th century and the fungus responsible for the modern death of Port Orford cedar trees in the Pacific Northwest.

Sudden oak death has killed tens of thousands of oaks, hitting coast live oak, black oak, tan oak and Shreve oak. It also has been found to harm redwood, Douglas fir, rhododendron, manzanita, California huckleberry, madrono, California bay laurel, California buckeye, big leaf maple, California coffeeberry, toyon and California honeysuckle, as well as arrowwood in Europe.

The scientists, with their assistants, have continued making critical discoveries about new host trees and shrubs, how the microbe survives, spreads and attacks. No treatment has been found to kill the disease or stem its spread, but they remain hopeful. “I’m grateful every day that we have them,” says Susan Frankel, the USDA Forest Service official who first hired them to work on sudden oak death. “We were really lucky. We were looking for the best people.”

Rizzo and Garbelotto are technical advisers to the board of the California Oak Mortality Task Force, which Frankel chaired during its first year. The work on sudden oak death is possibly their greatest challenge yet. Not only is it an exotic forest pest, but people want more answers faster than the scientific process can yield them. “The scientists are being asked to give answers for things that they still don’t know enough about,” Rizzo says.
Then there are the funding and lab staff levels that would make them the envy of many university scientists. “There’s pressure out there,” Rizzo says.

“[But] if you get frustrated easily, you wouldn’t be in this business.”
Scientists expect to find more than the current 17 hosts; they have found the disease also harms redwoods, a development that is a blow to the environment as well as logging and recreation. With many California vineyards planted in oak habitats, scientists are beginning experiments to find out whether grapevines could be victims. “The growing list started making us think, awhile back, we should start looking at economically important crops in the areas where this is breaking out to make sure it won’t cause damage,” says Steve Swain, sudden-oak-death project coordinator for the U.C. Cooperative Extension office in Sonoma County.
“One of the scary things about plant diseases is they aren’t always host-specific and can be fairly adaptable,” Rizzo says. Observations in Sonoma County have turned up infected bay laurels in a far more widespread area than oaks. Scientists suspect the disease moves from place to place on bay laurel leaves and then kills oaks along the way.

Another concern is the role El Niño-spawned winter storms might play in spreading sudden oak death. Rizzo says the microbe that causes the disease moves into areas during cool, wet months. So it could take off during El Niño periods, when rainfall is greater and lasts longer. Scientists also suspect it spreads through the air, by rain splash and by humans.

While its spread is daunting, sudden oak death faces tough foes in Rizzo and Garbelotto. Swain figures he knows something about diseased trees, but is often amazed at how the pair consistently gets good results quickly.

“Water-stained bark, that’s not much to go on,” he notes. “A lot of this is intuitive, seat-of-the-pants stuff.”

Rizzo is a wizard in the woods. He has designed experiments to learn that the fungus-like organism survives in rainwater, soil and leaf litter.

Garbelotto is a genius in the lab. He has developed techniques to read the micro-organism’s genetic code quickly and with near perfection. He has designed probes to get its DNA out of wood and bark ooze. “I’ve worked in collaborations a lot, and it’s like you’re always guarding a little bit. We’ve never had any reservations. We completely trust each other,” Garbelotto says. “He’s an extremely knowledgeable person, and he’s very humble about it. When he talks I really listen to him,” he says.

This is the first disease Rizzo can remember knowing nothing about in the beginning, so he welcomes Garbelotto’s expertise. “I don’t think we could have gotten this far along this quickly. We can divide up a large workload to answer a lot of questions. We still have the problem of learning what we’re dealing with,” Rizzo says.

In presenting findings, Rizzo often does the writing and Garbelotto the talking. “I think I’m a lot more boring than him,” Rizzo says. “But we both love to go out into the woods.”

Growing up in the suburbs of Philadelphia, Rizzo spent hours walking through woods and fishing along streams. “I used to spend all day out in the woods, collecting frogs, collecting snakes, that kind of stuff — not fungi. I didn’t start getting into those until college,” he recalls. Rizzo didn’t travel much in his youth, and the Shenandoah Valley was an exotic eye opener when he visited JMU. “It is surrounded by mountains and forests on both sides. Hiking, backpacking and taking class field trips in these areas led to an interest in forests and forest ecology,” Rizzo recalls. “Seeing different diseases in the field was just an extension of an interest in the forests,” he says. Bodkin’s biology class hikes into Virginia’s woods and forests were particularly revealing, Rizzo says.

“There’s more to biology than in a test tube,” Bodkin says. “I didn’t believe in dragging stuff into the lab. We were out in the field a lot. Some students look and don’t see. Not Dave,” Bodkin recalls.

A shrublike growth that Rizzo noticed on those journeys was all that remained of what once had been North America’s largest hardwood tree. Rizzo was eager to learn about the demise of that American chestnut, a sturdy tree that could reach 10 to 12 feet in diameter. Chestnut blight had decimated the tree across the East. Trees were cut down in an effort to check the spread of the disease.

What captured Rizzo’s curiosity were the sprouting stumps. “I thought that was interesting that a disease could wipe out a whole forest,” he recalls. “Little did I know that I would be working on a disease that might do the same thing.”


Story by Michael Coit
Photos by Gary Krueger


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