See:
Rhododendron - Ramorum Leaf Blight and Shoot Dieback
Cause The most common hosts of the fungus-like organism Phytophthora ramorum found infected in Oregon nurseries in order of importance are Rhododendron, Viburnum, Pieris, Camellia, and Kalmia. All species and hybrids of these "filthy five" genera are included in the regulated host list. Each year the disease has been found in a few PNW nurseries based on extensive sampling by state departments of agriculture. All nurseries exporting hosts/associated hosts must now participate in the federally mandated certification program to export product interstate and internationally.
This fungus-like organism has several spore types that help it survive and spread. Spores may survive in potting media for 6 to 12 months. They can even survive in composed municipal green waste. Spores landing on wet leaves or stems germinate and infect the plant with as little as 1 to 6 hours of wetness. Young leaves are especially susceptible. Within a few days, sporangia are produced, which release tiny swimming spores (zoospores). The sporangia themselves can also detach, germinate, and infect. Sporangia and zoospores can be moved with windborne rain, irrigation water, or with water splashed onto foliage. Long distance transport occurs via the movement of infected nursery stock or infested soil.
Symptoms On Pieris, infected leaves turn a dark brown. Young shoots and leaves are very susceptible to infection. On mature plants, the lower leaves may become infected and fall off. Note that Phytophthora citrophthora will also cause similar leaf spots on margins and tips of leaves.
Cultural control Management efforts in Pacific Northwest nurseries are focused on preventing new introductions, new infections and eradicating the pathogen where it is found. Early detection is vital to preventing disease spread. Practices useful in managing other foliar Phytophthora diseases are helpful in protecting plants from infection by P. ramorum.
- If the disease is found, nurseries are required to enter a certification program and are not released until state inspections are negative for 3 consecutive years.
- In Oregon, notify the ODA's Nursery and Christmas Tree Program when importing nursery stock from any out-of-state source.
- Purchase nursery plants only from nurseries certified as free from P. ramorum according to federal regulatory standards. A certified nursery will have an official certification shield.
- Shipments of known nursery hosts should be placed in an isolated area at least 7 ft from other plants. Regularly observed or inspected for disease symptoms. This may take as long as 3 to 6 months for symptom suppressing chemicals to wear off or occurrence of favorable weather.
- Contact your Department of Agriculture if you see or suspect plants with symptoms of this disease.
- Do not transport firewood, other susceptible plant material, or soil from diseased areas in California or southwest Oregon.
- Wash shoes, vehicles, bikes, etc. if you visit infested areas of California or southwest Oregon.
- Propagate cuttings only from plants known to be free of disease for several months. When using a disinfectant, use on mother stock before cutting and/or after sticking. Use clean, pathogen-free potting media and clean, new pots.
- Manage the application of irrigation water to reduce the length of time that foliage is wet. If possible, increase the interval between waterings to at least 1 hour or more. Improve drainage to avoid puddling and splashing. Place pots on well-draining surfaces.
- Use only clean water for irrigation. Treat irrigation water to kill spores of P. ramorum and other Phytophthora species. Remove and destroy any fallen leaf material, dead branches or plants.
- Provide good drainage for plants in beds, fields, or containers. Place containers on gravel beds (4-inches or more deep) or raised benches to allow drainage. Do not place containers on poly sheets; they can prevent containers from draining into soil and allow contaminated drainage water to spread from the base of one container to another.
- Remove and burn all leaves and branches of dead plants. Disinfest tools and shoes that may have been in contact with contaminated plants or potting media. Incinerate contaminated pots and trays or treat them with aerated steam (122°F for 30 min.) to kill the pathogen. Do not compost and reuse infected material.
Chemical control Use fungicides as general preventative treatments. If you actually find this disease in your nursery stock, your state department of agriculture will work with you to eradicate this quarantine pathogen. Most fungicides used to manage Phytophthora do not kill this organism. They can only prevent establishment of the organism before it gets into the plant. They can also prevent continued growth if the organism is already inside the plant thereby preventing the development of symptoms. Once chemical activity has subsided with time, the organism can resume growth within infected plants.
Resistance management through the use of tank-mixes or alternating products with different modes of activity will be very important. Limit the use of any one group during crop production.
- Mancozeb plus a spreader-sticker was effective as a foliar spray on Douglas-Fir seedlings when used after bud-break but before artificial inoculation with the pathogen. Mancozeb-based products may be useful alone and/or as mixing partners with other materials. Group M3 fungicides. 24-hr reentry.
- Protect DF at 1 to 2 lb/100 gal water plus 2 to 4 oz spreader-sticker.
- Orvego at 14 fl oz/100 gal water. Group 40 and 45 fungicide. 12-hr reentry.
- OxiPhos at 1.3 to 4 quarts/100 gal water as a foliar spray. Group P7 fungicide. 4-hr reentry.
- Phospho-Jet is registered at 1 to 2 qt/100 gal water as a foliar spray. Group P7 fungicide. 4-hr reentry.
- Segway SC at 6 fl oz/100 gal water. Effective on Camellia, Pieris and Viburnum. Group 21 fungicide. 12-hr reentry.
- Stature SC at 12.25 fl oz/100 gal water. Group 40 fungicide. 12-hr reentry.
- Subdue MAXX at 0.5 to 1 fl oz/100 gal water plus another fungicide with a different mode of action. Group 4 fungicide. 48-hr reentry.
Reference Parke, J.L., Linderman, R.G., Osterbauer, N.K. and Griesbach, J.A. 2004. Detection of Phytophthora ramorum blight in Oregon nurseries and completion of Koch's Postulates on Pieris, Rhododendron, Viburnum, and Camellia. Plant Disease 88:87.
Elliott, M., Strenge, D., Hulbert, J. M. and Chastagner, G. A., 2021. Multiagency collaboration strengthens applied research and mitigation of Phytophthora ramorum at a botanical garden in Washington State. Plant Health Progress, https://doi.org/10.1094/PHP-02-21-0045-FI.