Land & Resources Management

Sudden Oak Death - Frequently Asked Questions

What is Sudden Oak Death?

Sudden oak death (SOD) is a canker disease of forest trees and shrubs, which was recently introduced to California from an unknown source. The disease symptoms were first reported in 1995 in Marin County, and has since been found in nine additional coastal counties in California (Alameda, Mendocino, Monterey, Napa, San Mateo, Santa Clara, Santa Cruz, Solano, and Sonoma), and in Southern Oregon (Curry County).

The pathogen, Phytophthora ramorum is a newly described fungus species. It was identified as the causal agent of SOD in the summer of 2000. In the brief time since, the pathogen has been detected in a number of California native plant species, besides oak. An extensive research team has been formed to look at detection, spread, how it interacts with different host species, possible control measures, biomass removal, etc. Results of these projects can be found at www.suddenoakdeath.org

What plants are host to Phytophthora ramorum?

Big leaf maple (Acer macrophyllum), black oak (Quercus kelloggii), California bay laurel / Oregon myrtle (Umbellularia californica), California buckeye (Aesculus californica), California coffeeberry (Rhamnus californica), California honeysuckle (Lonicera hispidula), coast live oak (Quercus agrifolia), huckleberry (Vaccinium ovatum), madrone (Arbutus menziesii), manzanita (Arctostaphylos manzanita), California rhododendron (Rhododendron macrophyllum), Shreve's oak (Quercus parvula var. Shrevei), tanoak (Lithocarpus densiflorus), toyon (Heteromeles arbutifolia), and viburnum (in Europe only; Viburnum x bodnantense).

What are the symptoms of Sudden Oak Death?

Symptoms of infection by P. ramorum that occur in tanoak, coast live oak, black oak, and Shreve's oak vary in appearance, both chronologically and physically, but once symptoms appear the diseased oaks may die within a few months: It is important to remember that each of these species is also affected by other diseases that may look similar to P.ramorum, which can make field diagnosis very difficult.

  • In coast live oak, black oak, and Shreve's oak, the earliest symptom is the appearance of a bleeding canker. Burgundy-red to tar-black thick sap oozes on the bark surface. This bleeding is a response to infection with P. ramorum and is typically found from the root crown (the area where the trunk fans out to the roots) to a height of 6 feet. Bleeding has occasionally been observed at greater heights. Bleeding is a non-specific host response to the disease and can be caused by other organisms or wounding. Small black conks and bark beetles may also occur.
  • In tanoak, new growth may droop or turn yellow to brown. Bleeding, though less viscous than in the true oak hosts, has been observed on tanoak; although, tanoak may not show the bleeding symptom at all. P. ramorum can also persist in leaf spots on tanoak and nearby bay, rhododendron and other species (listed below).

Other hosts may also be infected by P. ramorum, but symptoms are different from those of oaks:

  • California bay laurel symptoms are confined to leaf spotting, often surrounded by a chlorotic (yellowish) halo. Leaf spots are often at the leaf tip and may or may not have a blackened line at the border. California bay is one of the easier species to field recognize the disease, however, another disease, anthracnose, can also cause similar symptoms. Plant mortality in bay laurel by P. ramorum has not been reported.
  • On California rhododendron (R. macrophyllum), the symptoms include twig dieback and leaf spotting, usually not mortality. However, recently rhododendrons have been reported killed by the disease in Curry County, Oregon. On rhododendron leaves, brown spots occur having diffuse, fuzzy margins (rather than sharp margins as caused by sunburn injury) where water accumulates, and generally do not involve the midrib of the leaf. Additional symptoms are blackened shoots with or without foliage still attached
  • Huckleberry symptoms include twig dieback, which in advanced stages will kill canes down to the ground, killing all the above ground portions of the plant. The small, blackened twig cankers may be dried and/or wilted. Infected huckleberry plants have been observed in areas of tanoak infection.
  • On Pacific madrone the symptoms include purplish leaf spots, and stem cankers that appear as blackened areas on twigs. At the advanced stages, the entire leaf and shoot become black. In the case of madrone, there are other fungal pathogens (i.e., Botryosphaeria and Natrassia) that are known to cause symptoms very similar to those of SOD, making it very difficult to field recognize the disease. The disease affects both juvenile and adult plants.
  • Toyon, California coffeeberry, California honeysuckle, and manzanita are less understood hosts for P. ramorum (Garbelotto, et al., 2002). Dark foliar spots, at times demarked by a thick black line, may be one of the symptoms caused by P. ramorum on toyon. Less prominent foliar spots, branch lesion, and unusual death of entire plants have been reported. In general, lesions appear where water accumulates; they are dark in color, and have a fuzzy margin. Lesions with concentric rings have been reported in honeysuckle. The water-soaked appearance of the cankers and the absence of black fungal reproductive structures (pycnidia) may help to differentiate P. ramorum symptoms on manzanita from those caused by Botryosphaeria.
  • P. ramorum can infect the leaves, petioles and twigs of California buckeye trees. Early symptoms start as rounded individual spots that tend to coalesce later in the season. The symptoms are very similar to those of buckeye anthracnose. Darkened lesions on petioles and twigs indicate presence of P. ramorum. Early summer leaf loss in response to drought is normal, and should not be confused as being due to disease in California buckeye. This natural senescence can cause leaf distortion, which is similar to that caused by SOD, however.
  • In bigleaf maple, P. ramorum foliar infection appears more like a leaf scorch, usually starting from the edges of the leaf. Generally this scorching has irregular borders, not following the leaf contour. Coloration of lesions is variable from orange to brown. Even after leaves die and turn yellow, the lesions should still be visible for some time. It is currently unknown whether associated branch dieback is caused by the pathogen (Garbelotto, et al., 2002). There have been few observations of the disease on bigleaf maple.

What about Coast Redwood, is it a host?

In early 2002 the press prematurely reported coast redwood as a possible host. To confirm a new species as a host requires careful laboratory, greenhouse, and field-testing. This research is currently being completed and official results are expected this spring to determine if coast redwood can be a host to Phytophthora ramorum.

Who do I contact, if I suspect Sudden Oak Death is present?

Humboldt County
Humboldt County Agricultural Commissioner's Office
5630 South Broadway, Eureka CA 95501
Phone: (707) 445-7223 Fax: (707) 445-7220

Del Norte County
Del Norte County Agricultural Commissioner's Office
2650 Washington Blvd., Crescent City, CA 95531
Phone: (707) 464-7231 Fax: (707) 465-6044

Where can I learn more about Sudden Oak Death?

For the most current information about sudden oak death, including photos of the host species symptoms, see the California Oak Mortality Task Force website (www.suddenoakdeath.org).





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