- A common nursery plant may lead to increased complications
and possible new management practices in the fight to halt
Sudden Oak Death, a highly contagious fungal disease that
is killing California oak trees, University of California
researchers announced today (Wednesday, Jan. 10).
In a breakthrough
in the study of the disease, UC researchers discovered that
the rhododendron, a popular ornamental plant, can be infected
by the same fungus that is causing the oak disease. The fungus
has infected European rhododendrons and, as of yesterday,
the researchers confirmed that it also is affecting California
rhododendrons, suggesting a transcontinental link. Finding
this relatively new fungus in two different parts of the world
- and in two species - is unusual, the researchers said.
discovery gives insight to the potential origin and transmission
of this pathogen and may suggest new ways of spread. Previously,
the pathogen only was known in three other California oaks
- tanoaks, coast live oaks and black oaks.
know we have a host that could have carried the fungus a long
way," said Matteo Garbelotto, a plant pathologist and adjunct
professor in the Department of Environmental Science, Policy
& Management in UC Berkeley's College of Natural Resources.
"People don't really export oak trees across state lines or
around the world," he said, "but they export rhododendrons."
may have a major impact on how scientists manage the disease.
Co-investigator David Rizzo, assistant professor of plant
pathology at UC Davis, said it may result in new restrictions
on the rhododendron nursery industry. "The big concern is
that someone will transport a sick rhododendron to a place
where there are susceptible oak species," he said.
came when a Clive Brasier, a British researcher who had visited
UC Berkeley last summer, later noticed in Europe a fungus
that looked like one he'd seen in Garbelotto's lab. The European
fungus had been found on rhododendrons in Germany and the
Netherlands. Brasier contacted the UC scientists, and researchers
from all four countries determined together that the European
rhododendron fungus was identical to the California oak-killing
agent. This finding established that the fungus is not exclusively
found in California and has important implications for international
and Garbelotto needed more proof to confirm the link between
the two plant species, and yesterday they got it. Rizzo and
Steve Tjosvold, a Santa Cruz County farm advisor, found the
fungus in a rhododendron taken from a Santa Cruz County nursery,
and Garbelotto confirmed with DNA analysis that it was the
same fungus killing the oaks.
don't know whether the disease was transmitted from California
to Europe, or vice versa, or whether it traveled to both places
from a third, as yet unknown, location. The fungus, first
noted in European rhododendrons in 1993, has not been found
in European oaks. However, European scientists are concerned
that the disease will spread to European oak forests, particularly
those in areas with a climate similar to that of California.
discovery of the mysterious oak-killing illness in California
in 1995, researchers have been scrambling to understand the
disease and design strategies to stop its spread. It is not
known if the fungus recently was introduced into California,
or if it is a native fungus that recently became a tree-killer
because of environmental changes. Tens of thousands of oak
trees have succumbed to the disease, and the researchers have
reported up to 80 percent mortality in some infected groves.
molecular sleuthing, Rizzo and Garbelotto determined that
the disease was caused by a never-before-seen strain of fungi
from the genus Phytophthora (Phy-TOFF-thoruh). A relative
belonging to this 60-member group caused the Irish potato
famine, and another relative is linked to the dieback of cedar
trees in Northern California and southern Oregon, eucalyptus
trees in Australia and oaks in Mexico, Spain and Portugal.
Sudden Oak Death has been reported from Sonoma Valley in the
north to Big Sur in the south, a 190-mile range, as well as
east to the Napa County border, about 25 miles inland. The
hardest hit counties are Marin and Santa Cruz. The disease
affects tanoak (Lithocarpus densiflorus), coast live oak (Quercus
agrifolia), and California black oak (Quercus kelloggii) found
along the coastal belt in California. To date, the disease
has not been found in other oaks such as blue oak or interior
is alarming, researchers say, for its potential to disrupt
the coastal forest ecosystems. Oaks provide habitat for wildlife
and a food supply for small mammals and are frequently planted
as ornamentals in gardens and parks. Additionally, downed
dead trees create a fire hazard from the resulting buildup
of dry fuel.
similarities between the disease in oaks in California and
rhododendron in Europe. In both cases, the fungus attacks
above ground parts of the plants. In oaks, the fungus enters
through the trunk and causes the formation of bleeding cankers
on the trunk. On rhododendron plants, the fungus causes similar
cankers and spreads from twig tips to the stem base, according
to the European researchers.
have notified agricultural and ecosystem managers in the affected
areas of the rhododendron discovery. Research is underway
to determine if native rhododendrons - those that have not
been imported - are being infected. Research also is being
conducted to determine how many other susceptible species
may be affected by the fungus.
your rhododendrons for cankers (sores) that spread from
twig tips to the base of the stem.
not transport rhododendrons out of areas where Sudden
Oak Death has occurred. This area includes a 190-mile
stretch between Big Sur and Sonoma County, as well as
east to Napa Valley. As yet, Sudden Oak Death has not
been observed in the East Bay.
not transport wood products such as mulch, bark or firewood
out of Sudden Oak Death areas. Sudden Oak Death occurs
in tanoaks, coast live oaks and black oaks. It does not
occur in blue oaks or interior live oaks.
visiting Sudden Oak Death areas, clean the soil from your
shoes and remove any soil from vehicle tires and sporting
or construction equipment.
your oak tree or rhododendron that you think may be infected,
contact your county agricultural commissioner.