A Survey of Phytophthora spp. in Midwest Nurseries, Greenhouses, and Landscapes

A survey of nurseries, greenhouses, and landscapes was conducted from 2006 to 2008 in order to determine the prevalence and diversity of Phytophthora spp. From sites in Iowa, Michigan, Ohio, and, predominantly, Indiana, 121 Phytophthora isolates were obtained from 1,657 host samples spanning 32 host genera. Based on sequence of the internal transcribed spacer (ITS) region of the ribosomal DNA, 11 Phytophthora spp. and two hybrid species were identified. A majority of the isolates were P. citricola (35.9%) or P. citrophthora (27.4%). Six isolates were confirmed as hybrids (four of P. cactorum × hedraiandra and two of P. nicotianae × cactorum) by cloning and sequencing the ITS region. Three P. cactorum × hedraiandra isolates were obtained from the same site, from three Rhododendron spp., which are known hosts to the parental species. The fourth isolate, however, was recovered out of a different location in a Dicentra sp., which is not a known host to either parental species, suggesting an expansion of host range of the hybrid isolate as compared with either parental species.