Phytophthora ramorum
A Short Course
Presented by the
California Oak Mortality Task Force
Part 2 - Symptom
Recognition, Diagnosis, and
Sampling
Background Photo Credits:
Karl Buermeyer, UC Cooperative Extension (forest scene)
Stephen Eales, Plant Health and Seed Inspectorate, DEFRA, UK (mountain laurel)
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
Part 2 - Symptom Recognition,
Diagnosis, and Sampling




Bark Symptoms on Trees
Foliar Symptoms on Wildland and Nursery
Plants
Scouting and Sampling Symptomatic Plants
Review Questions
Part 3 - Regulations and Management
of Phytophthora ramorum
Part 2 - Symptom Recognition,
Diagnosis, and Sampling
Bark Symptoms on Trees
When Phytophthora ramorum spores germinate
on the bark surface of a host tree, hyphae
penetrate the bark. The pathogen then grows in
the inner bark and outer xylem. In response, the
tree “bleeds,” exuding a sticky reddish ooze. This
is the only outward symptom of the disease until
the tree turns brown and dies. In true oaks, these
symptoms are only found within about 8 feet of
the ground. Since other diseases cause similar
symptoms, bleeding alone is not sufficient for
diagnosis.
If the outer bark of an infected tree is scraped
away, cankers can be seen. Generally, red healthy
bark tissue will be separated from brown, dull,
diseased tissue by a distinct black zone line
marking the progression of the disease.
David Chambers,
UC Cooperative Extension
Karl Buermeyer,
UC Cooperative Extension
A common condition in oak trees, known as
“bacterial wetwood,” also causes bleeding
symptoms which can be mistaken for
Phytophthora ramorum. Associated with an
injury or crack, wetwood bleeding is watery,
foul smelling, and usually very copious, unlike
the sticky, spotty oozing caused by
Phytophthora ramorum.
Karl Buermeyer, UC Cooperative Extension
Ambrosia Beetle
Monarthrum species
Oak Bark Beetle
Pseudopityopthorus pubippenis
Once infected by Phytophthora ramorum,
trees are often attacked by secondary
organisms. Ambrosia and oak bark beetles
are attracted to the infected area of the
trunk, and can be detected by small piles
of boring dust (frass) around entry and exit
holes in the tree trunk. Ambrosia beetles
bore deeply into the xylem tissue, resulting
in light colored frass, while oak bark
beetles stay in the outer bark, and can be
detected by reddish frass.
Another secondary organism commonly
associated with Phytophthora ramorum is the
fungus Hypoxylon thouarsianum. It produces
hard, black, round fruiting bodies on the bark
surface, and can be found where there is dead
wood in a tree. Although not unique to
Phytophthora ramorum-infested areas, it is
often quite noticeable due to the large amount
of dead wood available.
Karl Buermeyer, UC Cooperative Extension
Boring beetles and Hypoxylon can
rapidly weaken trees, causing them to
fail. These failures often occur where
the cankers infected the trunk - the
area to which the beetles are
attracted. Ambrosia beetle galleries
can be seen in the picture to the left
where this tanoak failed.
Karl Buermeyer, UC Cooperative Extension
Symptoms on Foliar Hosts
Tanoak (Lithocarpus densiflora)
As mentioned in Part 1 of this course, tanoak is
both a bark and foliar host. This may explain
why it is the most vulnerable to mortality. Bark
infections may or may not cause the oozing
symptoms described above (slide 4). Tanoaks
of all ages can be killed by the pathogen,
whereas true oaks don’t seem to be at risk until
they are about 6” in diameter.
Pavel Svihra, UC Cooperative
Extension
Symptoms present in tanoak that are
not found in true oaks include the
wilting of new spring growth due to
disruption of water transport (shown
above), and necrotic lesions on the
twigs, resulting in the death of the
leaves as the lesions progress along
the twig (photo to the left).
Karl Buermeyer, UC Cooperative Extension
California bay laurel (Umbellularia californica)
In California’s wildlands, the most important
foliar host of Phytophthora ramorum is
California bay laurel. The pathogen sporulates
abundantly on the leaf surface. When it rains,
the built up inoculum is washed or blown off
the leaves onto the soil, other leaves, and the
boles of other trees.
Steve Tjosvold,
UC Cooperative Extension
California bay laurel symptoms include a brown leaf lesion with an
irregular margin, sometimes with a yellow chlorotic band in the live
portion of the leaf, and often separated by a uneven black zone
line. Symptoms usually appear on the lowest part of the leaf, where
water accumulates. Due to abundant sporulation, and the ease of
culturing from these leaves, this tree is used as an early indicator
and risk factor for the disease entering an area. Although infection
can sometimes kill small twigs, Phytophthora ramorum does not
significantly damage California bay laurel.
Toyon Heteromeles
arbutifolia
California buckeye
A number of other native
broad-leaf species harbor
Phytophthora ramorum in
California and Oregon (See
the complete list in Part
1.). Little is known about
the role of these species in
the life cycle and spread of
the disease. The pathogen
is difficult to culture from
many of these species, and
is difficult to diagnose
because of the presence of
other foliar diseases.
Aesculus californica
California honeysuckle
Lonicera hispidula
All photos: Garbelotto Lab, UC Berkeley
Pacific madrone
Arbutus menziesii
Evergreen huckleberry
Vaccinium ovatum
Bigleaf maple Acer
macrophyllum
Wood rose
Rosa gymnocarpa
Ornamental Nursery Hosts
Rhododendrons (Rhododendron spp.)
Rhododendrons are a significant Phytophthora
ramorum nursery host, and in European
gardens are suspected of spreading the
pathogen to nearby trees that were growing
amongst them. Rhododendrons may also play
an important role in spreading the disease in
wildland situations in Oregon.
Steve Tjosvold,
UC Cooperative Extension
Symptoms on rhododendrons
include necrotic areas with
diffused margins that are
similar in appearance on both
sides of the leaf.
Bruce Moltzan, Missouri Department of Conservation
Stem infections can also occur in
rhododendrons. Infection begins either in
the leaf or at bud scars on the stems,
and can travel either from the leaves
through the midveins to the petioles and
stems, or vice versa.
BBA Germany
Alan Kanaskie,
Oregon Dept. of
Forestry
B
A
C
(2nd
artificial
infection
point)
Steve Tjosvold,
UC Cooperative Extension
In the two-week sequence above right, an artificial infection at point
(A) travels down the leaf to infect the stem (B), and then travels
throughout the stem and into other leaves (C).
Camellias (Camellia spp.)
First confirmed as a host of Phytophthora
ramorum in Europe, 2003, camellias are
perhaps the most commonly detected host in
US nurseries. Numerous cultivars of Camellia
japonica, C. sasanqua, C. reticulata, and C. x
williamsii have been found to be infected.
Camellia symptoms are limited to leaves.
Lesions have diffused edges, and appear
primarily at leaf tips and margins. No twig or
branch infections have been observed in
camellias, perhaps because infected leaves
abscise readily when infected. When checking
camellias, look at dropped leaves around the
base of the plant, and for plants that have
thin crowns, as shown to the right.
Cheryl Blomquist,
CDFA
Pieris or Andromeda (Pieris spp.)
In pieris, Phytophthora ramorum causes
leaf necrosis and leaf abscision similar to
that on Camellia. Branch tip dieback is
also found.
Central Science Laboratory,
DEFRA, UK (Crown Copyright)
Jan Hedberg, Oregon Dept. of Agriculture
Viburnum (Viburnum spp.)
In viburnums, Phytophthora ramorum
can cause leaf necrosis, tip dieback,
and cankers. Viburnums can react
similarly to bark hosts, in that the
whole plant can die at once.
Jan Hedberg,
Oregon Dept. of Agriculture
Cheryl Blomquist, CDFA
Central Science
Laboratory, DEFRA, UK
(Crown Copyright)
Other Ornamental Hosts
A number of other ornamental plants
have been found to be susceptible to
Phytophthora ramorum. Most of these
have been found in the UK. Mountain
laurel (Kalmia spp.) is of particular
concern in the US due to its wide
distribution in eastern forests.
Mountain Laurel Kalmia latifolia
Drooping leuthocoe
Leuthocoe fontanesiana
Lilac Syringa vulgaris
Conditions that may be Confused with
Phytophthora ramorum on Ornamentals
Phytophthora ramorum symptoms vary
greatly between and within host species,
so there is no universal symptom
indicating that a plant has the disease.
Therefore, when surveying for the
presence of this pathogen, it is best to
be cautious and sample any suspicious
host leaf or stem lesion.
Tomas Pastalka, CDFA
However, when sampling, rule out plants
that only have sun scald symptoms. This
condition only appears on one side of
the plant (facing the sun), and unlike
Phytophthora ramorum, it tends to avoid
the leaf midvein. The photos to the right
show sunscald on camellia and
rhododendron.
Steve Tjosvold, UC Cooperative Extension
Scouting and Sampling for
Phytophthora ramorum
Whether looking for Phytophthora ramorum in wildlands
or nurseries, it is important to be familiar with host
species symptoms. Diagnostic guides to assist in
identification can be found at www.suddenoakdeath.org.
At this site, review the document “Sudden Oak Death
and Associated Diseases Caused by Phytophthora
ramorum,” by Davidson et al., for information on native
species where Phytophthora ramorum is present in west
coast wildland habitats. It also gives a general overview
of the disease and briefly covers ornamental hosts. For a
more detailed guide to nursery host identification and
management, access “Nursery Guide for Diseases of
Phytophthora ramorum on Ornamentals: Diagnosis and
Management,” by Tjosvold et al.
Surveying in Wildland Areas
When surveying for Phytophthora ramorum, a 100% survey
or strip survey can be used, checking all host species for
symptoms. For larger properties, a 20% strip survey can be
laid out by spacing transects 100m apart, and checking all
host plants within 10m of the line (20m total width). Spacing
the lines closer together will result in progressively more
intensive surveys. Since the objective is to detect the
disease, a suspicious plant just outside the transect border
should be sampled. Lines should be perpendicular to the
general contour of the property (uphill and downhill) so that
different slope positions and draws will not be missed. Note
the location of sampled plants on a map or with GPS so that
they can be relocated if found to be positive.
If surveys are to be done on a state or county level, 1/10 ha
plots can be randomly distributed in areas that are likely to
have host species, as identified by vegetation maps.
Scouting in Nurseries
A nursery layout map that includes the locations of
targeted species is useful to get oriented and develop a
survey strategy. A scouting map includes species and
cultivar names, locations, approximate quantity, and
sources of targeted plants in scouted areas. During the
walk-through, record the date, observations, and sampling
information directly onto the scouting map. The recorded
information should be reviewed and used to develop an
efficient strategy each time the nursery is scouted.
Begin the inspection with an overview of the area from the
crop perimeter or with a quick walk-through. Examine any
symptomatic plants more closely in an attempt to identify
the problem. If symptoms are not apparent, start by
walking a systematic path through the crop. Pay particular
attention to low areas that may collect water, as well as the
perimeter of the nursery if there are host species in the
surrounding landscape.
Bole Sampling for Phytophthora ramorum
Although it is easier to identify, sample, and culture
Phytophthora ramorum on the leaves of some foliar hosts, it is
often necessary to sample from the bark of potentially infected
host trees. While the technique is described here, it is highly
recommended that only trained individuals sample from bark.
Upon locating
symptomatic
tissue…
the canker
is exposed
with a hatchet
or chisel.
Bole Sampling for Phytophthora ramorum
A small section
of the margin
tissue is separated...
and removed
with tweezers.
Bole Sampling for Phytophthora ramorum
The sample is
imbedded in a PARP
plate and sent to the
lab for culturing
and DNA testing.
Photo series by Karl Buermeyer, UC Cooperative Extension
Leaf Sampling for Phytophthora ramorum
Select a fresh, representative sample of symptomatic
plant parts, including some associated leaves and stems.
Be sure to include some healthy tissue along with the
diseased. The sample should be placed in a plastic bag
and labeled with the date, genus, species, cultivar (if
appropriate), and location. The sample should be kept
cool, away from direct sunlight, and delivered to the
diagnostic laboratory within 24 hours. Send samples to
your state’s plant pathology diagnostics laboratory, an
approved private laboratory, or contact your local
Agricultural Commissioner’s office.
Review Questions
1. What is the outward manifestation of Phytophthora
ramorum on bark hosts?
A reddish, sticky ooze from otherwise intact and
healthy looking bark
2. In bark cankers, where is the disease active, and
from where on the tree should someone take a
sample to send in for laboratory testing?
The black zone line surrounding the diseased
tissue (found under the bark)
3. Name two organisms that are secondary
invaders on infected trees, and hasten their
decline.
Hypoxylon fungi and ambrosia beetles
Review Questions, continued
4. Where are symptoms of Phytophthora ramorum most
likely to occur on California bay laurel leaves, and why?
On the lowest most portion of the leaf tissue, as it
hangs, where water accumulates
5. Name two reasons why some hosts of Phytophthora
ramorum are not good for sampling or indicating the
presence of the disease.
It can be difficult to isolate the organism from
leaves;
Some hosts get several other leaf diseases
6. What is the single most distinguishing characteristic
of Phytophthora ramorum lesions on the leaves of most
ornamental plants?
Diffused margins of necrotic areas
Review Questions, continued
7. What genus of ornamental plant can be infected near
the base, so that the whole plant dies at once?
Viburnum
8. Why might it be difficult to find symptomatic leaves
on a camellia or pieris, even though it is infected?
Infected leaves often abscise quickly
9. Name two high risk areas for Phytophthora
ramorum in a nursery.
Low spots where water collects;
Along the perimeter if there are infected hosts
outside the nursery