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For nearly two decades, stream baiting of northern and central California coastal streams has been an important tool in the management of sudden oak death, a devastating forest disease caused by the oomycete Phytophthora ramorum. Phytophthora species are baited with floating rhododendron leaves, serving as an early detection tool for the presence of P. ramorum in watersheds across more than 800 km of California coastline. While this long-standing management tool is focused on a single species of Phytophthora, other species of Phytophthora have been baited alongside P. ramorum, and this study documents the presence and distribution of 22 Phytophthora and Nothophytophthora species across the northern and central coasts of California. Although P. ramorum was isolated at the greatest number of sites, several species in subgeneric clade 6 were also abundant and widespread, a common feature of Phytophthora stream baiting studies. Clade 3 species P. nemorosa, P. pluvialis, and P. pseudosyringae were also frequently isolated in northern coastal streams. The species Nothophytophthora caduca and the genus Nothophytophthora are reported for the first time in North America along with the first report of P. pluvialis in California. Two novel species, Nothophytophthora sp. californica and P. sp. aureomontensis (a member of the P. citricola species complex) are provisionally named. Mitochondrial sequences revealed multiple hybridization events between P. lacustris and P. riparia. Stream monitoring can serve as an important tool for monitoring ongoing Phytophthora invasions as well as establishing baseline pathogen communities, critical data for preventing future invasions. 

A collection of 30 Phytophthora cactorum and 12 P. pseudotsugae (subclade 1a) strains isolated from several recent surveys across California was phylogenetically compared to a worldwide collection of 112 conspecific strains using sequences from three barcoding loci. The surveys baited P. cactorum from soil and water across a wide variety of forested ecosystems with a geographic range of more than 1000 km. Two cosmopolitan lineages were identified within the widespread P. cactorum, one being mainly associated with strawberry production and the other more closely associated with apple orchards, oaks and ornamental trees. Two other well-sampled P. cactorum lineages, including one that dominated Californian restoration outplantings, were only found in the western United States, while a third was only found in Japan. Coastal California forest isolates of both Phytophthora species exhibited considerable diversity, suggesting both may be indigenous to the state. Many isolates with sequence accessions deposited as P. cactorum were determined to be P. hedraiandra and P. ×serendipita, with one hybrid lineage appearing relatively common across Europe and Asia. This study contains the first report of P. pseudotsugae from the state of California and one of the only reports of that species since its original description. 

Phytophthora is a long-established, well known and globally important genus of plant pathogens. Phylogenetic evidence has shown that the biologically distinct, obligate biotrophic downy mildews evolved from Phytophthora at least twice. Since, cladistically, this renders Phytophthora ‘paraphyletic’, it has been proposed that Phytophthora evolutionary clades be split into multiple genera (Runge et al. 2011; Crous et al. 2021; Thines et al. 2023; Thines 2024). In this letter, we review arguments for the retention of the generic name Phytophthora with a broad circumscription made by Brasier et al. (2022) and by many delegates at an open workshop organized by the American Phytopathological Society. We present our well-considered responses to this proposal in general terms and to the specific proposals for new genera; together with new information regarding the biological properties and mode of origin of the Phytophthora clades. We consider that the proposals for new genera are mostly non-rigorous and not supported by the scientific evidence. Further, given (1) the apparent lack of any distinguishing biological characteristics (synapomorphies) between the Phytophthora clades; (2) the fundamental monophyly of Phytophthora in the original Haeckelian sense; (3) the fact that paraphyly is not a justification for taxonomic splitting; and (4) the considerable likely damage to effective scientific communication and disease management from an unnecessary break-up of the genus, we report that Workshop delegates voted unanimously in favour of preserving the current generic concept and for seeking endorsement of this view by a working group of the International Commission on the Taxonomy of Fungi.