Phytophthora species associated with forest soils in Central and Eastern U.S. oak ecosystems.
The existence of native and exotic species of Phytophthora in soils of eastern and central oak ecosystems is largely unknown. This informational void and the potential threat of P. ramorum to eastern oak species provided the impetus for a multiple state survey of soils associated with oak cover types. The initial survey was conducted from April to June 2004 in oak forests in Illinois, Indiana, Maryland, Michigan, Minnesota, Ohio, Pennsylvania, Wisconsin and West Virgina. Sampling sites were chosen to avoid areas impacted by oak wilt, recent storm damage or major defoliation. Although most stands contained a diverse community of tree species, oaks generally predominated. Stands were greater than 40 years of age and located on moist sites. Eight sites were usually sampled in each state. At each site, four soil samples were taken 1.5 m from the base of an oak tree in four cardinal directions. Sub-samples from each tree were bulked; five trees were sampled per site. As of January 2005, a total of 96 sites were surveyed and soils from 499 oak trees sampled. An oak leaf baiting procedure was used whereby soils from each tree were placed in a container, mixed and flooded with distilled water. Three- to seven-day-old Quercus robur leaflets were floated on the water surface to bait Phytophthora. Leaf samples that trapped Phytophthora were plated on PARPNH-medium. When initial isolations attempts failed, soils were dried at room temperature and the isolation procedure repeated. Twenty-three percent of the samples from individual trees yielded Phytophthora; P. cinnamomi was the most frequently recovered species (77 percent). Other species recovered included P. europaea, P. cambivora, P. citricola and yet undescribed species. A comparable survey was conducted during fall 2004 to establish a more complete assemblage of Phytophthora species so that studies of their role in forest health could be initiated.