What is Ramorum Blight and Dieback?
Ramorum Blight and Dieback, also known as 'sudden oak death', is a serious disease caused by Phytophthora ramorum,
a fungal-like pathogen, that has killed a large number of oak trees in
California and Oregon since it was first detected in 1995. In North America, the disease
is currently established only in California and Oregon, mainly in forested
areas or in small remnants of forests in developed areas. The first report of
this disease in Canada was a single case of rhododendrons found in a B.C.
nursery in June 2003. Additional detections occurred in 2004 and 2005 at a few
nurseries and garden centres. Swift regulatory action by the Canadian Food
Inspection Agency (CFIA) has successfully eradicated these occurrences, with
the exception of one nursery that remains under regulatory control. Strict
regulatory control measures are in place to ensure the disease does not become
established in Canada.
The name Sudden Oak Death was used initially to describe the rapid dieback
and mortality in oak. More recently the name Ramorum Blight and Dieback has
been adopted to better describe the wide range of symptoms caused by the
pathogen on various hosts. Damage to ornamental
host plants such as camellia and rhododendron typically consists of foliar blights and leaf spots. Severity
is variable, ranging from cosmetic
damage to eventual death of the host plant.
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Figure 1. Leaf blight on Camellia
Photo courtesy: Steve Ashby, Department of Environment, Food and Rural
Affairs United Kingdom |
Figure 2. Leaf blight on Rhododendron
Photo courtesy Joseph O'Brien, USDA Forest Service, www.forestryimages.org |
Causal organism
The causal organism is a fungus-like organism (protist), Phytophthora ramorum
(P.
ramorum). Researchers at the University of California have determined that P. ramorum is very closely related to
Phytophthora lateralis, the cause of Port-Orford
cedar root rot. P. ramorum was first described as a pathogen of
ornamental Rhododendron and Viburnum in Germany and The
Netherlands.
There are two mating types of P. ramorum - European
isolates belong to the A1 mating type, while most North American isolates
belong to the A2 mating type. In Europe, the disease is not known to cause
mortality in oaks, but does cause mortality in some ornamental plants,
particularly Viburnum and Rhododendron.
Geographical and host distribution
The current known distribution of P. ramorum includes the western USA
and Europe. For the most recent information on the North American geographical
distribution of P. ramorum, please refer to the
Oak Mortality Task
Force Web Site.
The pathogen has a wide host range, with at least 97 species in 31 plant
families confirmed as susceptible. In Canada, all species of
69 genera are
currently regulated for P. ramorum by the CFIA, including Abies,
Acer, Arbutus, Camellia, Corylus, Fagus, Pseudotsuga, Quercus, Rhododendron,
Rubus, Vaccinium and Viburnum. New hosts are being investigated and
added to the list as new data become available.
Refer to the CFIA website for an up-to-date list of regulated species in
Canada. For an up-to-date list of known host species, please refer to the
Oak Mortality Task Force Web Site.
Life cycle of the organism
Ramorum blight and dieback has been observed in three main forest types in California: Mixed Evergreen-Bay-Arbutus, Tanoak-Douglas fir and Coast Redwood.
Researchers currently believe that infections on foliar hosts, such as arbutus,
bay laurel, huckleberry, rhododendron and buckeye, may contribute to a rapid
build-up of the pathogen in the environment, serving as a reservoir of inoculum
(spores)
which in turn infects woody tissues of oaks and tanoaks (California Oak
Mortality Task Force, July 2001). It is possible that the understory infection on the leaves of
susceptible hosts occurs prior to the infection of woody tissues of oak species.
There is evidence that P. ramorum spreads by airborne spores and with
wind blown rain. Sporangia and chlamydospores are commonly produced on infected
foliage.
P. ramorum is active in wet and cool climate with the optimum growth at
20oC. Disease development is favoured by cool temperatures with
relatively high moisture.
Symptoms
- Symptoms vary from leaf spots, undefined leaf lesions, leaf/twig/stem
blights to dieback, depending on the host plant. These symptoms can be
confused with other types of damage caused by other pathogens, frost,
sunburn, etc.
- On rhododendron, symptoms on leaves appear as dark brown to black lesions
with “fuzzy” margins, usually on leaf edges or tips. On some species, the
lesions may exhibit a concentric ring pattern (Figure 2). Blight on
twigs/shoots first appear as brown to black lesions which spread along the
twig to cause dieback.
- On camellias, brown to dark-brown lesions with diffuse margins usually
appear on leaf tips and edges (Figure 1).
- On viburnum, brown to black lesions can appear on leaves, but lesions
often occur at the stem base leading to wilting and then death of the plant.
- On oak, the disease is characterized by a rapid decline (dieback) and
leaves turn brown suddenly and stay on the branch for up to a year. Bleeding
or oozing cankers develop on the lower trunk and branches. Ooze from cankers
is sticky, very dark reddish and smells fermented (Figure 4). When the
surface of the cankered bark is chipped away, the infected bark tissue below
shows thick dark zone lines (Figure 3 ) clearly separating the affected
tissue from the uninfected area in the cankered region.
- For a complete list of symptoms and related information, visit the
California Oak Mortality Task Force homepage at
http://www.cnr.berkeley.edu/comtf/. An excellent diagnostic guide is
available on this site with descriptions and pictures of symptoms on many
host plants.
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Figure 3. Sudden oak death canker (caused by P. ramorum) showing
clear zone line on inner bark of tan oak (Lithocarpus densiflorus) |
Figure 4. Bleeding canker, caused by P. ramorum, on Coast Live Oak
(Quercus agrifolia) |
Potential for disease spread to BC
Strict regulatory controls are in place in California, Oregon, and Washington
to prevent the spread of P. ramorum, and Canadian quarantine regulations
have been established to help prevent the introduction of the disease into
Canada on nursery stock, soil or other host materials.There is a high risk
that Ramorum blight and dieback will have a serious impact on BC's horticulture
sector and environment, due to its wide host range and the presence of a
suitable climate. The introduction of this disease and its
subsequent spread could threaten the ornamental and berry industries as well as many native plants. Prevention of its
introduction and rapid eradication efforts, should the disease be introduced,
are key to protect these industries from losses caused by P.
ramorum.
How can you prevent the introduction of this disease into BC?
To help prevent the introduction of P.
ramorum, do not transport infected
or potentially infected host material and soil that are taken from areas where
the pathogen is known to occur. Contact the Canadian Food Inspection Agency's (CFIA)
local office regarding quarantine restriction information on movement of hosts
and associated materials. Be aware of the symptoms, visually inspect host
plants, and report possible cases to your local office of the CFIA, Canadian
Forestry Service (CFS), or Ministry of Agriculture & Lands (MAL). The Canadian Nursery and Landscape Association (CNLA) has
developed a comprehensive certification program for commercial nurseries. The
program was launched in the fall of 2004 and it annually certifies more than
250 nurseries in BC. The key components of the program are annual nursery
sampling and testing for P. ramorum, implementation of best management
practices, and independent third-party audits to ensure nursery compliance. The
program reduces the risk of P. ramorum being introduced and/or established at a nursery
and, thereby, protects the garden centres and landscapers that purchase
planting stock from the nursery.
Control
- Preventing the introduction and spread of P. ramorum is the key to
minimize the impact on the nursery and environment.
- Commercial nurseries are advised to adopt the recommended Best Management
Practices (BMPs) and the Phytophthora ramorum Nursery Certification
Program to safeguard the industry.
- Fosetyl-AL (Chipco Aliette T&O) and metalaxyl (Subdue MAXX) have gained
emergency registration for preventative use in nurseries and landscape
plantings.
- Scout for visible symptoms, particularly during spring, early summer and fall when the
pathogen is active. If suspected, immediately notify the local office of the
CFIA, CFS, or MAL.
Quarantines have been put in place in both California and Oregon to limit
the spread of the disease, and eradication is underway in Oregon. As a rule,
these regulations pertain to nursery plants of known hosts or their close
relatives, wood products and soil. An import policy D-01-01 titled, "Phytosanitary
Requirements to Prevent the Entry of Phytophthora ramorum" has been implemented to prevent the
introduction of sudden oak death into Canada (CFIA – revised Oct. 18/2005). The
regulations may be viewed at
http://www.inspection.gc.ca/english/plaveg/protect/dir/d-01-01e.shtml
Detection and Identification
In BC, disease and pathogen identification services can be obtained by submitting suspicious samples to
the BCMAL Plant Diagnostic Laboratory in Abbotsford, or to the Pacific Forestry Centre Laboratory in Victoria.
Further Information
Ministry Contacts:
Siva Sabaratnam, Ph.D.
Plant Pathologist
Food Safety & Quality Branch
Tel: 604 556-3029
Dave Woodske
Industry Specialist, Ornamentals
Industry Competitiveness Branch
Tel: 604 556-3044
May 2007
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