 |
Shown
above are the wilting branch tips of a Douglas fir tree
infected with the Sudden Oak Death pathogen. Photo
Matteo Garbelotto, UC Berkeley, and David Rizzo, UC Davis |
BERKELEY
- Two of California's most highly prized trees —
coast redwood and Douglas fir — are susceptible to Phytophthora
ramorum, the pathogen that causes Sudden Oak Death, University
of California researchers have confirmed. Over the past seven
years, Sudden Oak Death, a highly contagious fungus-like disease,
has killed tens of thousands of oaks and tanoaks along the
northern coast of the state.
Researchers
from UC Berkeley and UC Davis have isolated living cultures
of P. ramorum from the branches and needles of coast redwood
and Douglas fir saplings that had shown symptoms of infection.
The researchers first announced the discovery of P. ramorum
DNA in the trees earlier this year, but couldn't confirm that
the pathogen was causing infection until living cultures were
successfully grown from the field samples.
It is not
yet clear how seriously the disease will impact California's
coast redwood and Douglas fir trees, which are ecologically
and economically vital to the state, particularly to the timber,
nursery, landscape and construction industries.
The infected
redwood saplings were found at Jack London State Park in Sonoma
County and Henry Cowell State Park in Santa Cruz County. The
infected Douglas firs were found at another site in Sonoma
County.
The researchers
also conducted DNA tests on diseased sprouts growing from
the base of mature redwood trees in Marin, Alameda and Monterey
counties. The presence of the pathogen in the sampled trees
has been strongly suggested by repeated positive DNA identification.
These new
test results will be published online in October in the journal
Plant Disease.
The number
of identified species susceptible to the Sudden Oak Death
pathogen has steadily grown since the disease was first reported
in Marin County in 1995. With the addition of coast redwood
and Douglas fir, there are now 17 known species worldwide
susceptible to P. ramorum. Sixteen of them are found in California,
including the madrone, bay laurel and buckeye. One additional
host species, viburnum, has been found only in Europe.
"It seems
that some species are able to tolerate the pathogen better
than others," said David Rizzo, associate professor of plant
pathology at UC Davis. The research was a collaboration between
the laboratories of Rizzo and Matteo Garbelotto, adjunct assistant
professor of ecosystem science and a cooperative extension
specialist at UC Berkeley's College of Natural Resources.
"We see
a whole range of symptoms in the field, from nasty cankers
on the trunks of oaks to minor spots on the leaves of the
buckeye," said Rizzo.
It is unclear
what the new findings mean for the health of redwoods and
Douglas firs in the long run, said the researchers. "Since
we have not seen evidence of disease symptoms or death from
the pathogen in large, mature redwood or Douglas fir, we cannot
say what the effects of the infection will be long-term,"
said Garbelotto.
Garbelotto
noted that symptoms have been detected only on the needles
and very small branches of redwoods. "What was somewhat surprising
is that, for redwoods, we found the pathogen in all the places
we checked," he said. "In contrast, infected Douglas fir saplings
were found at only one site — in Sonoma County —
but they seemed to show a stronger reaction to infection.
The Douglas fir saplings were right under heavily infected
bay laurel trees. We don't know if there was something unique
about that site that made the Douglas fir more susceptible
to infection than in other areas."
In addition
to checking diseased trees in the field, the researchers conducted
a battery of lab tests to see how P. ramorum would affect
healthy trees and to confirm that the pathogen was the cause
of the symptoms observed in the field.
|
Shown
above are lesions and discoloration on individual needles
of a redwood sapling that has been infected with the Sudden
Oak Death pathogen. New foliage does not appear to be
affected. Photo Matteo Garbelotto, UC Berkeley, and
David Rizzo, UC Davis |
In one
test, they exposed 20 redwood seedling stems to the pathogen
and compared them to unexposed seedlings. After six weeks,
the pathogen-exposed seedlings exhibited lesions from infection,
unlike their unexposed counterparts. Branches on many of the
infected seedlings became yellowed and discolored, while branches
on the control seedlings retained their green color.
The same
series of tests were conducted for Douglas fir seedlings,
which generally developed larger lesions than their redwood
counterparts after exposure to the pathogen. Like the redwoods,
Douglas firs that were exposed to P. ramorum developed lesions
from infection.
"We essentially
confirmed in the lab what we observed out in the field," said
Rizzo.
The UC
researchers say it is unclear what caused the dieback of a
mature redwood tree in Mill Valley, widely reported earlier
this year as having been infected by P. ramorum. Although
the tree stump tested positive for P. ramorum using DNA tests,
Rizzo and Garbelotto found that the tree appeared to have
been plagued by three other fungal infections.
"The tree
had already been cut down to a stump by the time we conducted
the tests, so we don't know if P. ramorum came in after the
tree was cut or if the pathogen had infected the tree when
it was still alive," said Rizzo. "It is impossible to say
what ultimately was wrong with that particular redwood tree."
The discovery
of P. ramorum in the redwood — one of California's most
treasured symbols — hits a sensitive chord for many in
the state. The majestic trees can reach heights of more than
350 feet and live to be 600 to 2,000 years old.
"It may
take years before we can start answering questions about the
ecological impacts of the disease on coast redwood and Douglas
fir," said Rizzo. The researchers emphasize the need for further
study, noting that they have only been studying the biology
of P. ramorum in redwoods and Douglas firs for several months.
This research
was funded by the USDA Forest Service Pacific Southwest Research
Station, the USDA Forest Service Forest Health Management
and the Gordon and Betty Moore Foundation.
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