Introduced Species Summary
Oak Death (Phytophthora ramorum)
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Sudden Oak Death
Name: Phytophthora ramorum
Phylum or Division:
(Note that the Phytophthora genus has approximately 60 strains,
some of which are the known causes for the Irish potato famine, the dieback
of cedar trees in Northern California and southern Oregon, death of eucalyptus
trees in Australia, and oaks in Mexico, Spain and Portugal.)
are currently 17 known species of plants in the United States and 2 in Europe that have
been confirmed as hosts for Phytophthora ramorum.
The symptoms vary in identified plants. In
live oaks (black, coast live oak, shreve’s oak)
the first symptom is thick, dark reddish-brown sap oozing from the trunk.
In tanoaks, the first sign is dropping of new leaf growth. As the weakened
trees are in later stages of decline, decaying fungi are seen on the trunks. Death may occur in oaks and tanoaks within several
weeks. In redwoods and Douglas fir, symptoms
of the disease are yellowing and discoloration of the needles and small branches. Symptoms in other plants
include leaf spots, twig die-back and stem cankers.
As these symptoms are similar to those caused by other Phytophthora
species, laboratory tests of DNA samples are required to confirm the presence
of P. ramorum.
(For photographs of symptoms
in various plant species refer to http://cemarin.ucdavis.edu/symptoms.html)
In 1995, a large number
of tanoaks were observed dying in Mill Valley, Marin County, California. The
disease was also noted in Western Europe in rhododendrons as early
as 1993. The pathogen was identified in July
2000 by researchers from the University of California.
Phytophthora ramorum has been identified in 17
known species of plants in 12 counties and eight state parks in California. The
geographic distribution encompasses an area 190 miles by 25 miles and includes
the following counties in California: Alameda, Contra Costa, Humboldt, Marin,
Mendocino, Monterey, Napa, San Mateo, Santa Clara, Santa Cruz, Solano and Sonoma. The counties that have been
most affected are Marin and Santa Cruz. It
was recently cited in the southern portion of Oregon. The disease has also been
identified in rhododendrons and viburnum in Europe, (primarily Germany, the Netherlands, the UK and Poland).
(For a map of the current distribution
in the United States refer to http://kellylab.berkeley.edu/SODmonitoring/whereisSOD/movie/default.html )
Site and Date of Introduction:
Sudden Oak Syndrome was
first noted in Mill Valley, Marin County, California in 1995.
The disease was later identified as the same disease occurring in
nursery plants in Europe as early as 1993. It is believed
that the transport of these garden plants is the most probable introduction
of the disease. Scientists do not know if the disease was transported from
Europe to the United States, or vice versa, or to both
places from a third, unknown location.
Mode(s) of Introduction: Phytophthora
ramorum, the organism that causes Sudden
Oak Death, is spread through spores and cysts. These
spores and cysts can be carried in the plant material of hosts. The host will release the spores, which can travel
in moist soil and through the air. The spores
can accumulate in dead leaves beneath affected trees, which are easily spread. The spores can also be tracked to other areas via
people and animals.
(a detailed graphic of the
ecological spread of the disease can be viewed at http://cemarin.ucdavis.edu/ecology.html
Reason(s) Why it has Become
Established: It is unclear how the disease
became established and why it is fatal to some plants and not others. While it was probably introduced into the US, it could also be a native
pathogen that evolved in response to environmental changes.
The death of trees is a
normal process in a forest ecosystem. Trees die
as a result of both abiotic and biotic factors, and in many cases, a combination
of both. Trees that are disposed to stress from
adverse environmental factors (such as drought, air pollution, compacted
soil) are more susceptible to biological factors (such as disease and pests). However, with Phytophthora
ramorum, it does not appear that any pre-disposing
stress is necessary for infection as the disease infects and kills healthy
trees. The aggressive disease has an 80% mortality rate of infected oaks.
This alarming level of decline has the potential to severely disrupt the
coastal forest ecosystems. Oaks provide habitat
and a food supply for a variety of wildlife. In
addition, downed trees create a fire hazard.
Benefit(s): Pathogens are beneficial in
the continued growth of the forest. The death
of weak trees can improve the overall health of the forest. In addition, death of trees in a dominant species
can allow the regeneration of other species, resulting in increased diversity
of plant life.
Threat(s): As Phytophthora
ramorum appears to be an introduced species
with no known cure, the full ecological impact is yet unknown. It is anticipated that Sudden Oak Death will spread
to more areas of California and potentially other areas
of the United States. Two of the species currently identified as hosts,
pin oak and northern red oak, are widespread throughout the Eastern and Midwestern
United States. The US Forest Service recently
declared these areas as high risk. Since the disease appears to affect some
tress and plant species more severely than others and continues to inhabit
new hosts, controlling the disease could be problematic.
The economic impacts of the disease have already been felt through
the quarantine of popular nursery plants exported from California. It
is still unclear what impact the disease will have on redwoods and Douglas firs, both of which have a
substantial economic impact on the California nursery trade. Douglas firs are a major timber and
Christmas tree species (estimated at one billion dollars).
Redwood is also valued as timber and mulch for landscaping.
Control Level Diagnosis:
Control Method: Quarantine regulations designed
to slow the artificial spread of Phytophthora
ramorum have been implemented by various
state and federal agencies including the California Department of Food and
Agriculture, the California Department of Forestry and Fire Protection, Country
Agricultural Commissioners, and the US Department of Agriculture. Interstate
movement of all confirmed hosts from quarantined areas
have been imposed. Regulated materials
include forest stock (including nursery stock), bark chip, and mulch. In addition, several counties in California have implemented hazardous
tree assessment, removal, and restoration plans. As there is known cure for
the disease, it is recommended that trees appearing to be infected are closely
monitored. No action should be taken until tests
have been conducted to confirm the presence of Phytophthora
trees may escape the disease and those that do not survive should be examined
by certified arborist to determine the appropriate course of action. The movement of infected leaves, wood and soil should
also be prevented by humans and animals. People
who have been in infected areas should clean and disinfect their shoes and
(A complete list of quarantine
regulations can be obtained through http://www.aphis.usda.gov/ppq/ispm/sod/
Last Edited: February 17, 2003