Molecular Phylogenomics and Population Structure of Phytophthora pluvialis.

Phytophthora pluvialis is an oomycete that was first isolated from soil, water, and tree foliage in mixed Douglas-fir-tanoak forests of the U.S. Pacific Northwest (PNW). It was then identified as the causal agent of red needle cast of radiata pine (Pinus radiata) in New Zealand (NZ). Genotyping-by-sequencing was used to obtain 1,543 single nucleotide polymorphisms across 145 P. pluvialis isolates to characterize the population structure in the PNW and NZ. We tested the hypothesis that P. pluvialis was introduced to NZ from the PNW using genetic distance measurements and population structure analyses among locations between countries. The low genetic distance, population heterozygosity, and lack of geographic structure in NZ suggest a single colonization event from the United States followed by clonal expansion in NZ. The PNW Coast Range was proposed as a presumptive center of origin of the currently known distribution of P. pluvialis based on its geographic range and position as the central cluster in a minimum spanning network. The Coastal cluster of isolates were located at the root of every U.S. cluster and emerged earlier than all NZ clusters. The Coastal cluster had the highest degree of heterozygosity (Hs = 0.254) and median pairwise genetic distance (0.093) relative to any other cluster. Finally, the rapid host diversification between closely related isolates of P. pluvialis in NZ indicate that this pathogen has the potential to infect a broader range of hosts than is currently recognized.

Ecology and pathology of Phytophthora ITS clade 3 species in forests in western Oregon, USA.

Phytophthora species are widespread and diverse in forest ecosystems, but little is known about their ecology. We explore ecological attributes of the closely related clade 3 species that occur sympatrically in western North American forests. We address the population structure, pathology, and epidemiology of P. ilicis, P. nemorosa, P. pluvialis, P. pseudosyringae, and P. psychrophila. Phytophthora species were isolated from plant tissues, rainwater falling through the forest canopy, streams, and soils in forests in western Oregon. Species identifications were based on morphology in culture with molecular confirmation using COX spacer and internal transcribed spacer (ITS) sequences. All five clade 3 Phytophthora species are present in western Oregon forests, although P. ilicis (only 1 forest isolate) and P. psychrophila (only 12 isolates) are apparently rare. P. ilicis is known only from holly in horticultural situations and once from a naturalized seedling in an urban forest. The known distribution of P. nemorosa in forest settings coincides with the ranges of its principle hosts, tanoak and myrtlewood, in Oregon and California. Although it is regularly identified from streams within the tanoak range, it has not been recovered from streams beyond that range. P. pluvialis is primarily associated with Douglas-fir canopies. It was identified from scattered locations throughout western Oregon in rain traps beneath Douglas-fir plantations and from diseased needles. P. pseudosyringae is also isolated from tanoak and myrtlewood in southwest Oregon and California, but its distribution, in streams at least, extends throughout much of western Oregon. P. psychrophila in Oregon is known only from rain traps beneath tanoak trees. Little intraspecific variation was detected in the nuclear rDNA ITS of clade 3 species. Variation in the mitochondrial COX spacer region was more frequent, with 2 to 10 haplotypes identified in the clade 3 species, for which we had multiple isolates.

Contrasting microsatellite diversity in the evolutionary lineages of Phytophthora lateralis.

Following recent discovery of Phytophthora lateralis on native Chamaecyparis obtusa in Taiwan, four phenotypically distinct lineages were discriminated: the Taiwan J (TWJ) and Taiwan K (TWK) in Taiwan, the Pacific Northwest (PNW) in North America and Europe and the UK in west Scotland. Across the four lineages, we analysed 88 isolates from multiple sites for microsatellite diversity. Twenty-one multilocus genotypes (MLGs) were resolved with high levels of diversity of the TWK and PNW lineages. No alleles were shared between the PNW and the Taiwanese lineages. TWK was heterozygous at three loci, whereas TWJ isolates were homozygous apart from one isolate, which exhibited a unique allele also present in the TWK lineage. PNW lineage was heterozygous at three loci. The evidence suggests its origin may be a yet unknown Asian source. North American and European PNW isolates shared all their alleles and also a dominant MLG, consistent with a previous proposal that this lineage is a recent introduction into Europe from North America. The UK lineage was monomorphic and homozygous at all loci. It shared its alleles with the PNW and the TWJ and TWK lineages, hence a possible origin in a recent hybridisation event between a Taiwan lineage and PNW cannot be ruled out.

The Phytophthora species assemblage and diversity in riparian alder ecosystems of western Oregon, USA.

Phytophthora species were systematically sampled, isolated, identified and compared for presence in streams, soil and roots of alder (Alnus species) dominated riparian ecosystems in western Oregon. We describe the species assemblage and evaluate Phytophthora diversity associated with alder. We recovered 1250 isolates of 20 Phytophthora species. Only three species were recovered from all substrates (streams, soil, alder roots): P. gonapodyides, the informally described "P. taxon Pgchlamydo", and P. siskiyouensis. P. alni ssp. uniformis along with five other species not previously recovered in Oregon forests are included in the assemblage: P.citricola s.l., P. gregata, P. gallica, P. nicotianae and P. parsiana. Phytophthora species diversity was greatest in downstream riparian locations. There was no significant difference in species diversity comparing soil and unwashed roots (the rhizosphere) to stream water. There was a difference between the predominating species from the rhizosphere compared to stream water. The most numerous species was the informally described "P. taxon Oaksoil", which was mainly recovered from, and most predominant in, stream water. The most common species from riparian forest soils and alder root systems was P. gonapodyides.

Temporal Epidemiology of Sudden Oak Death in Oregon.

An effort to eradicate Phytophthora ramorum, causal agent of sudden oak death, has been underway since its discovery in Oregon forests. Using an information-theoretical approach, we sought to model yearly variation in the size of newly infested areas and dispersal distance. Maximum dispersal distances were best modeled by spring and winter precipitation 2 years before detection, and infestation size the year prior. Infestation size was best modeled by infestation size and spring precipitation the year prior. In our interpretation, there is a 2-year delay between the introduction of inoculum and onset of mortality for a majority of sites. The year-long gap in between allows ample time for the production of inoculum contributing to the spread of P. ramorum. This is supported by epidemic development following changes in eradication protocols precipitated by an outbreak in 2011, attributable to a 2009 treatment delay and an uncharacteristically wet spring in 2010. Posteradication, we have observed an increase in the total area of new outbreaks and increased frequency in dispersal distances greater than 4 km. Although the eradication program has not eliminated P. ramorum from Oregon forests, it has likely moderated this epidemic, emphasizing the need for prompt treatment of future invasive forest pathogens.

Strong genetic differentiation between North American and European populations of Phytophthora alni subsp. uniformis.

Alder decline caused by Phytophthora alni has been one of the most important diseases of natural ecosystems in Europe during the last 20 years. The emergence of P. alni subsp. alni -the pathogen responsible for the epidemic-is linked to an interspecific hybridization event between two parental species: P. alni subsp. multiformis and P. alni subsp. uniformis. One of the parental species, P. alni subsp. uniformis, has been isolated in several European countries and, recently, in North America. The objective of this work was to assess the level of genetic diversity, the population genetic structure, and the putative reproduction mode and mating system of P. alni subsp. uniformis. Five new polymorphic microsatellite markers were used to contrast both geographical populations. The study comprised 71 isolates of P. alni subsp. uniformis collected from eight European countries and 10 locations in North America. Our results revealed strong differences between continental populations (Fst = 0.88; Rst = 0.74), with no evidence for gene flow. European isolates showed extremely low genetic diversity compared with the North American collection. Selfing appears to be the predominant mating system in both continental collections. The results suggest that the European P. alni subsp. uniformis population is most likely alien and derives from the introduction of a few individuals, whereas the North American population probably is an indigenous population.

Four phenotypically and phylogenetically distinct lineages in Phytophthora lateralis.

Until recently Phytophthora lateralis was known only as the cause of dieback and mortality of Chamaecyparis lawsoniana in its native range in the Pacific Northwest (PNW). Since the 1990s however disease outbreaks have occurred increasingly on ornamental C. lawsoniana in Europe; and in 2007 the pathogen was discovered in soil around old growth Chamaecyparis obtusa in Taiwan, where it may be endemic. When the phenotypes of over 150 isolates of P. lateralis from Taiwan, across the PNW (British Columbia to California) and from France, the Netherlands and the UK were compared three growth rate groups were resolved: one slow growing from Taiwan, one fast growing from the PNW and Europe, and one of intermediate growth from a small area of the UK. Within these growth groups distinct subtypes were identified based on colony patterns and spore metrics and further discriminated in a multivariate analysis. The assumption that the three main growth groups represented phylogenetic units was tested by comparative sequencing of two mitochondrial and three nuclear genes. This assumption was confirmed. In addition two phenotype clusters within the Taiwan growth group were also shown to be phylogenetically distinct. These four phenotypically and genotypically unique populations are informally designated as the PNW lineage, the UK lineage, the Taiwan J lineage, and the Taiwan K lineage. Their characteristics and distribution are described and their evolution, taxonomic, and plant health significance is discussed.

Phytophthora borealis and Phytophthora riparia, new species in Phytophthora ITS Clade 6.

Phytophthora borealis and Phytophthora riparia, identified in recent Phytophthora surveys of forest streams in Oregon, California and Alaska, are described as new species in Phytophthora ITS Clade 6. They are similar in growth form and morphology to P. gonapodyides and are predominantly sterile. They present unique DNA sequences, however, and differ in temperature/growth relations and geographic distribution.

Phytophthora beyond agriculture.

Little is known about indigenous Phytophthora species in natural ecosystems. Increasing evidence, however, suggests that a diverse, trophically complex Phytophthora community is important in many forests. The number of described species has steadily increased, with a dramatic spike in recent years as new species have been split from old and new species have been discovered through exploration of new habitats. Forest soil, streams, and the upper canopies of trees are now being explored for Phytophthora diversity, and a new appreciation for the ecological amplitude of the genus is emerging. Ten to twenty species are regularly identified in temperate forest surveys. Half or more of this Phytophthora diversity comes from species described since 2000. Taxa in internal transcribed spacer (ITS) Clade 6 are especially numerous in forest streams and may be saprophytic in this habitat. Three ecological assemblages of forest Phytophthora species are hypothesized: aquatic opportunists, foliar pathogens, and soilborne fine-root and canker pathogens. Aggressive invasive species are associated with all three groups.

Phytophthora species in forest streams in Oregon and Alaska.

Eighteen Phytophthora species and one species of Halophytophthora were identified in 113 forest streams in Alaska, western Oregon and southwestern Oregon that were sampled by baiting or filtration of stream water with isolation on selective media. Species were identified by morphology and DNA characterization with single strand conformational polymorphism, COX spacer sequence and ITS sequence. ITS Clade 6 species were most abundant overall, but only four species, P. gonapodyides (37% of all isolates), P. taxon Salixsoil, P. taxon Oaksoil and P. pseudosyringae, were found in all three regions. The species assemblages were similar in the two Oregon regions, but P. taxon Pgchlamydo was absent in Alaska and one new species present in Alaska was absent in Oregon streams. The number of Phytophthora propagules in Oregon streams varied by season and in SW Oregon, where sampling continued year round, P. taxon Salixsoil, P. nemorosa and P. siskiyouensis were recovered only in some seasons.

Phytophthora-ID.org: A Sequence-Based Phytophthora Identification Tool.

Contemporary species identification relies strongly on sequence-based identification, yet resources for identification of many fungal and oomycete pathogens are rare. We developed two web-based, searchable databases for rapid identification of Phytophthora spp. based on sequencing of the internal transcribed spacer (ITS) or the cytochrome oxidase (cox) 1 and 2 spacer region, followed by BLAST searching the databases. Both databases are highly selective. For ITS, only sequences associated with published Phytophthora spp. descriptions or classic Phytophthora phylogenetics references are included. For the cox spacer region, only data obtained by resequencing select isolates reported in published work were included. Novel taxa tentatively named are selectively included in the database and labeled as Phytophthora taxon "X"; as in, for example, P. taxon "asparagi". The database was validated with 700 Phytophthora isolates collected from nursery environments during 2006 to 2009. This resource, found at www.Phytophthora-ID.org , is a robust and validated tool for molecular identification of Phytophthora spp. and is regularly being updated.

Phytophthora rosacearum and P. sansomeana, new species segregated from the Phytophthora megasperma "complex".

Phytophthora megasperma sensu lato was a conglomeration of morphologically similar but phylogenetically unrelated species. In this paper we continue the segregation of species from the old P. megasperma complex, formally naming two previously recognized isolate groups. Isolates recovered from rosaceous fruit trees (especially apple and cherry) are in ITS clade 6, related to but distinct from P. megasperma sensu strictu. They are named here Phytophthora rosacearum. They have been referred to previously as the "AC" or "high temperature small oospore" group of P. megasperma. A second group of isolates, earlier called "soybean race non-classifiable", recovered from soybeans in Indiana and other Midwestern states, are morphologically similar to P. megasperma sensu strictu but unrelated to that species, falling in ITS clade 8. They are named here P. sansomeana. Isolates recovered from Douglas-fir seedlings in nurseries in the Pacific Northwest and various weedy hosts in New York State, referred to in earlier work as "P. megasperma DF1", appear to be conspecific with the soybean isolates, although they include certain ITS DNA polymorphisms. Both new species are supported by a combination of new and previously published morphological, growth and molecular data.

The systematic position of Phaeocryptopus gaeumannii.

Phaeocryptopus gaeumannii, causal agent of the Douglas-fir foliar disease Swiss needle cast, is the only known pathogenic species of the genus. Current classifications place Phaeocryptopus in the Venturiaceae (Pleosporales), typified by the apple-scab pathogen Venturia inaequalis. All core members of this family have hyphomycetous anamorphs. We sought to confirm these relationships by means of phylogenetic analyses of the small (SSU) and large (LSU) subunits and internal transcribed spacer (ITS) region of nuclear ribosomal gene sequences (nrDNA). Analyses indicated that both the genus Phaeocryptopus and the family Venturiaceae, as currently defined, are unnatural groups. Phaeocryptopus nudus, type of the genus, is aligned in the Dothioraceae (Dothideales) and P. gaeumannii in the Mycosphaerellaceae (Capnodiales) near species of Mycosphaerella and Rasutoria. Core representatives of Venturiaceae formed an unambiguous clade but ordinal placement was unresolved. The family apparently is not included in the Pleosporales, Dothideales, Myriangiales or Capnodiales. Coelomycetous Rhizosphaera form-species are accepted generally as anamorphic states of Phaeocryptopus, however the relationship never has been established conclusively. Species of Rhizosphaera are closely related to P. nudus but not to P. gaeumannii, supporting an anamorph-teleomorph connection between Rhizosphaera and Phaeocryptopus and providing further evidence that P. gaeumanii is not congeneric with P. nudus.

Phytophthora austrocedrae sp. nov., a new species associated with Austrocedrus chilensis mortality in Patagonia (Argentina).

Phytophthora austrocedrae is a new species isolated from necrotic lesions of stem and roots of Austrocedrus chilensis. It is a homothallic species characterized by semipapillate sporangia, oogonia with amphigynous antheridia, and very slow growth (1-2 mm d(-1) on V-8 agar at 17.5 degrees C optimum temperature). Phylogenetic analysis of ITS rDNA sequence indicates that its closest relative is Phytophthora syringae, another species frequently isolated from soil and streams in A. chilensis forests.

Phytophthora siskiyouensis, a new species from soil, water, myrtlewood (Umbellularia californica) and tanoak (Lithocarpus densiflorus) in southwestern Oregon.

An unknown Phytophthora species was recovered in southwestern Oregon from rhododendron and tanoak leaf baits used for monitoring streams and soils for the presence of Phytophthora ramorum, from a blighted shoot of myrtlewood and from tanoak bark cankers. Isolates of this species yielded ITS-DNA sequences that differed substantially from other Phytophthora sequences in GenBank. Morphological features also differed from available descriptions of known Phytophthora species. Based on the combination of unique morphology and unique ITS sequences a new species is proposed. The new species, Phytophthora siskiyouensis, is homothallic with globose to subglobose oogonia, which may be terminal, sessile or laterally intercalary. Antheridia are capitate and mostly paragynous but sometimes amphigynous. Oospores are mostly aplerotic. Sporangia are variable but commonly ovoid to reniform, with apical, subapical or lateral semipapillae (occasionally more than one). Sporangia are terminal, subterminal or occasionally intercalary on unbranched sporangiophores, with basal, subbasal or lateral attachment. Sporangia are weakly deciduous, with variable length pedicels. This combination of characters clearly separates Phytophthora siskiyouensis from other known Phytophthora species.

Histopathology of Infection and Colonization of Susceptible and Resistant Port-Orford-Cedar by Phytophthora lateralis.

ABSTRACT Port-Orford-cedar (POC) root disease, caused by Phytophthora lateralis, continues to kill POC in landscape plantings and natural forests in western North America. POC trees resistant to P. lateralis have been identified and propagated. Cytological observations of P. lateralis in susceptible and resistant roots and stems were made with light and transmission electron microscopy to identify resistance mechanisms. No differences in infection pathway and initial colonization were observed between susceptible and resistant roots, although there were differences in the rate and extent of development. Germ tubes formed appressoria, and penetration hyphae grew either between or directly through epidermal cell walls; inter- and intracellular hyphae colonized the root cortex. In susceptible roots, hyphae penetrated into the vascular system within 48 h of inoculation. In contrast, hyphae in roots of resistant seedlings grew more slowly in cortical cells and were not observed to penetrate to the vascular tissues. In resistant roots, infection was marked by general thickening of cortical cell walls, wall appositions around penetrating hyphae, collapse of cortical cells, and accumulation of osmophillic granules around hyphae. In susceptible stems, hyphae grew inter- and intracellularly in all cells of the secondary phloem except fiber cells, but were concentrated in sieve and parenchyma cells in the functional phloem. The pattern of penetration and colonization of hyphae was similar in the resistant stems, except that hyphae were found in the fiber cells of the xylem. In resistant stems, there were fewer hyphae in the functional phloem, and cytological changes such as damaged nuclei and disintegrated cytoplasm were evident. Structural changes in resistant stems included collapsed cells, wall thickening, secretory bodies, apposition of electron dense materials, and crystals in cell walls.

Population structure suggests reproductively isolated lineages of Phaeocryptopus gaeumannii.

A survey of the genetic diversity and population structure of the Douglas-fir Swiss needle cast pathogen Phaeocryptopus gaeumannii was conducted with single-strand conformational polymorphisms (SSCP) to screen for variability in mitochondrial and nuclear housekeeping genes. Thirty host populations representing the natural range of Douglas-fir as well as locations where the tree was planted as an exotic were sampled. Sequencing of SSCP variants revealed that the method accurately detected both single nucleotide and indel polymorphisms. Sequence information was used to construct multilocus gene genealogies and to test various hypotheses of recombination (outcrossing) and clonality (selfing). We found that P. gaeumannii in the region of Oregon's Swiss needle cast epidemic exhibits strong multilocus gametic phase disequilibrium and is subdivided into two reproductively isolated sympatric lineages. Low genotypic diversity together with the presence of overrepresented genotypes in both lineages suggests a predominantly selfing reproductive mode. Genotypes of one lineage were found in isolates from a widespread geographic distribution, occurring throughout much of the Pacific Northwest as well as nonindigenous populations abroad that have historical reports of disease. Genotypes of the second lineage were detected only in isolates from Oregon's coastal region. Within the main epidemic area, abundance of this second lineage in young plantations appeared to be correlated with disease severity.

Phytophthora ramorum: integrative research and management of an emerging pathogen in California and Oregon forests.

Phytophthora ramorum, causal agent of sudden oak death, is an emerging plant pathogen first observed in North America associated with mortality of tanoak (Lithocarpus densiflorus) and coast live oak (Quercus agrifolia) in coastal forests of California during the mid-1990s. The pathogen is now known to occur in North America and Europe and have a host range of over 40 plant genera. Sudden oak death has become an example of unintended linkages between the horticultural industry and potential impacts on forest ecosystems. This paper examines the biology and ecology of P. ramorum in California and Oregon forests as well discussing research on the pathogen in a broader management context.

Phytophthora species from declining Austrocedrus chilensis forests in Patagonia, Argentina.

A survey of Phytophthora spp. in declining and healthy Austrocedrus chilensis forest was conducted to obtain an overview of the species that inhabit these forests. Seventeen declining and three healthy stands plus 11 associated streams were surveyed. Five Phytophthora species were recovered. P. syringae was the most common species isolated from soil and/or streams at nine declining sites and one healthy site. P. gonapodyides was isolated from streams only, at five declining sites. P. cambivora was isolated from soil and the undescribed taxa 'P. taxon Pgchlamydo' and 22 'P taxon Raspberry' were isolated from streams at one declining site each. The species were identified by ITS rDNA sequences and morphological features. Brief descriptions of each species and a discussion of their possible relationship with "mal del ciprés" are presented.

Multiple new phenotypic taxa from trees and riparian ecosystems in Phytophthora gonapodyides-P. megasperma ITS Clade 6, which tend to be high-temperature tolerant and either inbreeding or sterile.

Phytophthora isolates associated with Phytophthora major ITS Clade 6 were grouped into 11 phenotypic taxa. These comprised the described morphospecies P. gonapodyides, P. megasperma s. str. and P. humicola; four previously identified but so far undescribed taxa, informally designated here P. sp. O-group, P. sp. Apple-cherry, P. taxon Pgchlamydo, and P. taxon Walnut; and four previously unknown taxa, designated P. taxon Oaksoil, P. taxon Raspberry, P. taxon Forestsoil, and P. taxon Riversoil. With the exception of P. gonapodyides, each phenotypic taxon represented an unique ITS lineage. Two isolates morphologically identical to P. gonapodyides comprised a separate lineage and probably represent another taxon, designated here P. taxon Salixsoil, P. humicola, P. sp. O-group, P. sp. Apple-cherry and P. taxon Walnut grouped together as subclade I. Within subclade II, P. taxon Oaksoil, P. taxon Raspberry, P. taxon Forestsoil, P. taxon Riversoil and P. taxon Pgchlamydo formed a cluster of closely related but phenotypically distinct lineages basal to P. gonapodyides and P. megasperma, P. taxon Salixsoil being the most basal member. The taxonomy, adaptation and breeding systems of Clade 6 taxa are discussed. They show a strong association with forests and riparian ecosystems, only a limited association with agriculture and an ability to tolerate high temperatures. Also, in contrast to most other Phytophthora clades, Clade 6 taxa are predominantly sterile or inbreeding in culture. Only one taxon, P. sp. O-group, appears classically A1/A2 heterothallic.