See:
Rhododendron - Ramorum Leaf Blight and Shoot Dieback
Cause The fungus-like organism, Phytophthora ramorum, is considered established in the landscape of southwest Oregon. Quarantines and eradication efforts have focused on limiting spread in Curry County, OR through early detection, monitoring, and eradication of symptomatic tanoak trees. The disease has also been found in the wholesale nursery industry. The host list for this organism includes many different plants, however, camellia, Pieris, rhododendron cultivars, and viburnum have been found infected in nurseries most often. All nurseries exporting hosts/associated hosts must now participate in the federally mandated certification program to export product interstate and internationally.
The deciduous azaleas 'Northern Hi-lights' and R. occidentale are highly susceptible. Most other azaleas are susceptible but generally symptoms are weakly expressed.
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 Symptoms are very similar to those caused by sunburn and other foliar Phytophthora species. Leaf lesions penetrate through the plant tissue so that spots can be seen on both sides of the leaf. They can be triangular in shape and extend along the leaf petiole and midrib, or leaf spots can occur where water accumulates on the leaf margins. Leaf spots have diffuse margins and the appearance of watersoaking. Infections of both above- and belowground tissues may not produce symptoms making this a very difficult disease to detect.
Cultural control Management efforts in Pacific Northwest nurseries are focused 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. In Oregon, contact your ODA nursery inspector.
- 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.
- Use an appropriate amount of ammonium nitrogen fertility avoiding excess levels.
Chemical control Use fungicides as general preventative treatments. If you 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. Activity of these fungicides can also be reduced when plants experience water or salt stress after application.
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.
- Adorn at 1 to 4 fl oz/100 gal water plus another fungicide. Group 43 fungicide. 12-hr reentry.
- Fenstop at 14 fl oz/100 gal water. Greenhouse use only. Group 11 fungicide. 12-hr reentry.
- Insignia SC at 12 fl oz/100 gal water. Use preventively only. Group 11 fungicide. 12-hr reentry.
- 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.
- Fore 80WP Rainshield at 1.5 lb/100 gal water plus a spreader-sticker.
- Protect DF at 1 to 2 lb/100 gal water plus 2 to 4 oz spreader-sticker.
- Micora at 4 to 8 fl oz/100 gal water. Group 40 fungicide. 4-hr reentry.
- 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.
- Segway SC at 6 fl oz/100 gal water. Effective on Camellia, Pieris and Viburnum so may be effective on Rhododendron. Group 21 fungicide. 12-hr reentry.
- Sparra at 12 to 24 fl oz/100 gal water as a soil drench or 1 to 2 quarts/100 gal water as a foliar spray. Group P7 fungicide. 4-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.
- Vapam HL may be drenched on soil preplant (beds or rows) to eliminate the pathogen from production areas. Apply 37.5 to 75 gal per treated acre in enough water to soak at least 2-inches deep. A method to seal or cover the treated area would help efficacy. Can also be used to treat soilless media. See label for details specific to your application. Restricted-use pesticide.
Note: Treatment with phosphonate compounds (Group P7 fungicides) significantly and consistently reduced lesion size in California live oak, especially when injected into trees. However, tests with nursery plants such as rhododendron and Douglas-fir have not shown these products (such as Aliette or Phostrol) to be as efficacious as other materials with different modes of action.
Use of Anti-Stress, Zonix or Nu-Film plus Zonix may help protect plants while in quarantine but still allow the development of possible infections. Treated plants had lower inoculum production and can be scheduled for destruction if symptoms develop.
References Bostock, R. M. and Roubtsova, T. V. 2021. The effect of applied salinity and water stress on chemical suppression of Phytophthora ramorum from soilborne inoculum in Rhododendron. Plant Disease, 105: https://doi.org/10.1094/PDIS-09-20-1928-RE.
Tjosvold, S. A., Koike, S. T., Davidson, J. M. and Rizzo, D. M. 2002. Susceptibility of azalea (Rhododendron) to Phytophthora ramorum. In Proc. of Sudden Oak Death, a Science Symposium: The State of Our Knowledge.