Ramorum Blight and Dieback, also known as 'sudden oak death', is a serious quarantine disease caused by a fungal-like organism, Phytophthora ramorum. It has killed thousands of Tanoak (Lithocarpus densiflorus) and oak (Quercus spp.) trees, particularly Coast live oak (Quercus agrifolia) and California black oak, in California and Oregon since it was first detected in 1995. In North America, the disease is currently established in localized areas in California and Oregon, mainly in forested areas or in small remnants of mature forests. The first report of this disease in Canada was the detection of infected rhododendrons in a British Columbia nursery in June 2003. Since then, additional detections of P. ramorum have been found at a few nurseries and garden centres in the Lower Mainland and Vancouver Island. Swift regulatory action by the Canadian Food Inspection Agency (CFIA) has successfully eradicated these occurrences. Strict regulatory control measures are in place to ensure the disease does not become established in Canada and to facilitate the movement of host plants and plant products within Canada and internationally.
The name Sudden Oak Death was used initially to describe the rapid dieback and mortality of P. ramorum infected Tanoak and several oak species, particularly Coast live oak, in California. 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 severe leaf and stem blight, defoliation, canker, dieback and eventual death of the host plant.
|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
The causal organism Phytophthora ramorum (P. ramorum) is an oomycete (fungal-like organism) that belongs to the Kingdom Protista. Researchers at the University of California have determined that P. ramorum is 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, including Rhododendron and Viburnum.
Geographical and host distribution
The current known distribution (establishment) 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 California Oak Mortality Task
Force Web Site.
The pathogen has a wide host range with well over 200 species belonging to 75 plant genera, confirmed as susceptible to P. ramorum. Both in Canada and the USA, host plants are regulated based on the susceptibility of specific plant species within each genus. New hosts are being investigated and added to the list as new data become available. For an up-to-date list of regulated plant species in Canada, please refer to the CFIA website. For an up-to-date list of regulated plant species in the USA, please refer to the United States Department of Agriculture website. For additional information on P. ramorum, please refer to the California 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. Research indicates that infections on foliar hosts, such as arbutus, bay laurel, huckleberry, rhododendron and buckeye, do 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 tanoak. It is possible that the understory infection on the leaves of host plants occurs prior to the infection of woody tissues of oaks and tanoak.
There is evidence that P. ramorum spreads by airborne spores carried by wind-blown rain. Sporangia and chlamydospores are commonly produced on infected foliage. P. ramorum is active in wet and cool weather with the optimum growth at 20oC. Disease development is favoured by cool temperatures with relatively high moisture.
- Symptoms vary from leaf spots, undefined leaf lesions, leaf/shoot/stem blights, bleeding or oozing tree-trunk cankers 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 appears 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 (Figure 4) and smells fermented. 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.suddenoakdeath.org/. An excellent diagnostic guide is
available on this site with descriptions and pictures of symptoms on many
|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
Potential for disease spread to B.C.
Strict regulatory control measures are in place in California, Oregon and Washington to prevent the spread of P. ramorum, and Canadian quarantine regulations have been established to prevent the introduction of the disease into Canada on nursery stock, soil or other host materials. There is a high risk that P. ramorum will have a serious impact on B.C.'s horticulture and forestry sectors and the environment due to its wide host range and the presence of a suitable climate. 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 B.C.?
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. Plants exported from the regulated counties in the U.S. shall be accompanied by a phytosanitory certification. 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. On arrival and in-house, visually inspect host plants, and report possible cases to your local office of the CFIA, Canadian Forestry Service (CFS), or Ministry of Agriculture (BCAgri).
The Canadian Nursery and Landscape Association (CNLA) is running a comprehensive voluntary P. ramorum certification program for commercial nurseries. The program was launched in the fall of 2004 and it annually certifies more than 250 nurseries in B.C. The key components of the program are 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.
A federal compensation program (Phytophthora ramorum Compensation Regulations and amendment to the compensation regulation) is in place to provide partial compensation to persons or agri-businesses for financial costs to comply with regulatory controls ordered by the CFIA.
- Preventing the introduction and spread of P. ramorum is the key to minimize its impact on the nursery and environment.
- Commercial nurseries are advised to adopt the recommended Best Management Practices (BMPs) and the P. ramorum Nursery Certification Program to safeguard the industry.
- Plant species belonging to genus Rhododendron, Camellia, Viburnum, Pieris, Kalmia and Magnolia are considered highly susceptible to P. ramorum (i.e. high risk hosts) and nurseries are advised to take extra precaution when handling these plants.
- 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 BCAgri.
- Dimethomorph (Acrobat 50 WP), fosetyl-AL (Aliette) and metalaxyl-M (Subdue MAXX) are registered for preventative use in nurseries and landscape plantings. Please strictly follow manufacturer’s label instructions and BMPs when using these chemicals.
Both in California and Oregon, quarantines have been put in place for areas where P. ramorum is established to limit the spread of the disease, and eradication measures are underway in Oregon. 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" (http://www.inspection.gc.ca/english/plaveg/protect/dir/d-01-01e.shtml) has been implemented to prevent the introduction of P. ramorum into Canada (CFIA – revised September, 2010).
Detection and identification
In B.C., symptomatic plant samples for pest diagnosis, including P. ramorum, can be submitted to the BCAgri Plant Health Laboratory in Abbotsford or to the Pacific Forestry Centre Laboratory in Victoria.
For further information
Plant and Animal Health Branch
Tel: 604 556-3029
Policy & Industry Competiveness Branch
Tel: 604 556-3044
Updated May 2012