Sudden oak death syndrome (SOD) was first reported in
the Netherlands and Germany in 1993, causing leaf-blight, stem cankers,
and tip dieback on nursery grown rhododendrons and viburnums. SOD has
become the latest serious threat to plant materials across the United
States and was first spotted in the U.S. in 1995 on tanoak in a Marin
County, CA forest. It took University of California researchers until
2001 to positively identify the pathogen, after which the USDA imposed a
federal quarantine of 12 California counties.
During the summer of 2001, SOD was also found killing
wild rhododendrons in Brookings, OR. Infected plants were eradicated
soon after sites were found and this infestation remains confined within
a 9-mile quarantined area of forested land near Brookings. The initial
source of infection of these sites remains unknown.
In the spring of 2003, SOD was found in a wholesale
nursery in Portland, OR and also in a retail outlet in Washington that
received plants from the Oregon wholesaler. Genera affected were Pieris,
Viburnum, and Rhododendron. Infected stock and
other nearby host plants were destroyed. This outbreak is suspected to
have come from an international shipment of rhododendrons.
On March 25, 2004, SOD was confirmed on plants at
Monrovia Growers in Azusa, CA and Specialty Plants, Inc. in San Marcos,
CA. On March 29, 2004, the US Department of Agriculture put all 1,500
nurseries in California under a 60-day quarantine. Each nursery has been
inspected for symptoms of SOD and released if no symptoms were found.
To date, there are confirmed trace forwards on stock
from Monrovia in 14 states: California, Alabama, Florida, Washington,
Oregon, Texas, Colorado, Georgia, Louisiana, Maryland, North Carolina,
New Mexico, Tennessee, and Virginia (Situation Report #43, May 25,
2004).
Cause and Hosts. SOD is caused by the fungus-like organism
Phytophthora ramorum (P. ramorum), and causes different symptoms on
different hosts. Called sudden oak death when it occurs on susceptible
tree species, P. ramorum is characterized by bleeding cankers
that girdle the trunks. On affected shrub species, the disease is called
Ramorum leaf and shoot blight and is characterized by leaf
blights and shoot diebacks. Unfortunately, on many plant species
symptoms of P. ramorum may be indistinguishable from other common
Phytophthora fungal diseases.
Phytophthora ramorum is well adapted to cool, wet
conditions and tolerates heat and drought. Unlike other species in this
large fungal family that usually infect roots, P. ramorum is
mainly a foliar pathogen. It produces several types of spores, which
helps the organism survive and spread. Spores landing on wet leaves or
stems germinate and infect young plant tissues. Several of the spore
types can be moved with windborne rain, in irrigation water, or with
water splashed onto foliage by sprinkler irrigation. P. ramorum
is also thought to move in soil and on infected plant material.
As of this writing there are 60 hosts and associated
host species in at least 12 plant families (see Table 1 and Table 2
below).
Symptoms.
On trees:
P. ramorum can affect just the bark, as on
beech, or both the bark and the leaves, as on tanoak. It is also
possible that some trees like ash, which has highly susceptible leaves
experimentally (but has not yet been found to be a natural host) may
just be leaf hosts to the pathogen. Bark infections occur as large
cankers on the lower trunk that are brown to black discolored areas on
the outer bark and "bleed" dark red sap. Mottled areas of dead and
discolored inner bark with black lines around the edges are visible
when the bark is removed. A rapid change in the color and condition of
the foliage, followed by sudden death of the tree, occur when cankers
girdle the trunks of affected trees. Cankers can cause sudden death,
as in California tanoak, or a slow decline over several years, as in
American oak species. Cankers do not extend below the soil line and do
not appear to infect the roots.
On leaves:
Leaf infections appear as brown to black dead areas,
often at the tip or base of a leaf. Shoots can also be affected and
appear brown to black in color and may wilt.
Symptoms on specific species include:
: needle blight and dieback
of young shoots of Douglas fir, coastal redwood, and grand fir.
On yew
: needle blight of young foliage resulting
in aerial dieback.
On Rhododendron,
Pieris, Vaccinium,
and Hamamelis spp. (witchhazel)
: affected shoots or twigs
develop a brown to black discoloration that spreads along the twig and
can spread into the leaf via the petiole; characteristics symptoms are
a blackening of the petiole, leaf base, and leaf tip that may extend
along the mid-vein; twig cankers lead to wilting of affected shoots;
development of symptoms can be rapid.
On viburnum
: infection commonly occurs at the base
of the stem, causing wilting and then death. May also cause brown to
black leaf infections and can also affect flowers, especially on
evergreen species.
On Camellia, Kalmia, Syringa (lilac) and
Leucothoe spp
.: usually confined to leaf infections; leaf
lesions are brown to black and typically occur at the tip or edges of
the leaf; some shoots may become infected, leading to dieback.
Two websites with good symptom pictures on various host
plants are:
•
cropandsoil.oregonstate.edu/people/faculty/parke/OSUPramorum.pdf
•
www.defra.gov.uk/planth/pestnote/newram.pdf
Management. Management efforts have been focused on eradicating the
pathogen where it is found and preventing new infections. Early
detection is vital to preventing the disease from spreading.
In order to limit the spread of this disease, homeowners
with a suspected plant infected with P. ramorum should contact
your nearest Extension Office and ask them about collecting a sample of
the plant.
Idaho Growers may submit a sample directly. PLEASE
FOLLOW THE PROTOCOL LISTED BELOW.
1. Suspect plant material must be placed in double
ziplock bags and stored in a refrigerator awaiting shipment to a
diagnostic facility. The preferred method for shipment is triple
packaging: two ziplock bags and an outer container. The outer
shipping container should be an approved cardboard shipping box. The
seams of the box should be closed with approved shipping tape.
2. Include the following information with the
sample:
• Host.
• Collector’s name, and date.
• Number of hosts at that site.
• Facility/location name and address, contact person, and county.
• Other potential hosts at the site and any observations.
3. Samples should be shipped via overnight delivery
or hand delivered to:
S. Krishna Mohan
Professor of Plant Pathology
University of Idaho
29603 U of I Lane
Parma, ID 83660
Phone: (208) 722-6701 Ext. 218
Fax: (208) 722-6708
4. All tools and other equipment must be sanitized
and/or sterilized before re-use.
Control.
Phytophthora ramorum has only recently been
described and knowledge of how the disease develops and spreads is
limited. Currently, fungicides are being used as preventative treatments
on known host plants. Most of the fungicides used to control other Phytophthora species do not control this organism; they can only
prevent it from becoming established.
For more information on Phytophthora ramorum
go
to the USDA APHIS website at:
www.aphis.usda.gov/ppq/ispm/sod/
