Rhododendron-Ramorum Leaf Blight and Shoot Dieback

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Cause A fungus-like organism, Phytophthora ramorum, was first reported to cause leaf blight, stem canker, and tip dieback on nursery-grown rhododendrons and viburnums in Germany and the Netherlands in 1993. At about the same time, many tanoaks (Notholithocarpus densiflorus), and oaks (Quercus spp.) in the San Francisco Bay Area were dying from this same pathogen. In Oregon, the organism was found killing wild rhododendrons in a few isolated sites just north of Brookings, OR during the summer of 2001. Federal and international quarantines have been established to prevent the spread of this pathogen in soil and infected plant material. Since the initial discovery, the disease is considered established and Oregon has focused on limiting spread in Curry County through early detection, monitoring, and eradication of symptomatic tanoak trees. Despite these efforts, the quarantine area in Curry County has expanded many times since 2001 from 9 sq. miles to over 500 sq. miles. The rate of disease spread increased most dramatically in 2014 requiring the quarantine area to nearly double in 2015.

In the spring of 2003, the disease was discovered in a wholesale nursery in Oregon, a south-coastal nursery in British Columbia, and in Washington at a retail outlet. Each year the disease has been found in a few PNW nurseries based on extensive sampling by state departments of agriculture. The disease has not been found in any Christmas tree plantations in the PNW. In 2005 this became a federally quarantined pathogen. All nurseries exporting hosts/associated hosts must now participate in the federally mandated certification program to export product interstate and internationally.

The disease was also discovered in a botanical garden landscape on the north end of Bainbridge Island, WA in 2015. Several plants including were infected with the NA-1 clone including Camellia, Gaultheria, Mahonia, Pieris, Rhododendron, Vaccinium, Viburnum and Vinca. Although eradication was completed it is suspected that the pathogen will persist and efforts are focused on containment.

The host list for this organism includes many different plants. The rhododendron cultivars most often found positive for this fungus include Unique, Baden Baden, Anah Krushke, and Roseum Elegans. Tip dieback and/or leaf spotting occurs on bigleaf maple, camellia, cascara, coast redwood, Douglas fir, honeysuckle, horsechestnut, huckleberry, kinnikinnick, myrtlewood, Pacific madrone, Pieris, poison-oak (stem cankers), rhododendron cultivars, salmonberry, Viburnum and others (those in bold were found infected in nurseries most often). Lethal bark cankers develop on oaks and tanoak. Oregon white oak (Quercus garryanna) does not appear to be very susceptible when challenged with the pathogen in lab tests.

Phytophthora ramorum is a fungus-like organism well-adapted to the cool, wet conditions of the Pacific Northwest and tolerant of heat and drought. Several spore types are produced that help this organism 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. 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. Natural spread of spores can be up to 3-4 miles a year. Longer distance transport occurs via the movement of infected nursery stock or infested soil. Lesion size and infection frequency increased at higher N fertilizer rates in the susceptible cultivars 'English Roseum' and 'Cunningham's White'.

There have been 5 migrations of P. ramorum between continents and 3 distinct introductions into the North American west coast. There are four clonal lineages named NA1, NA2, EU1 and EU2. The North American clone, NA1, was the first clone to arrive in the US with plant imports of exotic ornamentals into nurseries and is responsible for SOD outbreaks in both California and SW Oregon forests. A second introduction, of the clone NA2, occurred into British Columbia and Washington nurseries. A third introduction of the European clone, EU1, likely arrived from Europe into Pacific Northwest nurseries. This single infected tanoak with EU1 clonal lineage was found in southeast Oregon in 2015 and a dozen more in 2016. Eradication of this site is underway.

Symptoms Symptoms on rhododendrons 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.

In southern Oregon, tip dieback was observed where shoot tips became progressively necrotic. Leaves on these shoots may remain green for a while but the petiole becomes necrotic and eventually the whole leaf collapses once the stem is girdled. If left unchecked, the pathogen can kill rhododendrons, particularly if the apical bud is infected.

Symptoms on the other hosts are difficult to distinguish from those caused by other pathogens. If you suspect you have seen a P. ramorum-infected plant in Oregon, call the Invasive Species Hotline, 1-800-INVADER (1-800-468-2337).

Cultural control Management efforts in Pacific Northwest nurseries are focused on eradicating the pathogen where it is found and preventing new infections. Early detection is vital to preventing disease spread. Practices useful in managing other foliar Phytophthora diseases should also be helpful in protecting plants from infection by P. ramorum.

  • In Oregon, notify the ODA's Nursery and Christmas Tree Program Supervisor 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 and 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. 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. 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 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.

  • 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 80 WP 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 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.
  • 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, Alude or Phostrol) to be as efficacious as other materials with different modes of action.

References Blomquist, C.L., Yakabe, L.E., Rooney-Latham, S., McRoberts, N., and Thomas, C. 2016. Detection of Phytophthora ramorum in Nurseries and Forest Lands in California in 2004 to 2009. Plant Disease 100:139-148.

Grünwald, N.J., Garbelotto, M., Goss, E.M., Heungens, K., and Prospero, S. 2012. Emergence of the sudden oak death pathogen Phytophthora ramorum. Trends in microbiology. 20:131-138.

Linderman, R.G., and Davis, E.A. 2008. Evaluation of chemical agents for the control of Phytophthora ramorum and other species of Phytophthora on nursery crops. Online. Plant Health Progress doi:10.1094/PHP-2008-0211-01-RS.

Tjosvold, S.A., Koike, S.T., and Chambers, D.L. 2008. Evaluation of fungicides for the control of Phytophthora ramorum infecting Rhododendron, Camellia, Pieris, and Viburnum. Online. Plant Health Progress doi:10.1094/PHP-2008-0208-01-RS.