Ramorum Blight and Dieback (Sudden Oak Death)

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.

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 57 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

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 20^oC. 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.

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

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?

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

• Sudden Oak Death Information Page (Canadian Food Inspection Agency)
  • Sudden Oak Death Import policy (Canadian Food Inspection Agency)
  • Sudden Oak Death Factsheet (Canadian Food Inspection Agency)
  • List of regulated species in Canada (Canadian Food Inspection Agency)
• Canadian P. ramorum Certification Program - Canadian Nursery Landscape Association
• The California Oak Mortality Task Force
• Sudden Oak Death in California (University of California, Davis)

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

March 2006