Sudden Oak Death (Phytophthora ramorum) - SOD (also known as Phytophthora canker disease), was originally identified in Germany and The Netherlands in the early 1990's on Rhododendrons . Since being discovered in the United States, it has been confirmed in forests from California, Oregon, Washington and British Columbia. The origin geographically of Phytophthora ramorum is unknown and before the early 1990's there were no reports in Europe or the United States. The areas that do exist in Europe and the United States are believe to have been originally transported from other areas or even the original site of origin. Phytophthora ramorum's very limited distribution related to the host's distribution suggests a more recent introduction versus a point of origin.
Image Citation: Joseph O'Brien, USDA Forest Service, Bugwood.org
Two types of disease are caused by Phytophthora ramorum, the first being bark cankers and the second being foliar blights. Bark cankers may eventually kill the host while foliar blights serve as a reservoir for the pathogen to remain within and be tranferred from the foliar host. The list of hosts (and foliar hosts) seems to grow with each new report and now includes Coast and Canyon Live Oak, Tanoak, California Black Oak, Coast Redwood, Douglas Fir, Rhododendron, Bay Laurel, California Buckeye, Madrone, Bigleaf Maple, Oregon Myrtle, Toyon, Honeysuckle, Arrowwood, Camellia, Californis Hazelnut, Mountain Laurel, Valley Oak, Poison Oak and Grand Fir. In lab testing it has been found that both Red and Pin Oaks are susceptible this opens up the potential for spread into the Eastern portions of the US as the Red Oak family is found in most of North America. In the field the White Oak family including the Valley, White and Blue Oaks have not been confirmed as hosts or even shown any symptoms- hopefully this means they are immune to Phytophthora ramorum or at least have a higher tolerance level.
Image Citation: Bruce Moltzan, USDA Forest Service, Bugwood.org
As with many diseases of woody plants the spread of Phytophthora ramorum most likely occurs from contact with foliar hosts, infected material, soil transfer and spreading by rainwater. Windy, cool and moist conditions are also thought to aide in the spread of the pathogen by further dispersing the spores from their foliar hosts. Transporting (for nursery sale, wholesale or production) of foliar hosts may also aide in the spread of this disease making it harder to control.
The symptoms of Sudden Oak Death are easily identified by large cankers on the trunk or main stem, browning of the leaves or even death of the entire plant/tree. Some infected trees also become host to Bark or Ambrosia Beetles, or Sapwood rotting fungus-these outside organisms may speed up or even contribute to the death of the host. Foliar host infection os harder to identify and may not be noticed until it is to late. With a foliar host you may notice deep gray or brown lesions on the leaf blades, vascular tissues, petiole, or stems of the host.