Identification and Frequency of Phytophthora Species Associated with Foliar Diseases in California Ornamental Nurseries.

Numerous ornamental nurseries in 32 California counties were surveyed for leaf spots as part of the California Department of Food and Agriculture mandated surveys targeting Phytophthora ramorum. Tissue collected during the 2005 and 2006 surveys was initially screened by a Phytophthora-specific enzyme-linked immunosorbent assay. All positives samples were further tested using polymerase chain reaction to determine if P. ramorum was present. P. ramorum was detected in 1% of the total number of samples taken during the surveys. A total of 377 isolates were identified as species of Phytophthora other than P. ramorum, and their identity was determined by internal transcriber spacer (ITS) sequences. Subsets of the putative ITS-species were further verified using accepted morphological characters. Thirteen species of Phytophthora were found: P. cactorum, P. cambivora, P. citricola, P. citrophthora, P. cryptogea, P. foliorum, P. gonapodyides, P. hibernalis, P. nemorosa, P. 'Pgchlamydo', P. pseudosyringae, P. syringae, and P. tropicalis. P. syringae and P. citricola made up 55% of the total number of isolates. Species thought to be strictly forest pathogens, P. nemorosa and P. pseudosyringae, each made up less than 4% of the isolates. To test pathogenicity of acquired isolates, subsets of different species of Phytophthora were inoculated onto leaves of selected host plant genera. Of the 66 pathogen-host genera combinations tested, 44 resulted in lesion formation. Disease symptoms appeared as dark, water-soaked lesions with irregular margins and were similar among Phytophthora species.