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Sudden Oak Death

What is Sudden Oak Death? 

Sudden Oak Death (SOD) is a forest disease, caused by a fungus known as Phytophthora ramorum that accumulates on host plants (such as Ericaceous plants, which include heathers, azaleas, and rhododendrons) which live in the under story of forests in cool temperatures with relatively high moisture.


Within nurseries and garden centers there is a high risk of P. ramorum spreading to alternative potential hosts. The inoculum might spread from nurseries and garden centers to natural vegetation either through direct transmission or through transplanted infected plants. The broad plant host range and the potential for widespread distribution of the pathogen in the nursery industry and nationwide through transportation and marketing of nursery plants make this a critical issue.


There is little concrete information on the life history of P. ramorum in affected U.S. sites, but researchers currently believe that infections on foliar hosts – such as arbutus, bay, huckleberry, rhododendron and buckeye – may contribute to a rapid build-up of the fungus in the environment, serving as a reservoir of inoculum which, in turn, infects woody tissues of oaks and tanoaks. 


The American strain of SOD was found for the first time in 1995 in Mill Valley, California. Since then, it has been detected in forests in 17 California counties. In 2001 it was found in Curry County, Oregon near the California border. Native Washington oaks appear to be resistant to the disease. Other plants common to the Pacific Northwest are susceptible to the disease, but are not likely to die. 

Since May 2003, the European strain of SOD has been detected in nurseries in California, Oregon, Washington, and British Columbia, Canada. However, there is no evidence that the European strain has moved beyond the nursery environment. 

P. ramorum causes two types of diseases: bark cankers that may kill the host, and foliar blights that may serve as a resevoir for the pathogen. The spread of SOD likely occurs through infected plant material, rainwater, and soil. Foliar hosts may play an important role in the transmission of P. ramorum to bark canker hosts.

The common name “Sudden Oak Death” refers to the apparent rapid decline of the entire tree crown; the foliage of trees affected often turns from an apparently healthy green color to brown in only a few weeks. However, the decline of a tree’s health can actually take several months or years after it’s been exposed to the fungus. It is expected that disease progression may vary based on different tree individuals, tree species, geographic regions, and other factors.


There is no pesticide registered by the U.S. Environmental Protection Agency that will eradicate the organism that causes SOD. The only way to stop the disease is to cut down and burn infected plants or trees.

History of Sudden Oak Death  

The geographic origin of P. ramorum is unknown. It was first identified in 1993 in Germany and the Netherlands on ornamental rhododendrons. Since its discovery in North America in 1995, P. ramorum has been confirmed in forests in California and Oregon and in nurseries in California, Oregon, Washington, and British Columbia.


In general, Sudden Oak Death (SOD) has been observed, to date, in three main forest types in California:

  • Mixed Evergreen-Bay-Arbutus - these are fairly dry open forests with a grass and dense shrub under story;
  • Tanoak - Douglas-fir - this is a wetter forest, frequently with Vaccinium (Huckleberry) being the major under story species;*
  • Coast Redwood - these sites are usually more open, often with tanoak as the major under story component.

*Oregon sites are ecologically quite similar to tanoak-Douglas-fir sites in California, although wetter.


Three species of native California oaks (Quercus spp.) and a close relative of oaks, called tanoak or tanbark oak (Lithocarpus densiflorus), may be killed by the disease. To date, only oaks in the black or red oak group have been found to be susceptible to the disease, white oaks are currently not known to be hosts. At least 11 other unrelated plant species may serve as hosts for P. ramorum. On many of these hosts, infection by the pathogen may not lead to death of the whole plant, but rather to leaf spots and/or twig and branch dieback. The Latin name of the pathogen means “destructor of branches” reflecting its discovery by plant pathologists in Europe in 1993 as the cause of a new disease of leaves and branches of ornamental rhododendrons. Foliar hosts can at times be killed, especially if host plants are shrubby or small. Progressive dieback of branches can also lead to plant mortality -- normally the case if conditions conducive to the disease persist for several years in a row.


Nurseries and Greenhouses: Economic Output  

According to the Economic Research Service of the U.S. Department of Agriculture, the nursery and greenhouse industry comprises the fastest growing segment of U.S. agriculture.  The U.S. is the world's largest producer and market for nursery and greenhouse crops and these crops represent an important and unique segment of agriculture whose impact is felt on the national, state, and community levels.

  • In terms of economic output, nursery and greenhouse crops represent the third most important sector in U.S. crop agriculture, ranking seventh among all commodities in cash receipts, and among the highest in net farm income. U.S. production of nursery crops was estimated at $8.9 billion in 2002 and at $9.4 billion in 2003.
  • Nursery and greenhouse crops are the top five commodities in 27 states, and the top 10 commodities in 42 states.
  • Seven states account for almost two-thirds of all nursery-crop output in the U.S.: California (24%), Texas (11%), Florida (9%), North Carolina (9%), Oregon (7%), Ohio (4%) and Maryland (3%).

Eighty-five million U.S. households spent $39.6 billion at lawn and garden retail outlets in 2002, according to the National Gardening Association and Harris Interactive, while more than 24.7 million households spent $28.9 billion on professional landscape, lawn and tree care services. Additional descriptive information of the U.S. nursery industry is provided by the American Nursery and Landscape Association.  The average per-household purchase of nursery plants in 2002 was $84.


Resources & the Current Situation of SOD  

  • Phytophthora ramorum, A Guide for Oregon Nurseries (pdf) – Oregon State University Extension Service
  • National Pest Alert: Sudden Oak Death - North Central Pest Management Center 
  • 2003 Diagnostic Guide: Sudden Oak Death and Associated Diseases Caused by Phytophthora ramorum (pdf)The Plant Management Network
  • CSREES Collaborates on Responses to Phytophthora ramorum (P. ramorum) - CSREES
  • Sudden Oak Death and the California Oak Mortality Task Forceunder the California Forest Pest Council -- This Web site contains California-specific information on infested counties, SOD symptoms, affected plants, current regulations and quarantines, and more. 

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    Last Updated:10/2/2009 1:46 AM

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