Phytophthora ramorum

1
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)
2
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
3
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.
4
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.
5
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
6
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 long-distance
pathogen spread to previously uninfested
locations.
7
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.
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.
8
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.
9
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.o
rg.
10
Host Species, part I
California bay laurel California black oak Canyon
live oak Coast live oak Coast redwood Douglas-fir
European yew Holm oak Shreves 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 Winters 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
11
Host Species, part II
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
Arctostaphylos columbiana Arctostaphylos
manzanita Western maidenhair fern California
maidenhair fern Scotch heather Camellia
spp. California coffeeberry Witch
hazel Toyon California honeysuckle False
Solomons 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 Andrews clintonia
bead lily Laurustinus
Host List as of April 2006
12
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 80F, with an optimum temperature
of 68F. 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.
13
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
tanoak-dominated forests in Oregon.
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.
14

• 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

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.
15

• 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

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.
16
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)
17
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
18
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