Since 1995, large numbers (up to 80% in some areas) of tanoaks (Lithocarpus densiflorus), coast live oaks (Quercus agrifolia) and black oaks (Quercus kelloggii) have been dying in California’s coastal counties. The epidemic, referred to as Sudden Oak Death (SOD), was first seen on tanoak in Mill Valley (Marin County) in 1995. Since then, it has spread north into ten central coastal counties of California and Curry County, Oregon. In 2003 the disease was also found for the first time in nurseries in British Columbia, California, Oregon, and Washington.
In June 2000, University of California researchers isolated a previously unknown species of Phytophthora (“Phy-TOFF-thoruh”), a fungus-like organism, from dying oak trees. Relatives of this “fungus” caused the Irish potato famine, Port-Orford cedar root disease in the Pacific Northwest and are causing oak dieback in many parts of the world. In January 2001 researchers reported that a new species of Phytophthora isolated as early as 1993 from ornamental rhododendrons in Germany and The Netherlands matches the newly discovered species found in California. This new species has since been officially named: Phytophthora ramorum. The name refers to the pathogen’s tendency to cause infection on branches.
Important research discoveries have continued since then. So far, Garry oaks haven’t been affected by SOD. Researchers also found that the plants most prone to the disease include several other oak species, blueberries, honeysuckle, huckleberry, rhododendrons, azaleas, viburnum, pieris, and camellia. Some species of Douglas-fir, arbutus, sumac, and maple are also at risk. Scientists determined that Phytophthora ramorum may be spread through infected wood, soil and rainwater. However, probably the most important way in which humans spread the pathogen around is by moving infected plants and plant parts. The leaves of hosts such as bays, madrones and rhododendrons contain large amounts of spores, which may be dispersed through the air under moist and windy conditions.
For more information on sudden oak death, visit the following web sites:
- Canadian Food Inspection Agency
- Canadian Nursery Landscape Association
- Oregon Department of Agriculture
- California Oak Mortality Task Force
- University of California Cooperative Extension in Marin County
- USDA Forest Service (Sudden Oak Death - A guide for forest managers, Christmas tree growers, and forest-tree nursery operators in Oregon and Washington)
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