Xiphinema Rivesi Research Articles

Commodity risk assessment of plants of 12 selected Prunus species from Moldova.

The European Commission requested the EFSA Panel on Plant Health to prepare and deliver risk assessments for commodities listed in Commission Implementing Regulation (EU) 2018/2019 as 'High-risk plants, plant products and other objects'. This Scientific Opinion covers plant health risks posed by defoliated 1- or 2-year old bare root plants for planting (grafted or not) of 12 Prunus species (Prunus armeniaca, P. avium, P. canescens, P. cerasifera, P. cerasus, P. davidiana, P. domestica, P. dulcis, P. fontanesiana, P. persica, P. salicina, P. tomentosa) imported from Moldova, taking into account the available scientific information, including the technical information provided by the applicant country. The evaluation identified three EU-quarantine pests, Erwinia amylovora (protected zone quarantine pest), Xiphinema rivesi non-EU populations and Xanthomonas arboricola pv. pruni (protected zone quarantine pest), which were selected for further evaluation, based on defined criteria, including their presence in the applicant country. It should be noted that there is uncertainty regarding whether all relevant pests have been identified due to a limited number of scientific publications and pest surveys in Moldova. For the three selected pests, the risk mitigation measures proposed in the technical dossier from Moldova were evaluated taking into account the possible limiting factors. For these pests, an expert judgement is given on the likelihood of pest freedom taking into consideration the risk mitigation measures acting on it, including uncertainties associated with the assessment. The degree of pest freedom varies among the pests evaluated, with Erwinia amylovora being the pest most frequently expected on the imported plants. The Expert Knowledge Elicitation indicated, with 95% certainty, that between 9823 and 10,000 bundles (comprising 10-20 plants per bundle) out of 10,000 bundles would be free from E. amylovora.

Commodity risk assessment of grafted plants of Malus domestica from Moldova.

The European Commission requested the EFSA Panel on Plant Health to prepare and deliver risk assessments for commodities listed in Commission Implementing Regulation (EU) 2018/2019 as ‘High risk plants, plant products and other objects’. This Scientific Opinion covers plant health risks posed by defoliated and in dormant phase, grafted bare rooted plants for planting of Malus domestica imported from Moldova, taking into account the available scientific information, including the technical information provided by the applicant country. A list of 1,118 pests potentially associated with the commodity species was compiled. The relevance of these pests was assessed following defined criteria and based on evidence. The EU‐quarantine pest Xiphinema rivesi non‐EU populations fulfilled these criteria and was selected for further evaluation. For this pest, the risk mitigation measures proposed in the technical dossier from Moldova were evaluated taking into account the possible limiting factors. For this pest, an expert judgement is given on the likelihood of pest freedom taking into consideration the risk mitigation measures acting on it, including uncertainties associated with the assessment. The Expert Knowledge Elicitation indicated, with 95% certainty, that between 9,991 and 10,000 plants per 10,000 would be free of X. rivesi.

Commodity risk assessment of Juglans regia plants from Moldova.

The European Commission requested the EFSA Panel on Plant Health to prepare and deliver risk assessments for commodities listed in Commission Implementing Regulation (EU) 2018/2019 as ‘High risk plants, plant products and other objects’. Taking into account the available scientific information, including the technical information provided by the applicant country, this Scientific Opinion covers the plant health risks posed by the following commodities: dormant, free of leaves grafted plants and rootstocks of Juglans regia imported from Moldova. A list of pests potentially associated with the commodities was compiled. The relevance of any pest was assessed based on evidence following defined criteria. None of the pests in the list fulfilled all relevant criteria and therefore none were selected for further evaluation. As a result, risk mitigation measures proposed in the technical dossier from Moldova were listed, but not further evaluated.

Pest categorisation of Xiphinemaamericanum sensu lato.

The Panel on Plant Health performed a pest categorisation of Xiphinema americanum sensu lato (Nematoda: Longidoridae) for the EU. Sixty‐one species in this group are recognised. They are polyphagous pests found in soil associated with a number of plant species. As a migratory ectoparasitic species, it punctures cells of plant roots. Nematodes were classified in four categories based on their distribution and ability to transmit viruses. Category I contains the seven virus vector species present outside the EU: X. americanum sensu stricto, X. bricolense, X. californicum, X. inaequale, X. intermedium, X. rivesi (non‐EU populations) and X. tarjanense. Category II contains the 28 species not present in the EU and not known to transmit any virus. Twenty‐six species are present in the EU and are not known to be virus vectors (category III). Category IV contains the species present in the EU, which is a virus vector (EU populations of X. rivesi). All nematodes known to be virus vectors occurring outside the EU (category I) satisfy all the criteria that are within the remit of EFSA to assess to be regarded as Union quarantine pests. This is mainly due to their association with non‐EU virus isolates. Categories II and III contain species that are not reported to transmit viruses or cause economic damage to crop plants. Although uncertainty concerning their ability to transmit viruses exists, those species do not satisfy all the criteria to be regarded as Union quarantine pests. Category IV contains the EU populations of X. rivesi. The species is a virus vector but current EU populations of X. rivesi have not been reported to be associated with any of the EU viruses or their non‐EU isolates under field conditions. Xiphinema rivesi (EU populations) is widespread in some Member States and does not satisfy all the criteria to be regarded as a Union quarantine. None of the species can be regarded as a regulated non‐quarantine pest.

Mitochondrial genome diversity in dagger and needle nematodes (Nematoda: Longidoridae)

Dagger and needle nematodes included in the family Longidoridae (viz. Longidorus, Paralongidorus, and Xiphinema) are highly polyphagous plant-parasitic nematodes in wild and cultivated plants and some of them are plant-virus vectors (nepovirus). The mitochondrial (mt) genomes of the dagger and needle nematodes, Xiphinema rivesi, Xiphinema pachtaicum, Longidorus vineacola and Paralongidorus litoralis were sequenced in this study. The four circular mt genomes have an estimated size of 12.6, 12.5, 13.5 and 12.7 kb, respectively. Up to date, the mt genome of X. pachtaicum is the smallest genome found in Nematoda. The four mt genomes contain 12 protein-coding genes (viz. cox1-3, nad1-6, nad4L, atp6 and cob) and two ribosomal RNA genes (rrnL and rrnS), but the atp8 gene was not detected. These mt genomes showed a gene arrangement very different within the Longidoridae species sequenced, with the exception of very closely related species (X. americanum and X. rivesi). The sizes of non-coding regions in the Longidoridae nematodes were very small and were present in a few places in the mt genome. Phylogenetic analysis of all coding genes showed a closer relationship between Longidorus and Paralongidorus and different phylogenetic possibilities for the three Xiphinema species.

Cryptic diversity and species delimitation in theXiphinema americanum-group complex (Nematoda: Longidoridae) as inferred from morphometrics and molecular markers

The Xiphinema americanum-group constitutes a complex of about 55 species of polyphagous plant-ectoparasitic nematodes with a worldwide distribution. This group of plant-parasitic nematodes is one of the most difficult dagger nematode species complexes for diagnosis because the morphology is very conservative and morphometric characters often overlap. We conducted nematode surveys in cultivated and wild olives in southern Spain from 2012 to 2014, from which we identified 16 nematode populations of the X. americanum-group, five of which were tentatively identified as belonging to three new species and are described herein as Xiphinema plesiopachtaicum sp. nov., Xiphinema vallense sp. nov., and Xiphinema astaregiense sp. nov., and 11 populations belonging to nine known species: Xiphinema brevisicum, Xiphinema duriense, Xiphinema incertum, Xiphinema luci, Xiphinema madeirense, Xiphinema opisthohysterum, Xiphinema pachtaicum, Xiphinema parapachydermum, and Xiphinema rivesi. A phenetic study based on multivariate factor analyses was developed to compare some of these related species by using morphometric features. In the factor analysis the first four factors accounted for 73.1% of the total variance of the selected characters, identifying body length, body length/maximum body width (a), body length/pharyngeal length (b), body length/tail length (c), and tail length/body width at anus (c′) ratios, distance from anterior end to vulva as percentage of body length (V), stylet length, oral aperture-guiding ring distance, and lip region width as key morphometric characters to differentiate a restricted set of species within the X. pachtaicum-subgroup that includes X. plesiopachtaicum sp. nov. and X. vallense sp. nov. Multivariate analysis of variance using these specific characters allowed to differentiate species in the X. pachtaicum complex or groups of them using morphometric characters (body length, a, b, c, c′, V, stylet length, lip region width, oral aperture-guiding ring distance, female tail length, and hyaline region length). Phylogenetic analyses based on nuclear ribosomal DNA genes [D2-D3 expansion segments of large ribosomal subunit 28S, and internal transcribed spacer 1 (ITS1)] and the protein-coding mitochondrial gene, cytochrome c oxidase subunit 1 (coxI) were congruent, showing two main clades separating most of the species of X. americanum-subgroup ‘sensu stricto’ from the X. pachtaicum-subgroup. Agreement between phylogenetic trees and some morphological characters (viz. total stylet length, vulva position, and a ratio) were tested by reconstruction of their histories on rRNA-based trees using parsimony and Bayesian approaches. Thus, integrative taxonomy, based on a combination of multivariate morphological and molecular analyses constitutes a new insight into the identification of X. americanum-group species. © 2015 The Linnean Society of London

First Report of Xiphinema rivesi (Nematoda, Longidoridae) in Washington State.

Dagger nematode, Xiphinema rivesi Dalmaso, 1969 reportedly transmits several viruses in North America and Europe (2) leading to severe yield reduction in crops. Soil samples were collected in March 2013 during a survey of cherry orchards in Chelan County, WA; these historically suffer from cherry rasp leaf disease, caused by Cherry rasp leaf virus (CRLV) (genus Cheravirus). Soil samples were transported to the WSDA nematology laboratory in Prosser, WA, where 250-cc subsamples were processed using sucrose centrifugal flotation (1). Dagger nematodes were hand-picked and stored in 0.1% sodium chloride before being sent to the USDA-ARS Nematology Laboratory in Beltsville, MD, for morphological and molecular identification. The morphological and molecular analysis of adult females identified the dagger nematode species as Xiphinema rivesi Dalmaso, 1969 (4). Morphological characters used for identification included female body, and total stylet length (odontostyle and odontophore), location of guiding ring from oral aperture, head and tail shape, various tail measurements, and vulva percentage in relation to body length. Measurements of females (n = 10) include a mean body length of 1,902 ± 162.4 (1,832 to 2,203) μm, odontostyle 83 ± 3.5 (80 to 90) μm, odontophore 54.8 ± 4.2 (50 to 65) μm, total stylet 137.8 ± 4.2 (130 to 145) μm, guiding ring from oral aperture 70 ± 5.1 (60 to 75) μm, tail 30.8 ± 2.5 (27.5 to 35.0) μm, body diameter at anus 24.7 ± 1.7 (22 to 28) μm, J (hyaline portion of tail) 6.0 ± 0.9 (5.0 to 7.5) μm, body diameter at beginning of J 8.5 ± 1.0 (7.5 to 10.5) μm, body diameter at 5 μm from tail terminus 7.5 ± 0.2 (7.0 to 8.0) μm, and V% 52.2 ± 1.8 (49.4 to 55.0) μm. Molecular diagnosis of X. rivesi was confirmed after DNA was extracted from two individual nematodes by mechanical disruption with a micro knife in 20 μl worm lysis buffer containing 500 mM KCl, 100 mM Tris-Cl (pH8.3), 15 mM MgCl2, 10 mM dithiothreitol (DTT), 4.5% Tween 20, and 0.1% gelatin. DNA extracts were stored at -80°C until needed, then thawed, 1 μl proteinase K (from 2 mg/ml stock) was added, and the tubes were incubated at 60°C for 60 min, followed by 95°C for 15 min. The 28S large ribosomal D2-D3 expansion segment was amplified with D2A (5'-ACAAGTACCGTGAGGGAAAGTT-3') and D3B (5'-TCGGAAGGAACCAGCTACTA-3'), and the internal transcribed spacer (ITS) region was amplified with primers TW81 (5'-GTTTCCGTAGGTGAACCTGC-3') and AB28 (5'-ATATGCTTAAGTTCAGCGGGT-3'), as previously described (3). To verify the identity of the sequences generated from PCR, sequenced products were subjected to a database search using BLAST. Sequences from the 28S region were >99% identical to several sequences of X. rivesi sampled from Spain (GenBank Accessions JQ990038, JQ990039, HM921357, and HM921358). Sequences from the ITS region were 97 to 98% identical to X. rivesi sequences (FR878063 to FR878066) obtained from the host Vitis vinifera from Italy. To the best of our knowledge, this is the first report of this nematode from the Washington. The quick and persistent spread of CRLV in most of the orchards visited calls for concern and there is need for urgent control measures against this vector nematode.

Phylogeny, diversity, and species delimitation in some species of the Xiphinema americanum-group complex (Nematoda: Longidoridae), as inferred from nuclear and mitochondrial DNA sequences and morphology

During nematode surveys in southern Spain and Italy 14 populations of Xiphinema species tentatively identified as Xiphinema americanum-group were detected. Morphological and morphometrical studies identified three new species and six known Xiphinema americanum-group species, viz.: Xiphinema parabrevicolle n. sp., Xiphinema parapachydermum n. sp., Xiphinema paratenuicutis n. sp., Xiphinema duriense, Xiphinema incertum, Xiphinema opisthohysterum, Xiphinema pachtaicum, Xiphinema rivesi, and Xiphinema santos. The Xiphinema americanum-group is the most difficult Xiphinema species group for diagnosis since the morphology is very conservative and morphometric characters often overlap. This group includes vectors of several important plant pathogenic viruses that cause significant damage to a wide range of agricultural crops. Molecular characterisation of these species using D2-D3 expansion regions of 28S rRNA, 18S rRNA, ITS1-rRNA and the protein-coding mitochondrial gene, cytochrome oxidase c subunit 1 was carried out and maximum likelihood and Bayesian inference analysis were used to reconstruct phylogenetic relationships among these species and with other Xiphinema americanum-group species.

Xiphinema rivesi from Chile Transmits Tomato ringspot virus to Cucumber.

The dagger nematode, (Xiphinema rivesi Dalmasso), a member of the X. americanum group, was first reported in 2002 in Chile (3). X. rivesi is a vector of at least four North American nepoviruses including Cherry rasp leaf virus (CRLV), Tobacco ringspot virus (TobRSV), Tomato ringspot virus (TomRSV), and Peach rosette mosaic virus (PRMV) (2). TomRSV, first reported in Chile in 1984, was associated with raspberry decline and lately with brownline disease in D'Agen prune trees (1), however none of the Xiphinema spp. found in Chile have been reported to transmit this nepovirus. Two virus isolates, TomRSV (prune brownline isolate PBL-08) and Grapevine fanleaf virus (GFLV) (Yellow mosaic isolate GFLV-012), from the virus collection of the Departamento de Sanidad Vegetal, Universidad de Chile were used in transmission tests with a population of X. rivesi found in Chile. X. rivesi is not known to transmit GFLV and this virus was included as a check. The nematodes were extracted from soil from a D'Agen prune orchard, and transmission tests were done in compliance with the criteria proposed by Trudgill et al. (4). Cucumis sativus cv. National Pickling were grown in a growth chamber at 25°C and used as acquisition hosts and transmission bait plants. Acquisition hosts were mechanically inoculated with GFLV or TomRSV, displaying systemic symptoms in 15 to 20 days. Noninoculated cucumber plants were included as controls. Virus infection was confirmed by double-antibody sandwich (DAS)-ELISA before the introduction of nematodes to the soil. After a 20-day acquisition feeding period, the nematodes were wet screened from the soil and added to the healthy bait plants and allowed a 20-day inoculation feeding period. X. rivesi transmitted TomRSV but not GFLV. TomRSV bait plants developed systemic symptoms 5 weeks after the nematodes were transferred. Transmission of TomRSV was confirmed by testing leaf and root sap of bait plants in a DAS-ELISA. High virus concentrations were detected in the roots and leaves of TomRSV symptomatic plants. Bait plants on which nematodes had been allowed to feed following virus acquisition from GFLV-infected or from virus-free control plants tested negative by ELISA. No symptoms appeared on bait plants used for GFLV transmission or the control bait plants. To our knowledge, this is the first report of transmission of TomRSV with a Xiphinema population from Chile and South America.

Xiphinema rivesi from Slovania Transmit Tobacco ringspot virus and Tomato ringspot virus to Cucumber Bait Plants.

The dagger nematode, Xiphinema rivesi Dalmasso, a member of the X. americanum group, was detected in 2002 for the first time in Slovenia and for the fourth time in Europe (4). X. rivesi is a vector of at least four North American nepoviruses including Cherry rasp leaf virus (CRLV), Tobacco ringspot virus (TRSV), Tomato ringspot virus (ToRSV), and Peach rosette mosaic virus (PRMV) (1,2). All of these viruses are included on the EPPO and EU lists of quarantine organisms, but none of the Xiphinema species found in Europe have been reported to transmit these nepoviruses. Three virus isolates, including TRSV (from Lobelia spp.; virus collection of the Plant Protection Service, Wageningen, The Netherlands), ToRSV (grapevine isolate PV-0381; DSMZ, Braunschweig, Germany), and Arabis mosaic virus (ArMV) (from Vinca spp.; virus collection of the Plant Protection Service), were used in transmission tests with a population of X. rivesi found in Slovenia. X. rivesi is not known to transmit ArMV and this virus was included as a check. The nematodes were extracted from peach orchard soil collected near the village of Dornberk, and transmission tests fulfilled the set of criteria proposed by Trudgill et al. (3). Cucumis sativus cv. Eva, grown in a growth chamber at 25°C, was used as acquisition hosts and transmission bait plants. The acquisition hosts were mechanically inoculated and showing systemic symptoms before the introduction of nematodes. Noninoculated acquisition plants were included as controls. After a 10-day acquisition feeding period, the nematodes were transferred to healthy bait plants and allowed a 14-day inoculation feeding period. X. rivesi transmitted TRSV and ToRSV but not ArMV. TRSV and ToRSV bait plants developed systemic symptoms 4 to 6 weeks after the nematodes were transferred. Transmission of TRSV and ToRSV was confirmed by testing leaf and root sap of bait plants in a double antibody sandwich (DAS)-ELISA. High virus concentrations were detected in the roots and leaves of TRSV and ToRSV symptomatic plants. DAS-ELISA on bait plants from nematodes that had been allowed to feed on ArMV-infected or the virus-free control acquisition plants gave negative results. No symptoms appeared on bait plants used for ArMV transmission or the control bait plants. To our knowledge, this is the first report of transmission of TRSV and ToRSV with a Xiphinema population from Europe.

Effects of preplant soil fumigation on nematode population densities, and on growth and yield of raspberry

The effect of preplant soil fumigation on population densities of Pratylenchus penetrans, Xiphinema rivesi, and Meloidogyne hapla and growth of red raspberry (Rubus idaeus) was investigated over a 4-year period. P. penetrans was the most abundant and probably accounted for most of the growth reduction. Soil fumigation increased raspberry fields by 98%, 59%, and 18% in the first, second, and third year of production, respectively. The incidence of crown gall (Agrobacterium tumefaciens) was not correlated with soil populations of P. penetrans or M. hapla.

First Report of the Dagger Nematode Xiphinema rivesi, a Member of the X. americanum Group, from Slovenia.

The dagger nematode, Xiphinema rivesi Dalmasso, is a virus-transmitting plant-parasitic nematode described from France (2) and also reported from Germany, Portugal, and Spain. While this species is only known from a few local sites within Europe, it is widespread in North America and also present in Peru and Pakistan (3). X. rivesi is able to transmit at least four North American nepoviruses, including Cherry rasp leaf virus, Tobacco ringspot virus, Tomato ringspot virus, and Peach rosette mosaic virus (1,4). In early spring 2002, nematodes of the X. americanum group were detected in soil samples taken near the roots of peach plants from an orchard in Bilje near Nova Gorica, Slovenia. Resampling in June and September at the same site and at an adjacent, approximately 30-year-old peach orchard confirmed the presence of the nematode in relatively high numbers (5 per 100 ml of soil). Adult females were isolated and sent for further identification to the Plant Protection Service in Wageningen, the Netherlands. The nematode was morphologically identified as X. rivesi and confirmed at the rDNA-level with internal transcribed spacer restriction fragment length polymorphism analysis. Because both sampling sites where the nematode was found are close to the Vipava River in an area of extensive stone and pome fruit production, it is likely that the nematode is also present in other orchards in this region. The origin of this quarantine nematode is unknown as there is no direct link with Slovenian orchards and import of plant material from abroad in the last decades. To our knowledge, this is the first report of X. rivesi in Slovenia and the fourth report of this species in Europe.

Occurrence of novel verrucomicrobial species, endosymbiotic and associated with parthenogenesis in Xiphinema americanum-group species (Nematoda, Longidoridae).

Numerous micro-organisms have been described as cytoplasmic symbionts of eukaryotes. Many so-called obligate endosymbionts rely exclusively on maternal (vertical or transovarial) transmission to maintain themselves, rendering them dependent on the host sex ratio, which they would tend to manipulate to their own advantage. The latter phenomenon is often associated with the presence of Wolbachia pipientis (alpha-Proteobacteria) in arthropods and nematodes. A potentially similar situation was discovered involving members of a new clade of Verrucomicrobia, another main line of descent in the Bacteria. Nematode species of the Xiphinema americanum group (Nematoda, Longidoridae), viz. Xiphinema americanum, Xiphinema rivesi and Xiphinema brevicollum, each harbour their own specific verrucomicrobial endosymbionts. They are exclusively maternally inherited and their hosts reproduce by thelytokous (mother-to-daughter) parthenogenesis, males being extremely rare. A new genus, 'Candidatus Xiphinematobacter' gen. nov., along with three new candidate verrucomicrobial species, 'Candidatus Xiphinematobacter americani' sp. nov., 'Candidatus Xiphinematobacter rivesi' sp. nov. and 'Candidatus Xiphinematobacter brevicolli' sp. nov., are described on the basis of transmission electron microscopy, scanning electron microscopy, DAPI (4',6-diamidino-2-phenylindole) epifluorescence microscopy and 16S rDNA sequence analysis. These are the first endosymbiotic species described among the Verrucomicrobia. They share a mean 16S rDNA similarity of about 93%, whereas similarity to their closest relative, clone WCHD3-88, is less than 87%. Thus, the endosymbionts form a homogeneous clade for which the new candidate genus 'Candidatus Xiphinematobacter' gen. nov. is proposed. The type species is 'Candidatus Xiphinematobacter brevicolli' sp. nov.

Parasitism of Xiphinema Americanum and X. Rivesi By Catenaria Anguillulae and Other Zoosporic Fungi in Soil Solution, Baermann Funnels, or Soil

In dilute soil extract, Xiphinema rivesi was more susceptible than X. americanum to Catenaria anguillulae; the virulence of six isolates of C. anguillulae to Xiphinema also differed. When added to Baermann funnels, zoosporic fungi infected significant numbers of Xiphinema spp. but did not affect the number of nematodes extracted. In a greenhouse test, soil naturally infested with X. americanum and X. rivesi was amended with zoosporic fungi and planted with sudan grass. After 3 months, the zoosporic fungi had not reduced nematode numbers but infected significant numbers of Xiphinema in Baermann funnels; thus, the fungal inoculum persisted for 3 months in soil.

Redescription and distribution of Xiphinema rivesi Dalmasso, 1969 and Xiphinema americanum Cobb, 1913 in Canada with a description of Xiphinema occiduum n. sp. (Nematoda: Longidoridae)

Xiphinema rivesi and Xiphinema americanum are redescribed and illustrated from several eastern Canadian localities. Xiphinema occiduum n. sp. is described and illustrated from western Canada. The new species is most closely related to X. utahense but differs by the shorter odontostyle, greater J length and width, more anterior guiding ring, smaller a value and less elongate tail. The distribution and hosts of each species in Canada are also provided.