W.D. Smith1, J.W. Coulston2, E.M. Goheen3, F. Sapio4, K.W. Gottschalk5, S. Frankel6, P. Dunn7, and B. M. Tkacz8
Sudden Oak Death, a newly identified forest disease caused by the pathogen Phytophthora ramorum, has been killing thousands of tanoak and oaks and infecting shrub species such as rhododendron and evergreen huckleberry in coastal areas of central California. Small infestations were recently found in southern Oregon and eradication efforts have begun. Laboratory investigations indicate that other oak species, including northern red and pin oak, are susceptible to the pathogen. Concerns regarding the risk of this disease to the Nation’s oak forests have led to development of a National Sudden Oak Death Detection Survey of forests through the Forest Health Monitoring program.
A Risk map was developed for detecting the possible presence of P. ramorum based on three factors: (1) distribution of known and suspected host species, (2) likely pathways of introduction to new areas, and (3) climatic conditions that favor or limit the development of the pathogen.
Host susceptibility studies have only recently begun; preliminary results suggest that members of the white oak group are not susceptible to P. ramorum; therefore, in addition to those species confirmed to be susceptible, the red oak, live oak, and willow oak groups and evergreen ericarious shrubs such as are assumed to be at risk.
The evergreen hosts such as rhododendron and huckleberry are currently considered to be very important inoculum sources. Introduction into the East is likely through Rhododendron Nurseries. Recent reports on the susceptibility of Douglas-fir suggest that Christmas tree farms in the east are an additional pathway of introduction.
Climatic factors were the maximum and minimum temperature that Phytophthora ramorum spores are unlikely to survive and the number of the months of adequate rainfall (5 inches or 3 inches and 1 one day of dense fog) during optimum temperature (60 and 80 degrees F) for infection.
An ordinal scale risk map was developed by the spatial intersection of the three factors identifying regions of high (X), Moderate (1/3X), and Low (1/64X) risk. This approach to will allow the participating agencies to focus the initial detection surveys in high-risk areas in order to maximize the efficiency of the limited resources available.
Locations of known infected areas are not used in the mapping process. Comparison of infected locations with expected risk provides an opportunity to evaluate the veracity of the procedure.
