Pacific Northwest 1998 An Online Guide to Plant Disease Control

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Rhododendron -- Ramorum Leaf Blight and Shoot Dieback
See Also: Tanoak -- Sudden Oak Death ; and Rhododendron -- Phytophthora Blight
 
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 (Lithocarpus densiflorus), and oaks (Quercus sp.) in the San Francisco Bay Area were dying from this same disease. In Oregon, the fungus was found killing wild rhododendrons in a few isolated sites just north of Brookings, OR during the summer of 2001. Eradication of infected plants was initiated soon after the sites were found and continued when new infections were found. The infestation has spread slowly in Oregon from isolated spots. The area of new infested sites and expansions of previous infestations includes many acres of forest land near Brookings. Federal and international quarantines have been established to prevent the spread of this pathogen in soil and infected plant material.

In the spring of 2003, the disease was discovered in a wholesale nursery in Oregon 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 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 host list for this organism includes many different plants. Tip dieback and/or leaf spotting occurs on bigleaf maple, cascara, coast redwood, Douglas fir, honeysuckle, horsechestnut, huckleberry, kinnikinnick, myrtlewood, Pacific madrone, poison-oak (stem cankers), rhododendron cultivars, salmonberry, Viburnum and others. Lethal bark cankers develop on susceptible members of the oak family, such as oak 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 PNW 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. 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 wind-borne rain, irrigation water, or with water splashed onto foliage. There are two mating types, designated A1 and A2. The forest isolates from California and Oregon are the A2 mating type, while the European isolates are mainly the A1 mating type. Both mating types have been isolated from Oregon nurseries but the A1 mating type has not been isolated in Oregon since 2003..

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.

In southern Oregon, tip dieback was observed where shoot tips became progressively necrotic. Leaves on these shoots may remain green for awhile 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).

This shows the typical tip dieback to Rhododendron spp. from this disease. Leaves will collapse soon.

The bark has been stripped back to expose the discolored vascular system below.

Leaves collapse once the stem has been girdled.

It is uncommon to see necrosis occur out to the leaf tip. Usually the necrosis affects the midrib, as pictured here, and then goes down the petiole.

The rhododendron cultivar Unique, grown in a nursery showing necrotic leaf spots along the margins of the leaves.

 
Cultural control: Management efforts in PNW nurseries and forests 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.
  1. In Oregon, the Nursery & Christmas Tree Program Supervisor at ODA must be notified if importing nursery stock from any source (out-of-state or international). Plants must be certified as free from P. ramorum and have a copy of the official certification shield.
  2. 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.
  3. Contact your Department of Agriculture if you see or suspect plants with symptoms of this disease.
  4. Do not transport firewood or other plant materials (bark chips, sawdust, mulch, etc.) or soil from diseased areas in California or southwest Oregon. However, these items may be transported if they have been kiln dried, heat treated, or otherwise sterilized or bear an official “Disease Free” certification. Harvest of forest products (such as timber, greens and mushrooms) from the quarantine area is strictly regulated and generally prohibited.
  5. Wash shoes, vehicles, bikes, etc. if you visit infested areas of California or southwest Oregon.
  6. Propagate cuttings only from plants known to be free of disease for several months. Sanitize cuttings to eliminate the pathogen by soaking them in a disinfectant before storage and/or sticking in rooting medium. Use clean, pathogen-free potting media and clean, new pots.
  7. 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.
  8. 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.
  9. 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 (122oF for 30 min) to kill the pathogen.
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.

  1. Fenstop at 14 fl oz/100 gal water. Greenhouse use only. Group 11 fungicide. 12-hr reentry.
  2. Insignia at 16 oz/100 gal water. Use preventively only. Group 11 fungicide. 12-hr reentry.
  3. 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.
    1. Fore 80 WP at 1.5 lb/100 gal water plus a spreader-sticker.
    2. Pentathlon DF at 1 to 2 lb/A or per 100 gal water plus a spreader-sticker.
    3. Protect DF at 1 to 2 lb/100 gal water plus 2 to 4 oz spreader-sticker.
  4. Segway at 6 fl oz/100 gal water. Do not make more than 4 applications per crop per season. Effective on Camellia, Pieris and Viburnum so may be effective on Rhododendron. Group 21 fungicide. 12-hr reentry.
  5. Stature SC at 12.25 fl oz/100 gal water. Do not make more than 2 sequential applications. Group 40 fungicide. 12-hr reentry.
  6. Subdue MAXX at 2.5 fl oz/1000 sq ft as a drench application, then irrigated in with 0.5 inch water. Group 4 fungicide. Zero hr reentry. Application as a foliar sprays is allowed only in Oregon and Washington (SLN OR-050004 and WA-060003) at 2 fl oz/100 gal water and with a 48-hr reentry.
  7. Vapam HL may be drenched on 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 soil-less media. See label for details specific to your application.
Notes: Treatment with phosphonate compounds 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:
The California Oak Mortality Task Force http://www.suddenoakdeath.org

Hansen, E. M., Parke, J. L., and Sutton, W. 2005. Susceptibility of Oregon forest trees and shrubs to Phytophthora ramorum: a comparison of artificial inoculation and natural infection. Plant Dis. 89:63-70.

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.

Content edited by: Jennifer Parke and Jay W. Pscheidt on January 1, 2009
 
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