Scientists scramble to
understand disease ravaging a California icon
By Richard A. Lovett
September 1, 2004
For nearly a decade,
scientists have been watching oaks die in Northern California's coastal forests.
In some years, the blight that is killing them spreads slowly, giving scientists
hope that they might figure out ways to control it before it spirals totally out
of control. But in other years, it has spread much more quickly until, by now,
its impact is readily apparent even to the casual observer.
San Francisco resident
Lana Beckett, for example, had read of the disease but hadn't experienced it
firsthand until this August, when she was driving a back road near Mount Diablo
State Park, in the East Bay. "I started seeing clumps of dead trees on the
hillsides," she says. "There were still plenty of live ones, but I
wondered how much longer would they be there."
The cause is a disease
called sudden oak death, which started cropping up in 1995 and has since killed
tens of thousands of trees. And that's just the beginning: More trees are dying
each month. Most are in the coastal mountains between Big Sur and northern
Sonoma County, but the disease has crept as far north as Oregon, and nearly as
far south as San Luis Obispo.
In places, the blight is
so extensive that entire hillsides have been devastated, says David Rizzo, a
plant pathologist at the University of California Davis.
In Southern California,
however, there's little risk that vast numbers of oaks will die. Even though
susceptible species of oaks are found here, the dry climate prevents the growth
of other tree species (particularly bay laurel), which play a role in spreading
the disease among the oaks.
Tanoak bark appears to be "bleeding," the classic look of infection with the fungus that causes sudden oak death
Tree and shrub nurseries,
however, are an artificially moist environment that is much more susceptible to
infection. Earlier this year, alarms were raised when the disease showed up in
nurseries in San Diego and Los Angeles counties. It has also been found in
Europe and in trees shipped from California to at least 17 other states, raising
concerns that, hopscotching through tree nurseries, the disease might rapidly
spread across the globe.
Sudden oak death is caused
by a fungus called Phytophthora ramorum, which invades the trunks of
tanoaks and coast live oaks. Tanoaks are common in redwood forests; coast live
oaks are the dominant species in much of the rest of the Coast Ranges. Pockets
of them extend as far south as Baja.
So far, only California's
coastal oaks appear vulnerable. Inland species appear to be immune, possibly
because Phytophthora is a "water mold" that needs moister climates
and the trees adapted to them in order to spread.
The name "sudden oak
death" is a bit of a misnomer. The disease actually infects trees for
several years, invading their trunks and causing them to ooze reddish sap that
looks disturbingly like blood. Eventually, the infection spreads completely
around the trunk, girdling the tree and preventing the flow of sap. That's when
the tree dies.
From a distance, it does
appear to be sudden because until the infection cuts off the sap, the tree looks
green and healthy. Then, its branches die all at once.
One of the problems with
controlling Phytophthora is that the fungus infects many plants other than oaks.
Some it kills there are indications that California's madrones may also be
at risk but others simply develop gray-black spots on their foliage. These
foliar infections don't kill the plant, but they produce vast numbers of spores
that may carry the disease to more susceptible species.
EDWIN R. FLORANCE
Microscopic image shows the fungus responsible for sudden oak death Phytophthora ramorum releasing spores.
To date, the infection has
been patchy hitting some areas strongly and others not at all. Rizzo
believes it became established during the wet El Niño years of the early 1990s
and continues to spread whenever the state gets a moist winter. Thus, after the
wet April of 2003, it invaded numerous new areas, including San Francisco's
Angel Island State Park. But the dry spring of 2004 may have temporarily put the
brakes on additional spread.
Meanwhile, scientists are
scrambling to understand the disease and find ways to control it. So far, much
of the news is bleak.
Early in August, at a
meeting of the Ecological Society of America in Portland, Ore., Letty Brown, an
environmental science graduate student at UC Berkeley, reported that the
infection might be even more destructive than had previously been thought.
Prior studies had found
that Phytophthora infected from 4 percent to 30 percent of the coast live oak in
any given patch, and from 20 percent to 70 percent of tanoak. But Brown found
that up to 27 percent of the coast live oak within her study plots died in the
two-year interval 2002-04. (Her research focused only on coast live oak, and not
the even-more-susceptible tanoak.)
If you add in recently
dead trees that were probably killed by sudden oak death, plus living trees
showing symptoms of Phytophthora infection, the death rate in her most heavily
infected plots may soon exceed 60 percent. And that's not counting any
still-healthy trees that might yet succumb to the disease.
Ecologically, the death of
that many oaks is an immense change to California's coastal woodlands. To start
with, all of that dead wood will provide fuel for potentially intense fires. In
addition, sick, dead and dying trees open the door for the spread of other
diseases. That's beginning to happen in some areas, where Rizzo says that a
native pathogen, oak root fungus, is already taking hold.
This doesn't mean that
California's coastal woodlands are on the verge of turning into deserts.
Eventually, some other species will move into the gaps most likely
California bay laurel, Brown says.
Brown's colleague Kyle
Apigian, also of UC Berkeley, suspects this will be bad news for birds. Such
species as oak titmice and chestnut-backed chickadees, which forage heavily on
coast live oak, keep away from bay laurels, Apigian reported.
Ranger John Kolsrud inspects a diseased tanoak in Austin Creek State Park, near Guerneville. Bay laurel trees, like those in the background, play host to a nonfatal form of the sudden oak death fungus.
Nobody's really sure where
sudden oak death originated, although some scientists speculate that it was the
Himalayas. Wherever it came from, most ecologists believe that it is an exotic
species for which California's trees have no native resistance.
Preventing such diseases
from spreading is extremely difficult. Other Phytophthora species have been
truly catastrophic: one caused the potato blight that led to famine in
19th-century Ireland; another is currently devastating entire forests in
The closest relative to
sudden oak death is a type of Phytophthora that causes root rot on Port Orford
cedars in Northern California and Southern Oregon. When scientists learned that
this disease's spores can be transported on muddy truck tires and hiking boots,
the U.S. Forest Service began instituting wet-season road closures. It also
began thinning trees from the most at-risk stands, and started requiring logging
trucks to wash mud from their tires. The disease continues to spread, although
the rate appears to have slowed.
Similar measures might
slow the spread of sudden oak death. In another paper presented at the
Ecological Society meeting in Portland, J. Hall Cushman of Sonoma State
University found that hikers and mountain bikers may be spreading the disease
around California parks. Examining soil samples from a nature preserve north of
San Francisco, Cushman found that trails were heavily laced with Phytophthora
spores, even in regions far from infected trees. In a separate study of 100
square miles of woodlands between Sonoma and Santa Rosa, he found that heavily
used areas were more likely to be infected.
The options for
controlling this aren't good, Cushman says, because they'll provoke conflict
between park managers and outdoor enthusiasts. One choice is to do nothing.
Hikers and mountain bikers might be happy with that in the short run, but in the
long run, it might contribute to the disease's spread.
managers could shut down trails during the wet season, when Phytophthora most
easily spreads: a solution that's guaranteed to be unpopular with park users.
A less Draconian solution
would be to require hikers and bikers to clean shoes and tires when leaving
infected areas. But that would take education, outreach, vigilance and tax
Worse, it's possible that
none of these will work. Cushman's team has yet to address the question of
whether deer, rodents, birds and other animals might be even more effective at
spreading disease spores than people.
UC Berkley ecologist Max
Moritz, however, believes that the future may not be completely bleak. In yet
another study presented at the ecology meeting in Portland, he argued that
controlled burns might someday help prevent spread of the disease.
Ecologists have long
believed that fire suppression has played a role in making ecosystems vulnerable
to disease outbreaks. The argument is that the buildup of excess vegetation
forces plants to compete too strongly for light, water and nutrients, reducing
their ability to fight off infection.
To see whether this
applies to sudden oak death, Moritz compared the locations of known infected
zones to those where fires have occurred since 1950. He found that infections
were strongly concentrated in regions where fire had been absent. Remarkably, he
says, fires that occurred 20, 30, 40 or 50 years ago decades before the
first case of sudden oak death was observed are affecting the current
resistance to it.
Moritz has several
hypotheses for why this might be the case. One is simply the weakening effect of
dense vegetation on plants' disease resistance. Another factor is that
California bay laurels (whose leaves play host to the nonfatal form of the mold)
change their chemistry as trees age. Young trees have leaves that produce large
amounts of aromatic chemicals called phenols, which serve as natural
antibiotics. But as the trees age, the amounts of phenols go down. Fire resets
the cycle by burning out the old bay laurel and allowing it to be replaced by
disease-resistant younger plants.
Another factor is that
soil chemistry changes with the length of time since the last fire. In
particular, fire increases the amount of calcium in the soil, and calcium,
Moritz says, is an important factor in disease resistance.
Rizzo agrees that fire is
probably an important part of the sudden oak death story, and he applauds
Moritz's efforts to figure out the link. He's concerned, though, that the full
story might prove to be quite complicated. His own studies, he says, have
indicated that the healthiest trees are the most susceptible to infection.
"It may turn out that areas that burned 30 years ago are more resistant to
invasion," he says, "but once the cat's out of the bag (i.e., the
disease has become established), fire may or may not help."
People concerned about the
future of California oaks point to other blights that have devastated American
forests. A few decades ago, Dutch elm disease swept across the East and Midwest,
demolishing trees that once shaded small-town lawns throughout the American
heartland. Earlier, chestnut blight wiped out the American chestnut, previously
a dominant species in Eastern hardwood forests.
It's easy to look at these
examples and become depressed. Who wants to lose yet another tree species?
But Rizzo doesn't paint a
totally bleak future. To begin with, he says, oaks won't go extinct. Fungicides
exist that can effectively protect individual trees in back yards, nurseries and
Nor will large chunks of
California be denuded. Wild tanoaks might be in trouble, but coast live oak
appears to be quite variable in its susceptibility to the disease. Some stands
quickly sicken and die; others don't. Why the difference?
"That's the $10,000
question," says Brown.
The woodlands themselves
will persist. Eastern forests are still green, despite the loss of elms and
chestnuts. Let us hope that California's oaks one of the signature species
of the Coast Ranges never join the elm and chestnut on the list of things
that future generations don't even know they're missing.
Lovett is a freelance writer in Oregon.