Widespread Detection of Phytophthora Taxon Salixsoil in the Littoral Zone of Lake Constance, Germany

An unnamed ITS clade 6 Phytophthora was frequently isolated from rhizosphere soil of reed (Phragmites australis) growing on the littoral zone of Lake Constance. The isolates closely resembled P. gonapodyides, having internally proliferating, non-papillate sporangia, a rather high temperature optimum for growth (30 °C), and being sexually sterile. ITS sequence analysis revealed that they were identical to the as yet unnamed Phytophthora taxon Salixsoil, originally isolated from Salix roots in the UK and Alnus debris in Denmark. The taxon was readily isolated from permanently as well as occasionally flooded reed sites using standard baiting procedures, indicating a wide distribution in the Lake Constance littoral zone. In an in vitro leaf inoculation assay P. taxon Salixsoil proved to be more aggressive towards Salix alba than P. gonapodyides. The new taxon may be of significance as a root pathogen of woody plants in moist or flooded situations occurring in alluvial forest/plant communities. We propose that due to its close resemblance to P. gonapodyides the taxon might have passed unnoticed in the past, and possibly is much more widely distributed than previously recognised.     

The morphology,behaviour and molecular phylogeny of Phytophthora taxon Salixsoil and its redesignation as Phytophthora lacustris sp. nov.

Since its first isolation from Salix roots in 1972, isolates of a sexually sterile Phytophthora species have been obtained frequently from wet or riparian habitats worldwide and have also been isolated from roots of Alnus and Prunus spp. Although originally assigned to Phytophthora gonapodyides on morphological grounds, it was recognized that these isolates, informally named P. taxon Salixsoil, might represent a separate lineage within ITS Clade 6. Based on phylogenetic analyses and comparisons of morphology, growth‐temperature relationships and pathogenicity, this taxon is formally described here as Phytophthora lacustris sp. nov. Isolates of P. lacustris form a clearly resolved cluster in both ITS and mitochondrial cox1 phylogenies, basal to most other Clade 6 taxa. Phytophthora lacustris shares several unusual behavioural properties with other aquatic Clade 6 species, such as sexual sterility and tolerance of high temperatures, that have been suggested as adaptations to riparian conditions. It appears to be widespread in Europe and has also been detected in Australia, New Zealand and the USA. It was shown to be weakly or moderately aggressive on inoculation to Alnus, Prunus and Salix. The extent of P. lacustris’ activity as a saprotroph in plant debris in water and as an opportunistic pathogen in riparian habitats needs further investigation. Its pathogenic potential to cultivated fruit trees also deserves attention because P. lacustris has apparently been introduced into the nursery trade.     

Phytophthora diversity and the population structure of Phytophthora ramorum in Swiss ornamental nurseries

Invasive oomycete pathogens have been causing significant damage to native ecosystems worldwide for over a century. A recent well‐known example is Phytophthora ramorum, the causal agent of sudden oak death, which emerged in the 1990s in Europe and North America. In Europe, this pathogen is mainly restricted to woody ornamentals in nurseries and public greens, while severe outbreaks in the wild have only been reported in the UK. This study presents the results of the P. ramorum survey conducted in Swiss nurseries between 2003 and 2011. In all 120 nurseries subjected to the plant passport system, the main P. ramorum hosts were visually checked for above ground infections. Phytophthora species were isolated from tissue showing symptoms and identified on the basis of the morphological features of the cultures and sequencing of the ribosomal ITS region. Phytophthora was detected on 125 plants (66 Viburnum, 58 Rhododendron and one Pieris). Phytophthora ramorum was the most frequent species (59·2% of the plants), followed by P. plurivora, P. cactorum, P. citrophthora, P. cinnamomi, P. cactorum/P. hedraiandra, P. multivora and P. taxon PgChlamydo. The highest incidence of P. ramorum was observed on Viburnum × bodnantense. Microsatellite genotyping showed that the Swiss P. ramorum population is highly clonal and consists of seven genotypes (five previously reported in Europe, two new), all belonging to the European EU1 clonal lineage. It can therefore be assumed that P. ramorum entered Switzerland through nursery trade. Despite sanitation measures, repeated P. ramorum infections have been recorded in seven nurseries, suggesting either reintroduction or unsuccessful eradication efforts.     

Phytophthora species distribution in South African citrus production regions

Several Phytophthora spp. are known to cause a range of symptoms on citrus, resulting in significant crop losses worldwide. In South Africa, Phytophthora remains a destructive citrus disease, but the species and their distribution have not been well documented. A total of 162 Phytophthora isolates was collected from 60 citrus orchards in seven provinces of South Africa (Eastern Cape, Kwazulu-Natal, Limpopo, Mpumalanga, Northern Cape, North West and Western Cape). Isolates were identified to the species level through PCR-RFLP (restriction fragment length polymorphism) analyses of the internal transcribed spacer region. The identity of a subset of the isolates was confirmed using morphological and sequence analyses. Phytophthora nicotianae was the predominant species (76 % of isolates) and occurred in 80 % of the orchards in all of the provinces, followed by P. citrophthora (22 % of isolates in 28 % of orchards). The P. citrophthora isolates were further subdivided into two previously identified subgroups, G1 and G2, with most (69 %) of the isolates belonging to the G1 subgroup. Other Phytophthora species included P. multivora in the Western Cape Province, and an unknown species in the Eastern Cape Province with high sequence similarity (98 %) to a putative new species submitted to GenBank as Phytophthora taxon Sisuluriver. Phytophthora palmivora, a known citrus pathogen, was not identified. Most of the P. nicotianae isolates (79 %) were of the A1 mating type. The P. citrophthora isolates were mostly sterile (64 %), including most of the G1 isolates (81 %). The remaining G1 isolates (19 %) belonged to the A1 mating type, whereas almost all G2 isolates belonged to the A2 mating type except for one isolate that was sterile.     

Prevalence of Phytophthora species in macadamia orchards in Australia and their ability to cause stem canker

In Australia, Phytophthora cinnamomi is the only species reported as the causal agent of stem canker and root rot in macadamia. In other countries, five Phytophthora species have been reported to cause diseases in macadamia, which led us to question if more than one Phytophthora species is responsible for poor tree health in macadamia orchards in Australia. To investigate this, samples were collected from the rhizosphere, stem, and root tissues of trees with and without symptoms, nurseries, and water sources from 70 commercial macadamia orchards in Australia. Phytophthora isolates were identified based on morphological characteristics and DNA sequencing. P. cinnamomi was the most predominant and widely distributed species, and was obtained from the different types of samples including symptomless root tissues. In addition to P. cinnamomi, only P. multivora was isolated from diseased tissue (stem canker) samples. Six other Phytophthora species were obtained from the rhizosphere samples: P. pseudocryptogea, P. citrophthora, P. nicotianae, P. gondwanense, P. sojae, and a new Phytophthora taxon. Only P. cinnamomi was obtained from macadamia nursery samples, while five Phytophthora species were obtained from water sources. Of the heterothallic Phytophthora species, mating type A2 isolates were dominant in P. cinnamomi isolates, whereas only mating type A1 isolates were obtained for P. nicotianae, P. pseudocryptogea, and P. citrophthora. Pathogenicity assays revealed that P. cinnamomi and P. multivora caused significantly larger stem and leaf lesions than P. citrophthora, P. nicotianae, and P. pseudocryptogea. Phytophthora sp. and P. sojae were nonpathogenic towards leaves and stems.     

Isolation and pathogenicity of Phytophthora species from declining Rubus anglocandicans

Rubus anglocandicans is the most widespread and abundant blackberry species within the European blackberry (Rubus fruticosus) aggregate in Western Australia (WA). European blackberry is also one of the 32 Weeds of National Significance in Australia. A disease recorded as ‘blackberry decline’ was first observed in some blackberry sites in WA in 2006. A disease survey was conducted in the Manjimup‐Pemberton region along the Warren and Donnelly River catchments in WA between 2010 and 2012. Phytophthora amnicola, P. bilorbang, P. cryptogea, P. inundata, P. litoralis, P. multivora, P. taxon personii, P. thermophila and a P. thermophila × amnicola hybrid were recovered from declining and adjacent decline‐free sites, as well as from streams and rivers. Phytophthora cinnamomi was isolated from dying Banksia and Eucalyptus species from two non‐decline sites. Of these species, P. bilorbang and P. cryptogea were more pathogenic than the others in under‐bark inoculations using excised stems (primocanes), in planta primocane inoculations in blackberry growing wild in native forest stands, and in glasshouse pot trials. It was concluded that blackberry decline is a complex syndrome and Phytophthora species, in particular P. bilorbang and P. cryptogea, together with temporary inundation, are major biotic and abiotic factors contributing to blackberry decline.     

Molecular analysis of Phytophthora diversity in nursery‐grown ornamental and fruit plants

The genetic diversity of Phytophthora spp. was investigated in potted ornamental and fruit tree species. A metabarcoding approach was used, based on a semi‐nested PCR with Phytophthora genus‐specific primers targeting the ITS1 region of the rDNA. More than 50 ITS1 sequence types representing at least 15 distinct Phytophthora taxa were detected. Nine had ITS sequences that grouped them in defined taxonomic groups (P. nicotianae, P. citrophthora, P. meadii, P. taxon Pgchlamydo, P. cinnamomi, P. parvispora, P. cambivora, P. niederhauserii and P. lateralis) whereas three phylotypes were associated to two or more taxa (P. citricola taxon E or III; P. pseudosyringae, P. ilicis or P. nemorosa; and P. cryptogea, P. erythroseptica, P. himalayensis or P. sp. ‘kelmania’) that can be challenging to resolve with ITS1 sequences alone. Three additional phylotypes were considered as representatives of novel Phytophthora taxa and defined as P. meadii‐like, P. cinnamomi‐like and P. niederhauserii‐like. Furthermore, the analyses highlighted a very complex assemblage of Phytophthora taxa in ornamental nurseries within a limited geographic area and provided some indications of structure amongst populations of P. nicotianae (the most prevalent taxon) and other taxa. Data revealed new host–pathogen combinations, evidence of new species previously unreported in Italy (P. lateralis) or Europe (P. meadii) and phylotypes representative of species that remain to be taxonomically defined. Furthermore, the results reinforced the primary role of plant nurseries in favouring the introduction, dissemination and evolution of Phytophthora species.     

Phytophthora pachypleura sp. nov., a new species causing root rot of Aucuba japonica and other ornamentals in the United Kingdom

Isolates of an unknown Phytophthora species from the ‘Phytophthora citricola complex’ have been found associated with mortality of Aucuba japonica in the UK. Based on morphological characteristics, growth–temperature relationships, sequences of five DNA regions and pathogenicity assays, the proposed novel species is described as Phytophthora pachypleura. Being homothallic with paragynous antheridia and semipapillate sporangia, P. pachypleura resembles other species in the ‘P. citricola complex’ but can be discriminated by its distinctively thick‐walled oospores with an oospore wall index of 0·71. In the phylogenetic analysis based on three nuclear (ITS, β‐tubulin, EF‐1α) and two mitochondrial (cox1, nadh1) DNA regions, P. pachypleura formed a distinct clade within the ‘P. citricola complex’ with P. citricola s. str., P. citricola E and P. acerina as its closest relatives. Phytophthora pachypleura is more aggressive to A. japonica than P. plurivora and P. multivora and has the potential to affect other ornamental species.     

Phytophthora acerina sp. nov., a new species causing bleeding cankers and dieback of Acer pseudoplatanus trees in planted forests in northern Italy

A severe dieback of Acer pseudoplatanus trees was noticed in planted forest stands in northern Italy in 2010. Affected trees showed collar rot and aerial bleeding cankers along the stems, leading to crown dieback and eventually death. An unknown Phytophthora species was consistently isolated from necrotic bark and xylem tissue and from rhizosphere soil. Based on its unique combination of morphological and physiological characters and phylogenetic analysis, this new taxon is here described as Phytophthora acerina sp. nov. Phylogenetic analysis of ITS, cox1 and β‐tubulin gene regions demonstrated that P. acerina is unique and forms a separate cluster within the ‘P. citricola complex’, closely related to P. plurivora. Phytophthora acerina is homothallic with smooth‐walled oogonia, thick‐walled, mostly aplerotic oospores with a high abortion rate, paragynous antheridia, and persistent, morphologically variable semipapillate sporangia. Four to 5‐week‐old cultures produced globose to subglobose, appressoria‐like and coralloid hyphal swellings and characteristic stromata‐like hyphal aggregations. Optimum and maximum temperatures for growth are 25°C and 32°C, respectively. Genetic uniformity of all 15 studied isolates and the apparent absence of this species in the extensive surveys of nurseries, forests and seminatural ecosystems conducted in the previous two decades across Europe indicate a recent clonal introduction to northern Italy. Under‐bark inoculation tests demonstrated high aggressiveness of P. acerina to A. pseudoplatanus indicating that this pathogen might be a serious risk to maple plantations and forests in Europe.     

Phytophthora boehmeriae boll rot: a new threat to cotton cultivation in the mediterranean region

A boll rot of cotton (Gossypium hirsutum L.) was observed for the first time in Greece in August 1993 in Larissa and Volos counties, and in August and September 1995 in Trikala and Phthiotis counties. Fungi of the genusPhytophthora were isolated from diseased plants. Morphological characteristics of the pathogen were recorded on mounts made directly from the infected tissues or after growth of the isolated fungus on corn meal agar or sterile distilled water. Colony morphology, growth rates, features of asexual and sexual structures and maximum growth temperatures were examined. APhytophthora species new to Europe,Phytophthora boehmeriae Sawada, attacking cotton bolls, was identified. The pathogenicity of the isolates was confirmed by artificial inoculations of detached cotton bolls. Analysis of α-esterase isozymes revealed unique banding patterns for isolates ofP. boehmeriae compared with those ofP. cactorum andP. parasitica, which arePhytophthora species with similar morphology.     

Phytophthora bilorbang sp. nov., a new species associated with the decline of Rubus anglocandicans (European blackberry) in Western Australia

A new homothallic Phytophthora species, isolated from rhizosphere soil and roots of declining or dead Rubus anglocandicans (European blackberry) in south-west Western Australia, is described as Phytophthora bilorbang sp. nov. It produces non-papillate sporangia, smooth-walled oogonia containing thick-walled oospores, and paragynous antheridia. Although morphologically similar to several species within ITS Clade 6 and sub-clade II, namely P. gibbosa, P. gregata and P. megasperma, phylogenetic analyses of the ITS, cox1, HSP90, BT and NADH gene regions demonstrate that P. bilorbang sp. nov. is a distinct species. Additionally, P. bilorbang differs from these species in its growth and colony morphology on several media. Pathogenicity tests indicate that P. bilorbang could be responsible for the decline syndrome of blackberry within the Warren and Donnelly River catchments in the south-west of Western Australia.     

Specific and Sensitive Detection of Phytophthora nicotianae By Simple and Nested-PCR

Phytophthora nicotianae Breda de Haan is one of the most important soil-borne plant pathogens. The identification of this pathogen based on morphological or physiological characters is time-consuming and labour-intensive and requires comprehensive knowledge of fungi. Molecular analysis of the internal transcribed spacer (ITS) regions of rDNA is a novel and very effective method of species determination. Based on this concept, conventional and single closed tube nested-PCRs were developed for the specific and sensitive detection of P. nicotianae. Two new specific primers, designed from the spacer regions ITS1 and ITS2, internal to the nucleotide sequence flanked by universal primers ITS4 and ITS6, were used. To evaluate the specificity of the method, 36 morphologically characterized isolates were tested. A positive reaction, characterized by an amplification product of 737 bp, was shown by all P. nicotianae isolates and two P. nicotianae/cactorum hybrids. No amplification product was observed when other Phytophthora species and genera were assayed. The sensitivity of this method was analysed by serial dilutions of a defined amount of fungal DNA in a healthy root extract. Nested-PCR was at least 1000 times more sensitive than conventional PCR. In addition, samples from different infection sites, origins and crops, samples from nutrient solution, water and the rockwool used in hydroponic cultures, were analysed to validate this method.     

Two new species,Phytophthora nagaii sp. nov. and P. fragariaefolia sp. nov., causing serious diseases on rose and strawberry plants,respectively, in Japan

A new disease of rose was noticed in Chiba Prefecture of Japan in 1968, and the pathogen was initially identified as Phytophthora megasperma based on morphological characteristics. Similar Phytophthora isolates have since been collected from rose plants in Chiba, Kanagawa, and Shizuoka Prefectures. In 2005, several Phytophthora isolates were recovered from crowns of strawberry plants in Hokkaido Prefecture. These were considered to be members of a new species. In this study, we re-examined all these isolates using morphological and physiological studies and a multilocus phylogenetic analysis. The rose and strawberry isolates were mostly similar morphologically and physiologically, with some exceptions. The rose isolates differed significantly from P. megasperma sensu stricto and other related Phytophthora species. The rose and strawberry isolates had external proliferation of sporangia, characteristic funnel-shaped oogonia, predominantly paragynous antheridia, and fast growth rates of 10.5 mm/24 h at an optimum temperature of 28 °C. In the multilocus phylogenetic tree constructed using sequences from the rDNA ITS regions, rDNA LSU, and the translation elongation factor 1-α, β-tubulin and coxI genes, they formed a distinct monophyletic group in clade 7 with strong bootstrap support. The rose and strawberry isolates separated into two distinct groups. The results indicate that the rose and strawberry isolates constitute two separate species, designated here as Phytophthora nagaii and P. fragariaefolia.     

Standard and Swedish variant types of the hybrid alder Phytophthora attacking alder in Hungary

A new Phytophthora disease of common alder (Alnus glutinosa) similar to that previously reported in several countries in Europe has been observed in Hungary. Based on these earlier studies, the alder Phytophthora was considered likely to be a hybrid between P cambivora and a P fragariae-like species: across Europe a range of new alder Phytophthora is spreading that comprise a range of heteroploid hybrids including a 'standard' hybrid type and several other hybrid types termed 'variants'. Phenotypic and molecular features of the pathogen in Hungary were characterised and compared with isolates from elsewhere. The morphologies of five isolates from one region (Hévíz) resembled the common, 'standard' type, whereas the three isolates from another region (Hanság) exhibited traits similar to those of one of the 'variant' types, ie the Swedish 'variant'. Molecular markers of these two groups of Hungarian isolates also represented a good fit to those of the standard type and the Swedish variant, respectively. Isozyme patterns and profiles of restriction fragments of the entire internal transcribed spacer (ITS) region or mitochondrial DNAs and of RAPD-PCR products did not differ within a group, but distinct polymorphisms were exhibited between the two groups of isolates. Southern analysis of random amplified polymorphic DNA (RAPD) revealed the homologous nature of co-migrating bands of P cambivora and the isolates of alder Phytophthora. Furthermore, restriction fragment profiles of the ITS region of ribosomal DNAs and the mtDNAs were consistent with reported biparental origin of alder Phytophthora. The hybrid status of these continuously evolving pathogens raises many issues and challenges concerning efficient control measures.     

Monitoring of <Emphasis Type="Italic">Phytophthora</Emphasis> species on fruit trees in Bulgaria

Disease on fruit trees in Bulgaria caused by Phytopthora cactorum and P. citrophthora was found in the period 1998–1999. Leaves of some trees become reddish during July, and later in the season fall off. Infected trees die during the same season, or the next season. Observations on symptom development and spread of Phytophthora root and crown rot of fruit trees was undertaken from 1999 to 2009. Disease incidence is between 2% and 14% in some gardens and nurseries. The disease was registered in the regions of Plovdiv, Kjustendil, Sliven, Yambol, Karnobat, Bourgas and Svishtov. Samples from infected plant tissues were taken and isolations were done on selective PARP media, or by applying a baiting bioassay. Based on morphological and cultural characteristics and temperature requirements the following Phytophthora species have been identified: Phytophthora cactorum, P. citrophthora, P. drechsleri, P. cryptogea, hybrid and Pythium. Pathogenicity of the isolates was tested on green apple fruits or one-year-old apple rootstocks. Laboratory studies of the effect of temperature on mycelia growth showed that most isolates can grow from 5° up to 30°C, with an optimum from 18° to 25°C. Only three strains grew at 35–36°C, two developed slowly, one grew well. The optimal pH for mycelia development was tested. Aiming at control of disease, in vivo pot trials have been carried out for studying resistance of rootstocks to P. cactorum. At the end of the growing season a good level of resistance has been shown in the rootstocks M29C, Gizela 6, and MAXMA 14.     

Molecular characterization of <Emphasis Type="Italic">Phytophthora porri</Emphasis> and closely related species and their pathogenicity on leek (<Emphasis Type="Italic">Allium porrum</Emphasis>)

White tip, caused by Phytophthora porri, is a destructive disease in the cultivation of European leek (Allium porrum). P. porri and closely related species such as P. brassicae, P. primulae and P. syringae belong to the phylogenetic clade 8b within the genus Phytophthora. The objectives of this study were to establish the position of P. porri and closely related species within the Phytophthora clade 8b; to study genetic variation among P. porri isolates from leek and closely related species and to test the hypothesis that host-driven speciation has occurred within this clade. AFLP analysis could clearly make a distinction between isolates of P. porri from Allium species and related Phytophthora species such as P. brassicae, P. syringae and P. primulae. DNA similarity and cluster analysis based on 353 markers demonstrated little genetic diversity within the P. porri population from Allium species although Belgian and Dutch P. porri isolates from leek could be distinguished from Japanese P. porri isolates from other Allium species and the P. porri isolate from carrot. Our results point to incipient speciation within the P. porri isolates, which could have been driven by the host plant or by geographic isolation. ITS sequence analysis confirmed the results obtained by AFLP and showed a close relationship between P. porri isolates from Allium and P. primulae and between the P. porri isolate from carrot and P. brassicae. We hypothesize that interspecific hybridization has occurred within this clade.     

Natural hybrids of resident and introduced <Emphasis Type="Italic">Phytophthora</Emphasis> species proliferating on multiple new hosts

Several atypical Phytophthora strains, isolated from a range of horticultural hosts, were tentatively identified as P. cactorum. Numerous abortive oospores were observed in these strains and isozyme analysis showed all were heterozygous for the dimeric malic enzyme (MDHP). More detailed comparisons indicated that their MDHP alleles matched those of both P. cactorum and P. hedraiandra. Cloning and sequencing of the nuclear ribosomal internal transcribed spacer (ITS) regions of the atypical P. cactorum strains demonstrated the presence of sequences characteristic for both P. cactorum and P. hedraiandra. It was concluded that the atypical strains represented hybrids between the resident P. cactorum and the apparently recently introduced P. hedraiandra. Most strains had the mitochondrially inherited cytochrome oxidase I (Cox I) gene typical of one putative parent P. hedraiandra, while one single strain had that of the other putative parent, consistent with the hybrid hypothesis. Our data also suggest that the hybrids are evolving. The hybrids have proliferated on multiple new hosts in the Netherlands.     

Towards a best practice methodology for the detection of Phytophthora species in soils

The genus Phytophthora contains species that are major pathogens worldwide, affecting a multitude of plant species across agriculture, horticulture, forestry, and natural ecosystems. Here, we concentrate on those species that are dispersed through soil and water, attacking the roots of the plants, causing them to rot and die. The intention of this study was to compare the soil baiting protocol developed by the Centre for Phytophthora Science and Management (CPSM) with two other baiting methods used in Australia. The aim was to demonstrate the effectiveness of each protocol for soil baiting Phytophthora species in different substrates. Three experiments were conducted: the first to test the sensitivity of each method to detect Phytophthora cinnamomi, the second to test the effect of substrate type (sand or loam), and the third to test the detection of species (P. cinnamomi, P. multivora, or P. pseudocryptogea). The specificity of different plant species baits was compared within and between the methods. Substrate type influenced isolation in all methods; however, the CPSM method was superior regardless of substrate, albeit slower than one of the other methods for one substrate. Comparing bait species between the three methods, Quercus ilex was the most attractive bait for P. cinnamomi, particularly in the CPSM method. The choice of protocol affected the isolation associated with each bait type. Overall, the multiple bait system used by CPSM was shown to provide the most sensitive and reliable detection of Phytophthora species from soil samples.     

Diversity of Phytophthora species in natural ecosystems of Taiwan and association with disease symptoms

In 2013 a survey of Phytophthora diversity was performed in 25 natural and seminatural forest stands and 25 rivers in temperate montane and subtropical lowland regions of Taiwan. Using baiting assays, 10 described species and 17 previously unknown taxa of Phytophthora were isolated from 71.5% of the 144 rhizosphere soil samples from 33 of 40 tree species sampled in 24 forest stands, and from 19 rivers: P. capensis, P. citrophthora, P. plurivora, P. tropicalis, P. citricola VII, P. sp. × botryosa‐like, P. sp. × meadii‐like and P. sp. occultans‐like from Clade 2; P. palmivora from Clade 4; P. castaneae and P. heveae from Clade 5; P. chlamydospora and P. sp. forestsoil‐like from Clade 6; P. cinnamomi (Pc), P. parvispora, P. attenuata nom. prov., P. flexuosa nom. prov., P. formosa nom. prov., P. intricata nom. prov., P. × incrassata nom. prov. and P. × heterohybrida nom. prov. from Clade 7; P. sp. palustris and five new hybrid species from Clade 9. The A1 mating type of Pc was widespread in both montane and lowland forests and rarely associated with disease, whereas the A2 mating type was limited to lowland forests and in some cases causing severe dieback. Most other Phytophthora species were not associated with obvious disease symptoms. It is concluded that (i) Taiwan is within the centre of origin of most Phytophthora taxa found, (ii) Pc A2 is an introduced invasive pathogen, and (iii) interspecific hybridizations play a major role in speciation and species radiations in diverse natural ecosystems.     

Phytophthora caryae sp. nov., a new species recovered from streams and rivers in the eastern United States

Members of the Phytophthora citricola complex (Phytophthora clade 2c), such as P. plurivora, are destructive pathogens of trees and shrubs in nursery, landscape and forest settings worldwide. During surveys of Phytophthora species from streams and rivers in Massachusetts and North Carolina, a novel species in the P. citricola complex was recovered. Based on sequences from three nuclear (ITS, β‐tub and tef1) and two mitochondrial (cox1 and nadh1) loci, morphological characters, temperature–growth relationships and host plant inoculations, this novel species is described as Phytophthora caryae sp. nov. Phytophthora caryae resembles several other species in the P. citricola complex, demonstrating homothallism and producing paragynous antheridia and semipapillate and noncaducous sporangia. However, P. caryae exhibits smaller sexual structures, higher rates of oogonia with a tapered base and sporangia with an offset attachment of the sporangiophores. Phylogenetic analyses using maximum likelihood and Bayesian inference placed isolates of P. caryae into a unique clade with significant statistical support. Based on the mitochondrial dataset, P. caryae is most closely related to P. pini and P. citricola III, which are believed to be native in eastern North America. Inoculations of P. caryae on 1‐year‐old twigs of 12 tree species representing nine genera resulted in under‐bark lesions on species of Carya and Juglans. Sapling inoculations under greenhouse conditions suggest that P. caryae may be pathogenic to shagbark hickory (Carya ovata) but not to black walnut (Juglans nigra).