Carlavirus isolates from cultivated Allium species represent three viruses

From 12 cultivated and mostly vegetatively propagatedAllium species and varieties tested for carlavirus infections, 94 virus isolates were obtained which varied greatly on indicator hosts.Chenopodium amaranticolor, C. quinoa, Celosia argentea var.plumosa Geisha,Nicotiana hesperis accession 67A andN. occidentalis accession P1 proved valuable for detection, isolation and propagation of part of the isolates. The latter three species are new experimental hosts for carlaviruses ofAllium species. Other isolates could only be transmitted toAllium species such as crow garlic (A. vineale) leek (A. ampeloprasum var.porrum) and onion (A. cepa var.cepa). The isolates were grouped into three viruses by differential hosts and host reactions and their reaction with four antisera.Shallot latent virus (SLV) was found in ever-ready onion (A. cepa var.perutile), grey shallot (unidentifiedAllium species), multiplier onion (A. cepa var.aggregatum), pearl onion (A. ampeloprasum var.sectivum), rakkyo (A. chinense), shallot (A. cepa var.ascalonicum), and Welsh onion (A. fistulosum). Virus isolates from garlic and Asian shallot, fully reacting with antiserum to SLV but differing in host reactions from the SLV type-isolate, are now described as garlic strain (SLV-G) and Asian shallot strain of the virus, respectively. The garlic latent virus from garlic described in Japan is now considered identical with SLV-G.A carlavirus almost universal in garlic, and also found in great-headed garlic (A. ampeloprasum var.holmense), in an unidentifiedAllium species, and occasionally in leek, did not react with the antisera to SLV and the Japanese garlic latent virus, and is now described as the new garlic common latent virus (GCLV). It appeared identical to a virus erroneously identified in Germany as garlic latent virus.The new Sint-Jan's-onion latent virus (SjoLV) from Utrechtse Sint-Jan's onion (unidentifiedAllium species) from the Netherlands and similar crops originating from other countries, did not induce reactions in test plants and could only be detected by electron microscope decoration tests. It reacted equally wellwith the antisera to SLV and GCLV. It was also present together with SLV in ever-ready onion, pearl onion, rakkyo, shallot, and Welsh onion. Garlic latent virus reported in Japan from hosts other than garlic should be regarded as SLV, SjoLV, or a mixture of these viruses.The carlaviruses were not detected in wild plants of ramsons (A. ursinum), and of the predominantly vegetatively propagated crow garlic (A. vineale), field garlic (A. oleraceum), and sand leek (A. scorodoprasum), collected in the Netherlands.Severe reactions in the indicator hosts incidentally revealed soil-borne viruses in shallot (the nepovirusesArabis mosaic virus (ArMV) and tomato black ring virus) and crow garlic (ArMV and the tobravirus tobacco rattle virus). Tobacco necrosis virus (necrovirus) was detected in roots of shallot.     

Tomato infectious chlorosis virus — a new clostero-like virus transmitted byTrialeurodes vaporariorum

A previously undescribed virus disease of tomato, other crops and weed hosts was found in California. Affected tomato plants exhibited interveinal yellowing, necrosis and severe yield losses. Leaf dips and purified preparations contained closterovirus-like long flexuous, filamentous particles approximately 12×850–900 nm. The virus, designated as tomato infectious chlorosis virus (TICV), is transmitted in a semipersistent manner by the greenhouse whitefly,Trialeurodes vaporariorum. The host range of the virus is moderate (26 species in 8 plant families) but includes some important crops and ornamental species including tomato, (Lycopersicon esculentum), tomatillo (Physalis ixocarpa), potato (Solanum tuberosum), artichoke (Cynara scolymus), lettuce (Lactuca sativa) and petunia (Petunia hybrida). The virus has been found in a number of different locations in California and has a number of potential vehicles of movement including greenhouse grown ornamentals, tomato transplants, artichoke cuttings and potato seed. The virus has the potential to spread to other growing regions with resident populations of the greenhouse whitefly. The host range, particle size, insect transmission, and serology clearly distinguish TICV from previously described viruses.     

Identification and characterization of biologically distinct isolates of Iris yellow spot virus (genus Tospovirus, family Bunyaviridae), a serious pathogen of onion

Iris yellow spot virus (IYSV) causes an economically important disease in onion bulb and seed crops. While considerable information on the genetic diversity of the virus is available, little is known about the biological variability of the virus. Using two experimental hosts, Nicotiana benthamiana and Datura stramonium, IYSV from naturally infected onion fields was evaluated to determine the existence of biologically different isolates using the following criteria: ability to establish infection and become systemic, and the severity of the disease caused in inoculated plants. Additionally, the nucleocapsid gene of these biologically distinct isolates of IYSY was characterized at the molecular level.     

Survey and characterization of potyviruses and their strains of Allium species

Nearly 5700 plants of 14 cultivated and 8 wildAllium species and varieties from the Netherlands and other parts of the world, were tested for infection with aphid-borne potyviruses by ELISA, electron microscope decoration tests and/or inoculation onto test plants. This resulted in the detection of two known viruses, viz. leek yellow stripe virus (LYSV) and onion yellow dwarf virus (OYDV), and the discovery and characterization of two new viruses, viz. shallot yellow stripe virus (SYSV) and Welsh onion yellow stripe virus (WoYSV), and of six strains of these viruses. ‘Garlic mosaic’, ‘barlic yellow streak’, ‘onion mosaic’, ‘shallot mosaic’, ‘shallot X’, and ‘shallot yellows’ viruses, incompletely described in the literature, are now reidentified as well-known viruses or as strains or mixtures of such viruses. ‘Garlic yellow stripe virus’ is also a complex containing a potyvirus possibly differing from the viruses found in this survey. The symptoms of the potyviruses studied varied widely and ranged from mild to severe chlorotic to yellow striping of leaves, and they are of little diagnostic importance.LYSV was found in vegetatively propagated pearl onion (A. ampeloprasum var.sectivum) from Europe and Asia. It has decreased in leek crops (A. ampeloprasum var.porrum) in the Netherlands since the 1970, apparently due to resistance in new cultivars. OYDV was common in onion (A. cepa var.cepa) from the former USSR and North Africa, and in European cultivars of shallot (A. cepa var.ascalonicum), with the exception of the highly resistant ‘Santé’, but was not detected during this survey in Asian shallot. European samples of ever-ready onion (A. cepa var.perutile), multiplier onion (A. cepa var.aggregatum) and tree onion (A. cepa var.viviparum) contained OYDV. It was also found in sand leek (A. scorodoprasum) from european gene collections. A strain of OYDV from onion and shallot in Morocco and Spain was virulent on onion and shallot cultivars resistant to common OYDV, as reported early for a similar isolate in the USA.Asian shallot appeared generally infected with the new SYSV, similar to OYDV in host range and symptoms but serologically distinct. It was not detected in onion and shallot from Europe or North Africa. A virulent strain of this virus caused striping in sap-inoculated garlic (A. sativum) and Formosan lily (Lilium formosanum). The new WoYSV, infecting Welsh onion in Indonesia and Japan, was earlier described in Japan as OYDV from rakkyo and Welsh onion. It appeared serologically closely related to SYSV and distantly to OYDV, but differed in its host range.Host-specific strains of LYSV and OYDV were detected in garlic, wild garlic (A. longicuspis), an unidentifiedAllium species (suffix-G), and great-headed garlic (A. ampeloprasum var.holmense) (suffix-GhG)., LYSV-G and OYDV-G infected on average 45% and 73%, respectively, of the garlic samples of worldwide origin. Symptoms of isolates of both strains varied in severity, implying the necessity of serological tests for disease diagnosis and health certification. LYSV-GhG was the cause of yellow striping in 93% of the great-headed garlic plants tested, mainly from the Mediterranean area. One sample was also infected with OYDV-GhG.Many samples from vegetatively propagated crops grown from non-certified planting stock contained a few plants free of potyviruses, implying the possibility to obtain healthy (and possibly resistant) selections of such cultivars avoiding meristem-tip culture. Cross-protection of garlic sets by a mild potyvirus isolate seems to be an alternative to the use of vulnerable virus-free sets.Generally, viruses and virus strains could not be transmitted to anyAllium species other than their natural host, except to the highly susceptible crow garlic (A. vineale). This species, and other predominantly vegetatively propagating wildAllium spp. (field garlic,A. oleraceum; ramsons,A. ursinum; sand leek), were found not to be reservoirs of viruses that might infectAllium crops in the netherlands. Streaking in vegetatively propagated wild leeks (A. ampeloprasum and closely related species) originating from the Mediterranean area and Asia was due to an undescribed miteborne virus. The survey confirmed that spread of potyviruses inAllium crops in the Netherlands is from planting sets, and from a neighbouring crop only if of the same species.     

Molecular and biological characterization of two potyviruses infecting lettuce in southeastern France

Several potyviruses affect lettuce (Lactuca sativa) and chicory (Cichorium spp.) crops worldwide and are important constraints for production because of the direct losses that they induce and/or because of their seed transmission. Here, the molecular and biological properties are described of two potyviruses that were recently isolated from lettuce plants showing mosaic or strong necrotic symptoms in an experimental field in southeastern France. The first potyvirus belongs to the species Endive necrotic mosaic virus and is present in a large number of wild plant species, especially Tragopogon pratensis. It is unable to infect lettuce cultivars with a resistance to Turnip mosaic virus that is present in many European cultivars and probably conferred by the Tu gene. The second potyvirus belongs to the tentative species lettuce Italian necrotic virus and was not observed in wild plants. It infected all tested lettuce cultivars. Wild accessions of Lactuca serriola, Lactuca saligna, Lactuca virosa and Lactuca perennis were identified as resistant to one or the other potyvirus and could be used for resistance breeding in lettuce. No resistance against these two potyviruses was observed in the tested Cichorium endivia cultivars. In contrast, all tested Cichorium intybus cultivars or accessions were resistant.     

Tomato leaf curl Guangxi virus is a distinct monopartite begomovirus species

Three begomovirus isolates were obtained from tomato plants showing leaf curl symptoms in Guangxi province of China. Typical begomovirus DNA components representing the three isolates (GX-1, GX-2 and GX-3) were cloned and their full-length sequences were determined to be 2752 nucleotides. Nucleotide identities among the three viral sequences were 98.9–99.7%, but all shared <86.7% nucleotide sequence identity with other reported begomoviruses. The sequence data indicated that GX-1, GX-2 and GX-3 are isolates of a distinct begomovirus species for which the name Tomato leaf curl Guangxi virus (ToLCGXV) is proposed. Further analysis indicated that ToLCGXV probably originated through recombination among viruses related to Ageratum yellow vein virus, Tomato leaf curl China virus and Euphorbia leaf curl virus. PCR and Southern blot analyses demonstrated that isolates GX-1 and GX-2 were associated with DNAβ components, but not isolate GX-3. Sequence comparisons revealed that GX-1 and GX-2 DNAβ components shared the highest sequence identity (86.2%) with that of Tomato yellow leaf curl China virus (TYLCCNV). An infectious construct of ToLCGXV isolate GX-1 (ToLCGXV-GX) was produced and determined to be highly infectious in Nicotiana benthamiana, N. glutinosa, tobacco cvs. Samsun and Xanthi, tomato and Petunia hybrida plants inducing leaf curl and stunting symptoms. Co-inoculation of tomato plants with ToLCGXV-GX and TYLCCNV DNAβ resulted in disease symptoms similar to that caused by ToLCGXV-GX alone or that observed in infected field tomato plants.     

Lettuce ring necrosis,caused by a chytrid-borne agent distinct from lettuce big-vein ‘virus’

Ring necrosis is a serious disease of lettuce (Lactuca sativa) with often coalescing necrotic rings and ring-like patterns on middle leaves of plants or groups of plants in glasshouses during winter. Affected leaves may decay and plants rapidly become unmarketable. The disease was shown to be soil-borne and transmitted by the zoospores ofOlpidium brassicae. Symptoms in lettuce do not appear before seven weeks after inoculation via the soil. Additives to the inoculum and chilling of source leaves, inoculum buffer and utensils enabled mechanical transmission of a pathogenic agent toChenopodium quinoa, C. amaranticolor, Nicotiana benthamiana, N. clevelandii, N. hesperis, andN. occidentalis but not to lettuce. TheChenopodium spp. reacted with local lesions, infection was symptomless inN. clevelandii and mostly so inN. benthamiana, butN. hesperis andN. occidentalis reacted with leaf spotting and plant stunting. With zoospores of an originally pathogen-free fungus culture further cultivated on the roots of cuttings from sap-inoculated plants ofN. clevelandii andN. occidentalis, the agent could be transferred back to lettuce and the symptoms of ring necrosis be reproduced. The agent biologically resembles those of lettuce big-vein (LBV) and freesia leaf necrosis and the tobacco stunt virus. In lettuce it often occurs together with LBV virus but differs in longer incubation period, type of symptoms and symptom appearance only during winter. It could be separated from a mixture with LBV virus by serial transfer always selecting plants without LBV symptoms. So far cultural hygiene, including soil disinfection addressing the vector, is the main means of control.     

Host Range Studies for Tomato chlorosis virus, and Cucumber vein yellowing virus Transmitted by Bemisia tabaci (Gennadius)

The Bemisia tabaci (Gennadius) biotype B transmitted host range of Tomato chlorosis virus (ToCV), genus Crinivirus, Family Closteroviridae, and Cucumber vein yellowing virus (CVYV), genus Ipomovirus, Family Potyviridae, was studied. New experimental hosts were identified for each of these viruses. Seventeen species in eight plant families were assessed as potential hosts for ToCV. Infection in asymptomatic Anthriscus cereifolium (chervil) test plants by ToCV was confirmed by using a Real-Time PCR assay designed for ToCV. The presence of readily transmissible, infectious ToCV virions in A. cereifolium was confirmed by re-isolation of the virus via whitefly-transmission from A. cereifolium to Lycopersicon esculentum and A. cereifolium. This is the first report of the experimental transmission of ToCV by B. tabaci to a species within the Umbelliferae. All other hosts assessed for the presence of ToCV were found to be uninfected. Ten species in five families were assessed as potential hosts for CVYV. The CVYV host range identified included some important crops and common weeds, such as L. esculentum, Nicotiana tabacum, A. cereifolium, Datura stramonium, Nicotiana benthamiana, Nicotiana clevlandii and Cucumis sativus. Symptoms were present on D. stramonium, N. benthamiana and C. sativus control plants. The presence of infectious whitefly transmitted CVYV virions was confirmed solely for D. stramonium and N. tabacum, following re-isolation of the virus via B. tabaci transmission from all infected species to C. sativus. This is the␣first report of experimental CVYV transmission by B. tabaci to non-cucurbitaceous crop and weed hosts belonging to the Solanaceae or Umbelliferae.     

Phylogenetic investigations in the downy mildew genus <Emphasis Type="Italic">Bremia</Emphasis> reveal several distinct lineages and a species with a presumably exceptional wide host range

Bremia lactucae is one of the most devastating and widespread pathogens in lettuce production worldwide. Despite its economical importance, uncertainty prevails about the species delimitation in the genus Bremia. Commonly, Bremia is considered to be monotypic, containing only Bremia lactucae, while taxonomists have described additional species, and molecular phylogenetic studies have shown significant sequence divergence between accessions from different hosts. Here, we report that several previously described species are genetically highly distinct from Bremia lactucae parasitic to Lactuca sativa. These include Bremia lapsanae, Bremia sonchicola, and Bremia taraxaci. In addition to these host-specific species, a plurivorous species is revealed, which infects hosts from three different tribes in the Asteraceae subfamilies Asteroideae and Carduoideae. The broad host range of clade 1 is exceptional for downy mildews and only paralleled by Pseudoperonospora cubensis, which infects a broad range of Cucurbitaceae. The taxonomic status of Bremia cirsii and of Bremia centaureae remains unresolved, as the accessions from Cirsium and Centaurea, respectively, did not form a monophylum but were partly contained in the plurivorous clade 1. Bremia lactucae was found to be restricted to Lactuca sativa and Lactuca serriola. Thus, it can be assumed that Bremia infections on weeds apart from Lactuca species do not pose a significant risk for lettuce production. However, it is unlikely that breeding resistance genes from Lactuca serriola into Lactuca sativa will result in durable resistance of lettuce to downy mildew disease, because the current study provides additional evidence that Bremia accessions from both hosts form a population continuum.     

Host range comparison of the causal agents of pepper yellow vein and lettuce big vein

A number of solanaceous and composite plant species were tested as hosts for the causal agents of pepper yellow vein (PYVA) and lettuce big vein (LBVA), transmitted by a pepper and a lettuce isolate ofOlpidium brassicae, respectively. The agents had the following artificial hosts in common:Lycopersicon esculentum, Solanum melongena, Physalis floridana, Nicandra physaloides, Lactuca sativa, Sonchus oleraceus andL. virosa. Capsicum annuum, S. villosum, S. nigrum, Crepis vesicaria andSenecio vulgaris were infected by PYVA, but not by LBVA.Cichorium endivia andTaraxacum officinalis were not infected by any of the two agents.N. physaloides, although not colonized by the pepper isolate ofOlpidium, still became infected by PYVA.     

Host–pathogen interaction between Phytophthora infestans and Solanum nigrum, S. villosum, and S. scabrum

Potato and tomato are the two major hosts for Phytophthora infestans causing late blight. The susceptibility of leaves and whole plants of Solanum nigrum, S. villosum, and S. scabrum to infection by P. infestans was tested under laboratory conditions. Out of 39 plants representing 38 different S. nigrum accessions, 16 were highly resistant (seven accessions did not show any symptoms of infection, nine were highly resistant showing necrotic lesions in the place of infection), and 23 plants of S. nigrum were colonized by, at least, 1 of the 2 isolates of P. infestans (17 accessions were infected with two P. infestans isolates, and 6 accessions showed different reactions depending on the isolate used for inoculation). Three accessions of S. villosum, and one accession of S. scabrum were tested and did not show any symptoms of infection. The majority of S. nigrum accessions infected by P. infestans in a detached leaf assay were also infected in the whole plant assay. The reaction of field- and greenhouse-grown plants to inoculation with P. infestans in detached leaf assays was similar, but in some cases leaves from field-grown plants reacted as resistant in comparison with the leaves from greenhouse-grown plants, which were susceptible.     

Agroinoculation Shows Tobacco leaf curl Yunnan virus is a Monopartite Begomovirus

We demonstrated that only 2 out of 15 isolates of Tobacco leaf curl Yunnan virus (TbLCYNV) were associated with the satellite DNAβ molecules. To investigate the infectivity of this virus, an infectious clone of TbLCYNV isolate Y143 (TbLCYNV-Y143) was agroinoculated or whitefly transmitted into Nicotiana benthamiana, N. glutinasa, Petunia hybrida and N. tabacum. TbLCYNV-Y143 alone was able to induce severe upward leaf curling, vein thickening or stunt symptoms in these plants. Co-inoculation of TbLCYNV-Y143 with DNAβ molecules associated with other begomoviruses induced similar symptom types on these plants. This indicates that TbLCYNV is a monopartite begomovirus. The relevance of results that only two isolates of TbLCYNV were associated with DNAβ molecules is discussed.     

Characterisation of Rhizoctonia solani Anastomosis Groups Causing Bottom Rot in Field-Grown Lettuce in Germany

Bottom rot caused by Rhizoctonia solani is an increasing problem in field-grown lettuce in Germany. During the growing seasons of 1999 and 2000, 95 isolates of R. solani from lettuce plants with bottom rot symptoms were collected from eight locations. The isolates were characterised using hyphal anastomosis, pectic zymograms and morphological characteristics. Ninety-three isolates were identified as anastomosis group (AG) 1-IB, one as AG 1-IC and one as AG 2-1. Optimum hyphal growth was measured over a temperature range of 20–30 °C with an optimum at 25 °C. Aggressiveness of the AG 1-IB isolates varied from weak to strong when tested on detached lettuce leaves. The pathogenic potential of six AG 1-IB isolates was determined on 14 plant species in comparison with lettuce under conditions favourable for the fungus. Radish, broccoli, kohlrabi, spinach and millet seedlings were as severely infected as lettuce seedlings. The same isolates caused little symptoms on maize, tomato and onion. Knowledge about the host range of AGs of R. solani are important for planning an effective crop rotation as part of a control management system.     

Further observations on variation of lettuce mosaic virus in relation to lettuce (Lactuca sativa), and a discussion of resistance terminology

Two potyvirus isolates from endive, originating from southern France (Ls252) and from the Netherlands (Ls265), that were highly and poorly pathogenic on lettuce, respectively, were compared with a common isolate (Ls1) of lettuce mosaic virus (LMV) and with two highly deviant Greek isolates fromHelminthia (Picris) echioides (Gr4) and endive (Gr5), earlier recognized as LMV. The isolates could not be distinguished by particle morphology and serology, and were all identified as LMV. Leaf curling, plant stunting and necrosis were more characteristic of the virus than mosaic. The isolates studied varied considerably on differential host species and a range of lettce cultivars including pathotype differentials of Pink et al. [1992b]. Ls1 and Ls265 reacte largely as pathotype II, including the common strain of the virus, but Ls265 was least pathogenic on lettuce. Ls252 fitted pathotype IV and was very similar to LMV-E (the Spanish strain). The Greek isolates were very similar to each other in causing very severe symptoms on some non-lettuce hosts and a number of lettuce cultuvars. In lettuce variectal reaction Gr4 resembled pathotype I, but Gr5 severely affected Salinas 88, resistant to pathotypes I, II and III, and it appears to be a novel pathotype. Genetic interaction between lettuce and LMV is not following a simple yes-or-no pattern, and it is not a mere matter of resistance versus susceptibility. Adoption of a more realistic resistance terminology is proposed. None of the lettuce cultivars tested was resistant to the most pathogenic isolate Ls252, but resistance to it was found in 2 out of 12 wildLactuca species tested (Lactuca perennis andL. tatarica) while the symptomless plants ofL. perennis clearly reacted in ELISA.     

Sequence analysis of 43-year old samples of Plantago lanceolata show that Plantain virus X is synonymous with Actinidia virus X and is widely distributed

Plantain virus X was first recognized by the ICTV as a species in the genus Potexvirus in 1982. However, because no sequence was available for plantain virus X (PlVX), abolishing the species was proposed to the Flexiviridae working group of the ICTV in 2015. This initiated efforts to sequence the original isolates from Plantago lanceolata samples. Here we report the full-genome sequencing of two original isolates of PlVX, which demonstrate that the virus is synonymous to Actinidia virus X, a species previously reported from kiwifruit (Actinidia sp.) and blackcurrant (Ribes nigrum). PlVX was previously noted to be widespread in the UK in P. lanceolata. This report additionally presents novel data on the distribution and diversity of PlVX, collected at the same site as the original UK isolates, and from three independent surveys, two in the Netherlands and one in Belgium. This study also includes two new host records for PlVX, Browallia americana and Capsicum annuum (sweet pepper), indicating the virus is more widespread and infects a broader range of hosts than previously reported. This stresses the importance of surveys of noncultivated species to gain insight into viral distribution and host range. This study also demonstrates the value of generating sequence data for isolates retained in virus collections. Additionally, it demonstrates the potential value in prepublication data sharing for giving context to virus detections such as the four independent studies here which, when combined, give greater clarity to the identity, diversity, distribution, and host range of plantain virus X.     

Mite-borne virus isolates from cultivated Allium species,and their classification into two new rymoviruses in the family Potyviridae

While testing several samples of onion and of vegetatively propagated garlic, sand leek and shallot from a number of countries, virus isolates with unusually flexuous particles were obtained by mite (Aceria tulipae) or sap transmissions. No aphid-borne poty-or carlavirus was transmitted by mites, and mite-borne virus isolates could not be transmitted by aphids. The mite-borne isolates did not react with antisera to aphid-borne potyviruses ofAllium spp. or with the Agdia potyvirus group monoclonal. In contrast to the mite-borne onion and garlic mosaic viruses reported in the literature, our mite-borne isolates induced no visible or only very mild symptoms inAllium spp., except isolates from shallot ‘Santé’ which caused diffuse striping. Heavily mite-infested test plants or plant samples showed streaking and malformation due to mite feeding (tangle-top). The mite-borne virus isolates could be classified with test plants and a discriminating antiserum into three groups, representing two viruses and a strain of one of them. They are tentatively named onion mite-borne latent virus (OMbLV), garlic strain of this virus (OMbLV-G), and shallot mite-borne latent virus (SMbLV). Mite transmission, length of virus particles (ca. 700 to 800 nm), and the presence of granular inclusion bodies in infected tissue indicate that the viruses belong to the mite-borne genusRymovirus of the familyPotyviridae. OMbLV from shallot and onion, and OMbLV-G from garlic and sand leek, can be assayed onChenopodium murale but differ in their natural hosts. They are very common. SMbLV, to whichC. murale does not react, was isolated from shallot originating from Asia and Russia.     

Susceptibility of different plant species and tomato cultivars to two isolates of <Emphasis Type="Italic">Pepino mosaic virus</Emphasis>

As Pepino mosaic virus has become a pathogen of major importance in worldwide tomato production, information is needed on possible differences between the sensitivity of cultivars towards infection. Furthermore, it is important what hosts other than Solanaceae may be virus reservoirs and are, therefore, threats for tomato cultivation. Two PepMV isolates (PepMV-Sav, E397, a European tomato isolate and PV-0554, a Peruvian pepino isolate) differing in their origin and virulence were used for several experiments to investigate these issues. The response to mechanical inoculation with PepMV was studied using 25 tomato cultivars, seven indicator plant species, and nine other possible horticultural host plants. Symptom development after infection with PepMV was monitored and the virus was detected by DAS-ELISA and IC-RT-PCR. Garlic and broad bean were shown to be additional hosts of PepMV depending on the virus isolate. Nicotiana benthamiana seems to be the most sensitive indicator among all tested indicator plants developing symptoms. Both PepMV isolates infected all tested tomato cultivars. Development of disease symptoms depended on the cultivar and the virus isolate but symptoms were not visible in all cases. None of the cultivars showed tolerance against the two isolates but two responded with a lower susceptibility at an absorbance level of 0.2 (healthy control 0.09). It was observed that some cultivars grown hydroponically showed also lower losses in biomass and yield. Data indicated a correlation between absorbance level in DAS-ELISA and reduction in total tomato growth.     

Occurrence and genetic diversity of Blackcurrant reversion virus found on various cultivated and wild Ribes in Latvia

A large‐scale survey was carried out to assess the occurrence, natural host range and genetic diversity of Blackcurrant reversion virus (BRV) in cultivated and wild Ribes in Latvia using RT‐PCR and sequence analyses of 3′ NTR of BRV RNA2. The virus was detected in all surveyed habitats in most of the studied Ribes, except gooseberries, Ribes sanguineum, Ribes laxiflorum and crossbreeds between blackcurrants and gooseberries. The overall occurrence of BRV was 27%, although it varied significantly among the surveyed Ribes habitats, exceeding 40% in home gardens and germplasm collections. Among cultivated Ribes, blackcurrants were the most infected and BRV was detected in all commonly grown cultivars. The virus was detected for the first time in Ribes aureum, Ribes fragrans, Ribes nigrum var. pauciflorum and Ribes fasciculatum var. chinense. The sequence identities of the studied fragments of RNA2 3′ NTR varied from 92.8% to 99.7% among 26 BRV isolates from various cultivated, ornamental and wild hosts from Latvia and from 91.1% to 97.1% when they were compared with 27 corresponding sequences from GenBank. Phylogenetic analyses revealed that the major clustering of isolates was not related to host, origin or symptoms. Grouping of BRV isolates based on host or location was identified within the phylogenetic subclusters. Several well‐supported clades were formed within the subclusters, including a group of BRV isolates from redcurrants that had unique nucleotide substitutions. Five putative recombinants were identified for the first time among BRV isolates from Latvia, Finland, Scotland and the Czech Republic.     

Pathogenicity of <Emphasis Type="Italic">Stemphylium vesicarium</Emphasis> from different hosts causing brown spot in pear

Stemphylium vesicarium (teleomorph: Pleospora herbarum) is the causal agent of brown spot disease in pear. The species is also able to cause disease in asparagus, onion and other crops. Saprophytic growth of the fungus on plant debris is common. The objective of this study was to investigate whether isolates of S. vesicarium from different hosts can be pathogenic to pear. More than hundred isolates of Stemphylium spp. were obtained from infected pear fruits, dead pear leaves, dead grass leaves present in pear orchard lawns as well as from necrotic leaf parts of asparagus and onion. Only isolates originating from pear orchards, including isolates from dead grass leaves, were pathogenic on pear leaves or fruits in bioassays. Non-pathogenic isolates were also present in pear orchards. Stemphylium vesicarium from asparagus or onion, with one exception, were not pathogenic to pear. Analysis of the genetic variation between isolates using Amplified Fragment Length Polymorphism (AFLP) showed significant concordance with host plants. Isolates from asparagus or onion belonged to clusters separate from the cluster with isolates from pear or grass leaves collected in pear orchards. Multilocus sequencing of a subset of isolates showed that such isolates were similar to S. vesicarium.