Phytophthora ramorum
A Short Course
Presented by the
California Oak Mortality Task Force
Part 1 - Introduction to
Phytophthora ramorum
and Sudden Oak Death
Background Photo Credits:
Karl Buermeyer, UC Cooperative Extension (forest scene)
Jan Hedberg, Oregon Department of Agriculture (viburnums in nursery)
Steve Tjosvold, UC Cooperative Extension (rhododendron)
Course Outline
Part 1 - Introduction to Phytophthora
ramorum and Sudden Oak Death




History
Biology
Impacts - Past, Present, and Future
Review Questions
Part 2 - Symptom Recognition,
Diagnosis, and Sampling
Part 3 - Regulations and Management of
Phytophthora ramorum
Part 1 - Introduction to Phytophthora
ramorum and Sudden Oak Death
History
In the mid-1990s, large
numbers of tanoaks and coast
live oaks began to die in the
coastal counties of central
California.
The cause of death was
unknown, and due to the
rapid browning of the foliage,
the condition was named
“Sudden Oak Death.”
In July 2000, scientists
discovered the cause of
Sudden Oak Death to be a
newly identified species of
Phytophthora.
This pathogen was the same as
one that had been observed on
rhododendrons and viburnums in
nurseries and gardens in Europe
since 1993. The new species was
named Phytophthora ramorum.
Rizzo Lab,
UC Davis
As of April 2006, U.S. wildland
Phytophthora ramorum
infestations have been
confirmed in 14 California
counties and Curry County,
Oregon.
Once considered only a forest
disease in North America,
nursery detections are now
made annually in dozens of
nurseries throughout the nation.
Distribution List as of April 2006
From 1993 to 2003, Phytophthora
ramorum in Europe appeared to be
limited to nursery and garden
situations. Affected countries included
Belgium, Denmark, France, Germany,
the Netherlands, Norway, Poland,
Republic of Ireland, Slovenia, Spain,
Sweden, and the UK.
However, in the fall of 2003, several
tree species in the Netherlands and UK
that were adjacent to diseased
rhododendrons became infected. Along
with the increasing number of nursery
finds in North America, this raised
concern that nursery host movement
and out-planting could facilitate longdistance pathogen spread to previously
uninfested locations.
Plant Protection Service, The Netherlands, 2003
Biology
Phytophthora ramorum belongs in the
kingdom Chromista (Stramenopiles),
and is related to diatoms and brown
algae. The genus Phytophthora (“plant
destroyer”) has more than 60 species,
many of which are virulent plant
pathogens. They are Oomycetes, or
“water molds;” water is crucial to their
lifecycle and management.
Phytophthora ramorum is
microscopic. It develops hyphae
(collectively called mycelium),
which grow through bark and
leaf tissue. It also produces
asexual reproductive structures
called sporangia and
chlamydospores. Sporangia
release zoospores which have
two flagella that propel them
through water.
Jennifer Parke, Oregon State University
Rizzo Lab, UC
Davis
Chlamydospores are
hardy structures that
protect the pathogen
during adverse
conditions, such as
heat and drought, but
little is known about
their role in disease
progression or the
conditions leading to
their germination.
Phytophthora ramorum is heterothallic; it requires two
different mating types (A1 and A2) for sexual
reproduction through structures called oospores.
In Europe, the Phytophthora ramorum population is
primarily the A1 mating type. While only the A2 mating
type is found in North America, the European A1 mating
type has been intercepted and destroyed in a small
number of Pacific Northwest nurseries. So, while sexual
reproduction has not been observed outside of the
laboratory, there is concern about the two mating types
coexisting in nature, as sexual reproduction could occur
and potentially produce more virulent and adaptable
pathogen strains.
Phytophthora ramorum grows and sporulates on the
surface of leaves and twigs of a number of plant species
known as “foliar hosts.” It can also grow into the
cambium and outer xylem of “bark hosts” and
effectively girdle the tree; this is “Sudden Oak Death.”
Thus, the same pathogen causes two different diseases.
Slide 10 is a list of the known Phytophthora ramorum
hosts. Plants that may be killed by the pathogen are
underlined.
There are numerous species, hybrids, and cultivars of
ornamental plants that have been found to be infected
by Phytophthora ramorum. For a complete up-to-date
list by species and hybrids (not cultivars), consult the
California Oak Mortality Task Force website:
www.suddenoakdeath.org.
Host Species, part I
California bay laurel
California black oak
Canyon live oak
Coast live oak
Coast redwood
Douglas-fir
European yew
Holm oak
Shreve’s oak
Southern red oak
White fir
Grand fir
Red fir
Striped bark maple
Evergreen maple
Planetree maple
Horse chestnut
Portuguese laurel
cherry
European turkey oak
Sessile oak
Northern red oak
Pacific yew
Yew
California nutmeg
Strawberry tree
Sweet chestnut
Winter’s bark
European beech
Bigleaf maple
California buckeye
Madrone
Oregon ash
Bay laurel
Southern magnolia
Star magnolia
Loebner magnolia
Saucer magnolia
Michelia doltsopa
Michelia maudiae
Michelia wilsonii
Roble beech
Victorian box
Cascara
European ash
Griselinia
Tanoak
Persian ironwood
Continued on next slide…
Host Species, part II
Arctostaphylos columbiana
Arctostaphylos manzanita
Western maidenhair fern
California maidenhair fern
Scotch heather
Camellia spp.
California coffeeberry
Witch hazel
Toyon
California honeysuckle
False Solomon’s seal
Red tip photinia
Mountain Andromeda
Himalaya Andromeda
Japanese Pieris
Formosa firethorn
Rosa ‘Meidiland’
Vine maple
Rugosa rose
Salmonberry
Goat willow
Poison oak
Redwood ivy
David Viburnum
Fragrant Viburnum
Wayfaringtree Viburnum
European cranberrybush
Viburnum
Burkwood Viburnum
Viburnum
Prague Viburnum
Alleghany Viburnum
Ardisia
Spicebush
Andrew’s clintonia bead lily
Laurustinus
California hazelnut
California wood fern
Spreading euonymus
Salal
Hybrid witchhazel
Chinese witchhazel
Mountain laurel
Fetterbush
Drooping leucothoe
Sweet Cicely
Osmanthus
Rhododendron spp.
Wood rose
Lilac
Western starflower
Evergreen huckleberry
Bodnant Viburnum
Doublefile Viburnum
Host List as of April 2006
The pathogen spreads primarily through sporulation on
leaves and twigs of foliar hosts; little sporulation has been
observed in bark hosts. So, although infections may not
cause significant damage to foliar hosts, they are vital to
the life cycle and spread of the pathogen.
Phytophthora ramorum exists within
a temperature range of 36 to 80°F,
with an optimum temperature of
68°F. Spore structures form on foliar
hosts in as little as 24 hours after a
wet period. Spores are then
transported in water droplets to the
soil, the bark of surrounding trees,
and other leaves. In California, the
organism sporulates prolifically on
California bay laurel (Umbellularia
californica). Host plant proximity to
infected California bay laurel is
considered the highest risk factor for
infection.
Steve Tjosvold, UC Cooperative Extension,
2004
Impacts of Phytophthora ramorum-caused diseases
Since it was first noticed, Sudden Oak
Death has caused the death of tens of
thousands of tanoaks and true oaks. The
disease has impacted California coastal
evergreen forests, redwood forests with
tanoak understories, and tanoakdominated forests in Oregon.
Karl Buermeyer, UC Cooperative Extension,
2002
Preliminary research indicates that resistance to the disease is
present in all bark hosts, but more so in coast live oak than in tanoak.
In limited populations of tanoak that are geographically isolated,
almost total mortality has been observed.
The spread of Phytophthora ramorum in natural situations appears to
be limited to moist climates with moderate temperatures and the
presence of foliar hosts. Yet, nurseries create their own environmental
conditions and grow host species that would not naturally occur in the
surrounding environs. Therefore, nursery infestations are not limited
to naturally infested regions.
Long-term wildland impacts may include:
 Visual impacts from dead trees and altered
forest canopies
 Altered ecosystems due to loss of important
trees
 Increased fire hazard from dead, woody
material
 Shortages of food and habitat for wildlife
 Water quality impacts from loss of shade and
increased run-off
 Financial impacts of mitigation and quarantine
efforts
Karl Buermeyer, UC Cooperative
Extension, 2004
Homeowners in urban/wildland interface areas have suffered
aesthetic and property value loss as landscape trees have been
killed. These trees can become hazardous in a matter of months, due
to failures resulting from decomposition by secondary insect and
fungal organisms. Homeowners or public agencies incur substantial
costs to remove these trees.
The recent increase in nursery infestations
has greatly impacted the nursery industry in
Europe, Canada, and the U.S. monetary
impacts include:
 Phytophthora ramorum nursery and plant
shipment inspections, and disruption of
shipment schedules
 The implementation of mitigation measures
 The destruction of plants in infected
nurseries
 Loss of export potential due to quarantines
Canadian Food Inspection Agency,
2003
Trees close to infected rhododendron plantings in European gardens
recently became infected by Phytophthora ramorum, demonstrating
the potential for the pathogen to spread through the nursery trade and
into natural settings. Two species infected in this way were northern
red oak (Quercus rubra) and southern red oak (Q. falcata). Both are
native to the eastern U.S. and grow in forests with foliar host
understories, such as rhododendron and mountain laurel, in climates
favorable to Phytophthora ramorum. Other infected species, such as
European beech (Fagus sylvatica) and horsechestnut (Aesculus
hippocastanum) are commonly planted as ornamentals worldwide.
Review Questions
1. Phytophthora ramorum causes two basic diseases.
What are they?
Bark cankers on oaks, tanoaks, and other trees
(“Sudden Oak Death”);
Foliar and twig blight on a number of other plants from
a wide range of families (“Ramorum Blight”)
2. What environmental conditions are most favorable
for the spread and survival of Phytophthora ramorum?
Moderate temperatures (~68° F) with ample moisture
3. What tree species most readily succumbs to P.
ramorum?
Tanoak (Lithocarpus densiflora)
Review Questions, continued
4. What is the biggest concern about Phytophthora
ramorum in nurseries?
It is a potential vector for long-distance spread of the
disease into wildlands
5. What limits the genetic diversity and adaptability
of Phytophthora ramorum?
Lack of sexual reproduction
6. Why does Phytophthora ramorum do poorly in
dry conditions?
Free water is needed for spores to develop and move
Review Questions, continued
7. What costs, specific to the nursery industry, have been
incurred as the result of Phytophthora ramorum?
• Implementation of inspections and mitigation
measures
• Loss of export markets
• Destruction of infected plant blocks
8. Why is the eastern U.S. considered a risk for
natural infestations of Phytophthora ramorum?
There are susceptible red oak species with foliar
host understories
9. Why is Phytophthora ramorum not as geographically
limited in nurseries as in wildlands?
Nurseries create their own climatic conditions and
host species distribution