Download Recognizing symptoms of Phytophthora ramorum, a new pathogen

California Oak Mortality Task Force and the
USDA Forest Service, Pacific Southwest Region, State and Private Forestry, Forest
Health Protection
Recognizing symptoms of Phytophthora ramorum,
a new pathogen causing Sudden Oak Death
Identifying Phytophthora ramorum on the currently recognized 10 hosts is not easy or clear
cut. There are limited distinctive, diagnostic symptoms on many of the hosts so confirmation
in a laboratory is required. The purpose of this handout is to provide a few pointers and
photos to use as a guide to recognizing symptoms of Phytophthora ramorum infection. Even
experienced researchers can’t always tell whether a declining tree is infected with
Phytophthora ramorum.
If you need assistance with diagnosis or sampling of Sudden Oak Death, contact the local
office of your county Agricultural Commissioner, UC Cooperative Extension, or California
Department of Forestry and Fire Protection. Useful information may be found at the California
Oak Mortality Task Force website www.suddenoakdeath.org. Sampling information is contained
in Sudden Oak Death: Diagnosis and Management, (Pest Alert 5) March 2001, Marin County
University of California Extension.
Recommended reading:
Oaks of California by, Bruce M. Pavlik, Pamela Muick, Sharon G. Johnson and Marjorie Popper.
Published by Cachuma Press and the California Oak Foundation. 1991. Available from the CA
Oak Foundation.
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Symptoms of Phytophthora
ramorum infection on oaks
Figure 2.
- Coast live oak (Quercus agrifolia),
- California black oak
(Quercus kelloggii),
- Shreve oak
(Quercus parvula var. shrevei)
These oaks, all in the red oak group,
show similar symptoms due to infection
by P. ramorum. Typically, the first
symptom is bleeding from the lower bole
to 6 feet or higher. You may see dead
moss in areas where the ooze has killed
it. The ooze may be dried up and
hardened (Figure1). It can dry up
completely and disappear or the outside
bark may remain darkened. Following
rainstorms, the bleeding often is washed
off and difficult to find.
Figure 1. Evidence of previous bleeding
on coast live oak.
Photos by Joseph O’Brien, USDA Forest
Service.
The crown symptoms on oaks can be a
generally thinning crown, or a complete
crown color change – where all the
leaves turn brown over a few weeks. In
the early stages of disease the tree may
have a healthy appearing crown. In
later stages of decline the trees often are
covered with sawdust and frass due to
bark beetle attack. Hypoxylon, a
common saprophtytic fungus may fruit
abundantly on the dead portions of the
tree.
If you chip away at the edge of the
blackened or oozing area dead patches
of the outer bark surrounded by black
zone lines will be present. (Figure 2.)
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Figure 3. Coast Live Oak dying from Sudden Oak Death in a mixed-evergreen
forest in Marin County. Photo: P. Svihra, UCCE, Marin County.
On oaks, the leaves are usually not spotted and do not show other symptoms of
damage until the tree dies and they turn uniformly brown.
Note that P. ramorum has never been found on an oak tree in the middle of a
watered lawn. There are other Phytophthora species that infect oaks and other
fungi such as Armillaria that are common on watered oaks. P. ramorum is found
in forests and in the urban-wildland interface where remnant forests are still
present in what has become people’s backyards, parks, etc.
It is easiest to recognize P. ramorum on living trees so if you find a dead tree
carefully examine the neighboring trees to see if you can find one that is
bleeding but still has a green crown. Chip away the bark on the live tree and
look for dead patches of outer bark.
Another indicator that P. ramorum is present is to check to see if multiple tree
and shrub species in an area all show symptoms (i.e. bleeding oaks next to bay
trees with a leafspot at the tip).
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Symptoms of Phytophthora ramorum infection in tanoak
(Lithocarpus densiflorus)
Tanoak is the most susceptible host, it is common to find large numbers of
tanoaks, from regeneration to large trees, killed by P. ramorum.
Figure 4. Dying tanoaks in a redwood-tanoak forest in Marin County.
A common early symptom of P. ramorum infection in small (less than 20 foot
tall) tanoak is a wilting of the new shoots. The shoot tip dies back, often forms a
Shepard’s crook and then commonly breaks off. Several new shoots emerge
from the point of dieback giving the tanoak a bushy appearance.
P. ramorum has been isolated from completely brown tanoak leaves but there
are other pathogens that cause branch flagging on tanoak that may look similar.
Diplodia has been frequently isolated from declining tanoak twigs but we do not
understand its role in this pest complex.
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Figure 5. An early symptom on tanoak: wilting shoots.
Photos by Pavel Svihra, UCCE Marin County.
Eventually the tanoaks die – there often is a canker, just above the ground line
to 15 feet and above, present on trees that are in the process of turning from
yellow to brown. You can check for the canker by chopping away the outer bark
and looking for necrotic (dead areas) in the bark surrounded by thin black zone
lines. Look on the bole just above the ground particularly if you see a slightly
discolored, water soaked or depressed area in the bark. Bleeding does occur on
tanoak, but is more difficult to find than on the oak species.
As the above ground portion of the tanoak dies; it resprouts from the base but
typically these sprouts die as well.
Figure 6. Sprouts at base of dead tanoak. Photo by Joseph O’Brien, USDA-FS.
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Symptoms of Phytophthora ramorum on madrone
(Arbutus menziesii)
Madrone is the most difficult host to recognize Sudden Oak Death on. While the
pathogen has been confirmed using molecular probes and cultural isolation
techniques there are no distinctive symptoms to distinguish P. ramorum infection
from a number of other diseases of madrone. Further research is needed to
evaluate the environmental and biotic factors impacting madrone.
Because of the confusion stemming from all the other problems madrone is
suffering from, if you are in an area that is not known to have Sudden Oak Death
and see dying madrone – look for other susceptible species in the area (like bay,
or oaks) to see if you can find Sudden Oak Death symptoms. We can’t provide
good instructions for sampling for Sudden Oak death – these still need to be
developed.
This picture shows a madrone leaf that was inoculated with P. ramorum in Dave
Rizzo’s lab. (Figure 6)
In areas with Sudden Oak Death, madrones show tip die back and mortality. We
know some of this injury is caused by P. ramorum but we also know that not
every madrone tree in decline is infected. There are other pathogens that cause
similar symptoms, particularly Botryosphaeria
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Symptoms of Phytophthora ramorum on bay laurel
(Unbellularia californica) (also called myrtle trees in
Oregon)
Bay trees are not known to be killed by P. ramorum but they are thought to be
important reservoirs of inoculum since the pathogen readily sporulates on bay
leaves in wet weather. The typical symptom on bay is black tips of leaves often
with a chlorotic halo. This symptom does not distinguish P. ramorum infection
from other foliar diseases of bay, such as anthracnose.
Figure 9. Picture of bay with spots at
the leaf tips, typical of those caused
by P. ramorum.
Photo by Joe O’Brien, USDA Forest Service.
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Symptoms of Phytophthora ramorum on rhododendron
(Rhododendron spps).
Rhododendron in California, the Netherlands and Germany have been found
infected with P. ramorum. The plants are not usually killed but show branch
dieback and leaf spots.
It is very difficult to tell leaf spots caused by P. ramorum from those caused by
sunburn or other common pathogens, including other species of Phytophthora.
To distinguish damage from a pathogen from sunburn, look at the plant location
and consider how much direct sunlight it is getting. Look at the distribution of
the spots; if it is fairly uniform all over the sunnyside of the plant it may be
sunburn. Lastly, turn the leaf over and carefully look at the underside of the
leaf. If the spot has a diffuse margin it maybe caused by a pathogen, rather than
sunburn. The leaf needs to be sent to a lab for confirmation of P. ramorum.
Infested rhododendron plants in pots are known to releaf and show no outward
symptoms of damage after a several months. We do not know if the pathogen is
still present on these plants or if it dies out as well.
Figure 10. Comparison of leaf spots created by sunburn vs. P. ramorum.
Photo by Tim Tidwell, CA Dept of Food and Agriculture.
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Symptoms of Phytophthora ramorum on evergreen
huckleberry (Vaccinuim ovatum)
The most distinctive symptom of P. ramorum on huckleberry is a canker on the
cane. Look for a darkened area on the cane with dead tissues underneath. The
pathogen causes cane dieback and when severe can kill the entire above ground
plant. Typically infected huckleberry canes drop their leaves quickly so not much
is known about whether leaf discoloration and various leaf spots on huckleberry
are caused by P. ramorum.
Figure 11. Credit David Rizzo, UC-Davis.
Symptoms of Phytophthora ramorum on California Buckeye
(Aesculus californica)
P. ramorum is not known to kill buckeye trees. Since this host was just recently
discovered not much is known about it. The symptom is a leaf spot. This tree
typically loses its leaves in mid-summer, this is normal.
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Figure 12. Buckeye leaf spot symptom from infection by P ramorum.
Matteo Garbelotto photo, UC-Berkeley.
Figure 13. Inoculated buckeye leaves showing spots. The control which was
inoculated with water is on the right. Photo by David Rizzo, UC-Davis.
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Symptoms of Phytophthora ramorum on Viburnum x
bondnantense in Germany (arrowwood)
P. ramorum has only been isolated from Viburnum in Germany. The combination
of symptoms on arrowwood is different from all other known hosts. The
pathogen caused an overall wilting of the shrub and discoloration was found at
the base of the stem.
Figure 14. Arrowwood in Germany infected with P. ramorum.
Photo credit: Sabine Werres, Institute for Plant Protection in Horticulture, Braunschweig, Germany
Contributors to these guidelines: Thanks to David Rizzo, Jennifer Davidson and
Garey Slaughter all of UC-Davis and Timothy Tidwell, California Department of Food
and Agriculture, for their guidance and review of this handout.
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