Sudden oak death is a new disease capable of causing a range of symptoms from leaf spots to plant death on many woody hosts. Because sudden oak death is a new disease, much is unknown about the pathogen, host range, and the disease epidemiology.
CAUSAL AGENT and GEOGRAPHIC DISTRIBUTION
Sudden oak death is caused by Phytophthora ramorum (a water mold). The historical origin of the pathogen is unknown but it was first described in Europe on ornamental Rhododendron sp. and Viburnum sp. in 2001. In 2002 the pathogen was reported in California and Oregon and has since been found in western Canada.
Various symptoms have been produced on more than 40 species of native and cultivated ornamental plants infected with the pathogen. The list is not exclusive as many plant species have not yet been tested. It is likely that species closely related to susceptible hosts also could be infected by P. ramorum. Additional hosts will be identified as the pathogen is spread to new areas.
Symptoms of this disease vary from host to host; however, roots of plants infected with P. ramorum typically appear healthy. Symptoms may progress rapidly after infection or may not be visible for significant periods of time. Symptom progression is favored by temperatures near 20 degrees Celsius (68 degrees Fahrenheit).
SPREAD of the PATHOGEN and CONTROL OPTIONS
The fungus can be spread by movement of infected host material, infested soil, irrigation water, and wind-blown rain. Unintentional movement of infected but asymptomatic nursery stock is also a potential means of pathogen dissemination. Because this is a new pathogen, the best option for controlling spread of the disease is preventing its introduction and establishment in new areas. Quarantines and eradication programs in conjunction with extensive surveys are the most effective way to deal with potential introductions. Eradication efforts include burning and deep burial of infected plant material. Fungicides have not been evaluated for management of this disease. It is possible that fungicides that prevent and control diseases caused by other Phytophthora spp. may be effective, but no data are available at this time.
Philip F. Harmon, assistant professor, Plant Pathology Department. Carrie Harmon, plant pathology coordinator, Southern Plant Diagnostic Network, Plant Pathology Department, Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, 32611.