Kurt W. Gottschalk1, Randall S. Morin2, and Andrew M. Liebholds2
Sudden oak death syndrome is caused by the organism, Phytophthora ramorum, and was first discovered in central coastal California in 1995. The non-native disease has killed large numbers of oaks (coast live oak, Quercus agrifolia, California black oak, Q. kelloggii, and Shreve oak, Q. parvula var. shrevei), tanoaks (Lithocarpus densiflorus), and Pacific madrone (Arbutus menziesii) and has recently been found in coastal Oregon. The organism has not affected California species in the white oak group. Greenhouse tests of eastern oak species pin oak (Q. palustris) and northern red oak (Q. rubra) have shown that these species are just as susceptible to the pathogen as their west coast relatives. Based on this knowledge, we developed a preliminary map of the potential susceptibility to Phytophthora ramorum for Eastern forests in order to prepare for quarantine and management actions should the pathogen be introduced or move naturally to the Eastern U.S.
The map was developed using FIA (Forest Inventory and Analysis) periodic inventory plot data from the Eastwide FIA database. While only two species have been tested, based on the California data, we assumed that all species in the red oak and live oak groups in the East would be susceptible to the pathogen. The proportion of the total plot basal area that included these oak species was mapped for all Eastern state periodic FIA points. Kriging was used to develop a surface from the point data. A forest density layer was used to mask the surface to remove non-forested areas and to reduce the proportions based on forest density. The Northeastern states conducted a shrub inventory during these periodic inventories. This shrub data was recently obtained and binary classifications were used based on presence or absence of Phytophthora ramorum shrub hosts, primarily evergreen ericaceous species. Indicator kriging was used to generate a probability surface of shrub host presence.
The result of the overstory tree analysis is a map that shows potential susceptibility of forests containing those two oak groups for the eastern United States. This map shows that over half of the forests in the Eastern U.S. contain some susceptible hosts. The majority of these susceptible forests contain only a small percentage of susceptible hosts (less than 20 percent), so impacts on these forests may be less critical than in the areas that contain large percentages (up to 90 percent in some areas). Areas with high proportions of susceptible hosts include the Ozark-Ouachita Highlands, pin oak sand flats in the Lake States, and live oak areas in Florida. These susceptible areas can be further defined by using understory hosts. The Northeastern states shrub data is being used to refine the overstory susceptibility for that area. Climatic factors are also an important component of vulnerability of hosts to the pathogen. Future work will add these factors into the analysis.
The Eastern oak forests of the United States appear to have plenty of susceptible hosts, both overstory and understory, that would make the establishment of Phytophthora ramorum in these forests a very serious threat. In a worst case scenario, it could rival or exceed the chestnut blight in its impact on the forest ecosystems of the East.
