Phytophthora ramorum

Phytophthora ramorum was first recognized in the U.S. in 1994 and was likely introduced via international trade of commercial plants (Ivors and others 2006). Since its introduction, the pathogen has infected western live and red oaks in coastal forests of California and Oregon, sometimes causing mortality greater than 40 percent (Garbelotto and others 2001, Garbelotto and others 2003). In addition, P. ramorum infects dozens of commercial shrub host species that can yield large numbers of aerially dispersed spores (Davidson and Shaw 2003, Davidson and others 2002, Tooley and others 2004). Many of these shrubs, (e.g., rhododendrons, azaleas, camellias) are sold as nursery stock (Garbelotto and others 2001, Tooley and others 2004). In the past few years, wholesale nurseries on the west coast have unknowingly shipped infected plants to retail and wholesale outlets in roughly 40 States (Stokstad 2004), although surveys have not detected the pathogen in natural forests outside California and Oregon.

A large portion of the Eastern United States is considered at high risk for establishment of P. ramorum if it is introduced into forested areas. Much of the concern has to do with climatic conditions believed to be favorable for the pathogen. Growth, sporulation, and infection are all affected by moisture and temperature. Optimal temperatures for P. ramorum growth, based on laboratory analysis, appear to be between 64.4 °F and 71.6 °F (Werres and others 2001), but some growth occurs across a wider temperature range (up to at least 80 °F). Peak sporangia formation appears to occur at 59-68 °F (Davidson and others 2005). Persistent moisture on foliage is considered critical to spread. Laboratory inoculation trials on California bay laurel (Umbellularia californica), a major source of P. ramorum spores in California, suggest 9-12 hours of free moisture on leaf surfaces under appropriate temperatures are necessary for significant leaf infection (Garbelotto and others 2003). Further studies suggest that at least 24-48 hours of generally wet conditions are necessary for sporulation, with infection requiring additional time (Davidson and Shaw 2003, Davidson and others 2002, Rizzo and Garbelotto 2003). Fog and high relative humidity may be important for spread of aerial Phytophthora species within forest stands (Werres 2003), as high air moisture can keep leaf surfaces wet and enable spore production. Nevertheless, despite regular summer fog in California, P. ramorum sporulation and infection seem to be restricted to the winter-spring rainy season (Rizzo and others 2005). Isolated rains during otherwise dry summer months do not appear to facilitate spore production or dispersal (Davidson and others 2002). Ultimately, it is unknown how the pathogen’s behavior on the west coast will translate to the Eastern United States, where warm season and cool season precipitation are similar (Akin 1991).