Oak disease variant looms as new threat
Third strain may be offspring of U.S., European types
- Peter Fimrite, Chronicle Staff Writer
Saturday, January 22, 2005
A previously unknown strain of the tree-killing disease known as sudden
oak death has been found in a nursery in Washington state, a possible
mutant child of the fast-spreading pathogen. The discovery means that
the European version of the disease has not only found its way to America,
but may have mated with its California counterpart, which has killed
tens of thousands of oak trees in the state. "We detected a third
strain with traits from both the U.S. and European strains," said
UC Berkeley forest pathologist Matteo Garbelotto during the three-day
Sudden Oak Death Science Symposium in Monterey. "It has some genetic
traits
in the DNA that we've never seen. It's a unique strain."
The fear is that the product of any such union could end up being a fungal
version of Rosemary's baby. "The obvious risk," said Jonathan
Jones, who manages the sudden oak death program for the U.S. Department
of Agriculture's Animal and Plant Health Inspection Service, "is
that there could be sexual recombination, and we could
end up
with something worse than what we have."
The symposium, which ended Friday, marks only the second time the world's
top sudden oak death scientists have been brought together to discuss
the disease, known scientifically as Phytophthora ramorum. Garbelotto,
who helped discover Phytophthora ramorum and has been one of the top
scientists in the field, said there is no reason at this point to be
alarmed about the new strain, which, he said, has "not been particularly
aggressive
on oaks."
It was found over the summer on a plant in a nursery where P. ramorum's
American and European mating types -- the equivalent of a male and female
of
the
same species -- were known to exist.
The European strain has been deadly to England's ubiquitous rhododendrons
and beech trees, but it wasn't until 2003 that it was detected in a nursery
in British Columbia. It has since been found in nurseries in Oregon and
Washington.
The discovery of the third strain could mean the American and European
types mated and produced this new strain, but Garbelotto said that is
not necessarily the case. "One hypothesis is that it is the result
of recombination," he said. "The other hypothesis is that it's
a representative of the species that predates the split between the European
and U.S. mating types and it was introduced separately. It's interesting
because it offers us another clue into the origin
of this
Phytophthora species. Maybe we can find out where this plant came from
and trace the disease back." Scientists have long suspected that
the pathogen was an exotic species introduced to America and Europe from
some far off place, most likely Southeast Asia. Genetic evidence collected
as a result of the sequencing last year of the
P.
ramorum genome, supports that theory. In fact, said Garbelotto, the California
outbreak appears to have come from a single alien microbe that
hitched a
ride to the Golden State, cloned itself millions of times and then did
its
dirty
work.
"
All the evidence points to the fact that it was introduced," Garbelotto
said. "It doesn't have any genetic features of a native species.
There is very little genetic diversity in the species in North America.
They are essentially clones. In other words, everything can be traced
back to one individual." None of which offers any relief to the
nursery industry, whose earnings suffered a near-meltdown last year,
after the microbe was found in ornamental plants shipped from a Southern
California nursery. The infestation led to a nationwide quarantine of
nursery products from California. A recent federal
order
extended the quarantine to include Oregon and Washington. It is all costing
the nursery industry millions of dollars in lost sales revenue and inventory
destruction. Garbelotto and others expressed support at the conference
for measures that would halt shipments of nursery plants and material
in favor of seeds or cell cultures. "This situation is a good example
of why the movement of plants should be checked," he said. "Almost
every major plant and tree disease we've ever had
was
brought here by bringing plants and plant material."
Among the diseases that are believed to have been spread by the importation
of exotic plants was the chestnut blight, which killed entire forests
of American chestnut trees, and Dutch elm disease, which obliterated
American elms
in the northeast United States.
The Dutch Elm Phytophthora was, in turn, exported to England, where it
mutated into a more virulent form and wiped out elm trees throughout
the United Kingdom starting in the 1960s. The apparent movement to cut
off the ornamental plant trade was met with dismay by the nursery industry. "There
is a reason why California is the biggest nursery state in the
country: We grow and sell plant material that beautifies the environment," said
Steven Knudsen, who owns Knudsen Nursery Inc., an Oakdale (Stanislaus
County) business started by his father and uncle. "To suggest we
not sell plants across
the
state is, well, in my opinion it would shut down the entire industry." Some
researchers at the conference said that sudden oak death is merely a
symptom of worldwide forest decline and that P. ramorum has always been
around
to
attack weakened trees.
Lee Klinger, an independent scientist from San Anselmo, said the real
culprits are mosses and acidification due to, among other things, suppression
of
forest fires and the extermination of California Indians who once managed
the
oak
savannas.
His contention that P. ramorum has always existed, however, seems to
be directly refuted by the genetic evidence presented by Garbelotto.
And while everyone is talking about sudden oak death, a new, even more
virulent Phytophthora called kernoviae has emerged in Britain. Stephen
Hunter, the head of plant health for the Department for Environmental
Food and Rural Affairs in Britain, said the new, unrelated microbe has
attacked historic gardens near Cornwall elsewhere in southwest England,
where
it is
threatening to wipe out sparse woodlands.
"
The disease itself seems to be progressing more rapidly than ramorum," Hunter
said. "There is a lot of concern." E-mail Peter Fimrite at
pfimrite@sfchronicle.com.
For more information please contact
Dr. Gary Chastagner
WSU Puyallup Research and Extension Center
7612 Pioneer Way E.
Puyallup,
WA 98371-4998
Phone: 253-445-4528
E-mail: chastag@wsu.edu
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page last modified on
January 27, 2005
