The question of seed transmissibility of Plum pox virus

For many years, Plum pox virus (PPV) was considered to be transmissible by seed, increasing the fear of long-distance spread of the disease. In the late 1970s, it was claimed on the basis of biological transmission of the virus to herbaceous indicator plants and the development of serological diagnosis based on polyclonal antibodies, that PPV was seed-transmitted, with a different infection rate according to the plant species and part of the seed which was tested. In the 1990s, PPV was characterized into four different types, and specific monoclonal antibodies were produced for them. These new and more sensitive diagnostic techniques, together with RT-PCR with different sets of specific primers, were used to approach once again the problem of PPV transmission through seeds. The virus was detected in seed coats and cotyledons, but embryonic tissue and seedlings obtained from germinated seeds never showed symptoms, and gave negative results for PPV with both ELISA and PCR assays. No PPV isolate is currently recognized to be seed transmitted, so vertical transmission of PPV from infected mother plants to their progeny does not occur. Hypothetically, the only possibility of seed transmission would arise from a mutation in the helper component of the virus, associated with high susceptibility of the infected Prunus cultivar.     

Hosts and symptoms of Plum pox virus: ornamental and wild Prunus species

Several ornamental and wild Prunus species have been identified as natural and/or experimental hosts for Plum pox virus (PPV). The significance of natural vs. experimental hosts, graft or bud-transmitted infections vs. aphid-transmitted infections in ornamental or wild Prunus hosts, and their relevance in the field situation, are not clearly understood. However, since PPV is aphid-transmitted, any host in the field or nursery serves as a potential reservoir and source of inoculum and must be monitored and controlled in any PPV eradication or management programme.     

A preliminary account of the sanitary status of stone-fruit trees in Morocco

Field surveys were carried out in the main stone-fruit growing areas of Morocco to evaluate the sanitary status of commercial orchards, varietal collections and nurseries. The presence of virus and virus-like diseases was checked by ELISA, sap transmission to herbaceous hosts, testing on woody indicators and molecular hybridization (dot-blot and tissue-printing). 1211 samples (382 almond, 339 peach, 291 plum, 150 apricot and 49 cherry) were tested by ELISA for the presence of Prunus necrotic ring spot virus (PNRSV), Prune dwarf virus (PDV), Apple chlorotic leaf spot virus (ACLSV), Apple mosaic virus (ApMV) and Plum pox virus (PPV). The overall average of virus infection rate was 16.4%, whereas that of single species was: 22.6% for almond, 17.8% for plum, 15% for peach, 10.2% for cherry, and 2.7% for apricot. The following viruses were detected: PNRSV, PDV, ACLSV and ApMV. 565 samples were tested by dot-blot and tissue-printing hybridization for the presence of Peach latent mosaic viroid (PLMVd) and Hop stunt viroid (HSVd). 48 samples were infected, 41 by PLMVd and 7 by HSVd. In addition, nested-PCR tests identified Plum bark necrosis and stem-pitting associated virus (PBNSPaV) in a few almond trees affected by stem pitting.     

Identification of herbaceous hosts of the <Emphasis Type="Italic">Grapevine Pinot gris virus</Emphasis> (GPGV)

Grapevine Pinot gris Virus (GPGV) is a single stranded RNA of the genus Trichovirus infecting grapevine (Vitis vinifera) and associated with stunting, chlorotic mottling and leaf deformation symptoms. During a monitoring of GPGV infection in vineyards of the Trentino region in Italy, we have detected the virus in the herbaceous plants Silene latifolia subsp. Alba (Mill.) (bladder campion) and Chenopodium album L. (white goosefoot), which showed symptoms of viral infection. The full-length GPGV RNA genome, amplified from these infected hosts, was sequenced and a phylogenetic analysis revealed that its closest relative is the strain SK13, recently isolated in Slovakia. Our results indicate that herbaceous plants can be considered as a reservoir for the GPGV virus. This finding is important for studying the epidemiological aspects of GPGV disease and to formulate appropriate control measures.     

Hosts and symptoms of Plum pox virus: woody species other than fruit and ornamental species of Prunus

The presence of Plum pox virus (PPV) in woody hosts other than in fruit and ornamental Prunus species was evaluated. PPV symptoms, and their variability and intensity, are described in leaves and fruits of Prunus cerasifera , Prunus spinosa , Ligustrum vulgare and Euonymus europaea . Juglans regia is not confirmed as a new host of PPV.     

A Single Amino Acid Mutation in the Plum pox virus Helper Component-Proteinase Gene Abolishes Both Synergistic and RNA Silencing Suppression Activities

ABSTRACT The effects on symptom expression of single amino acid mutations in the central region of the Plum pox virus (PPV) helper component-proteinase (HC-Pro) gene were analyzed in Nicotiana benthamiana using Potato virus X (PVX) recombinant viruses. PVX recombinant virus expressing the wild-type variant of PPV HC-Pro induced the expected enhancement of PVX pathogenicity, manifested as necrosis and plant death. Recombinant virus expressing a variant of PPV HC-Pro containing a single point mutation ( HCL(134)H) was unable to induce this synergistic phenotype. The RNA silencing suppressor activity of PPV HC-Pro was demonstrated in a transient silencing suppression assay. In contrast, the HCL(134)H mutant showed no such activity. These results indicate that a unique point mutation in PPV HC-Pro impaired its ability to suppress RNA silencing and abolished its capacity to induce synergism, and clearly shows for the first time the link between these two functions in potyvirus HC-Pro. Additionally, we compared the effects on virus accumulation in N. benthamiana plants infected with either the PVX recombinant constructs or with native viruses in double infection experiments. PVX (+) and (-) strand genomic RNA accumulated at similar levels in plants infected with PVX recombinants, leading to an increase in PVX pathology, compared with plants infected with PVX alone. This finding confirms that the enhancement of pathogenicity associated with synergistic interaction is not a consequence of more efficient PVX replication due to RNA silencing suppression by PPV HC-Pro.     

Alternative hosts and seed transmissibility of soybean blotchy mosaic virus

Soybean blotchy mosaic virus (SbBMV) is an important virus of soybean in the warmer regions of South Africa. The presence of the virus is associated with blotchy mosaic symptoms on soybean leaves and significant annual yield losses. The virus is a member of the genus Cytorhabdovirus and persists between soybean growing seasons. In this study, multiple specimens of indigenous tree species, other crops and herbaceous weeds surrounding soybean fields with high disease incidences of SbBMV were tested for the presence of SbBMV by RT-PCR in order to determine whether the presence of alternative hosts facilitates the seasonal carry-over of the virus. Commercial soybean cultivars commonly grown in the region were also evaluated for seed transmissibility of the virus. A total of 487 accessions representing 27 different species were screened and one accession each of Flaveria bidentis, Lamium amplexicaule and Gymnosporia buxifolia tested positive for the presence of SbBMV and may serve as possible alternative hosts of SbBMV, allowing over-wintering of the virus when soybean is absent. Symptoms associated with SbBMV infection were not present in any of the 2, 829 seedlings collected from naturally infected SbBMV plants, and none of the 21 seedlings showing various abnormalities and tested by RT-PCR were positive. SbBMV does not appear to be seed transmissible in soybean at an incidence above that which numbers screened would have detected the virus. It was concluded that the presence of alternative plant hosts, functioning as viral reservoirs during the soybean off-season might allow for the re-emergence of the disease early in the soybean production season each year. Future work will investigate the role of Peragallia caboverdensis, the leafhopper vector of SbBMV, and specifically the possible propagative transmission of the virus in the persistence of the disease.     

Potential role of almond in sharka epidemics: susceptibility under controlled conditions to the main types of plum pox potyvirus and survey for natural infections in France 1

The changes in cropping conditions of almond and the development of efficient tools for the detection and characterization of plum pox potyvirus (PPV) populations have led us to reassess the potential susceptibility of this species to sharka disease and its role as a virus reservoir. The susceptibility of almond cv. Aï to nine isolates, representative of the known diversity among PPV populations, was assessed under controlled conditions. Most isolates were able to infect almond, by graft or aphid inoculation, causing generalized stable infections without any obvious sharka symptoms. These infected almonds were found to constitute a potential source of virus for aphid vectors, mainly in the case of M isolates. Surveys were carried out in the south of France, in foci of M and D strains of PPV, to evaluate the presence of natural infections of almond. No typical symptoms were observed and the virus was never detected. It can be assumed that the actual limited prevalence of PPV in France does not lead to a sufficiently high inoculum pressure to allow almond trees to be infected.     

小麦黄花叶病(WYNW)的研究

 小麦黄花叶病的病状是黄色花叶。病株细胞内有1至数个内含体。病毒质粒为线状,稍弯曲,大小为13×100-600毫微米,存在于细胞质中。内含体呈风轮状。病毒仅感染小麦属(Triticum)10多种,能通过病根、病土及它们侵出液传染。介体为禾谷多粘霉(Polymyxa graminis),其寄主有小麦等28种禾本科植物。病土加灭菌土1000倍混合仍有致病性,病田表土致病性最强,深到31-40厘米处还会发病。田间3月中旬以后病株不再增加。予防本病以选育选育抗病品种为主,辅以轮作,增施肥料和迟播等。     

Significance of the heterologous encapsidation of zucchini yellow mosaic potyvirus in transgenic plants expressing plum pox potyvirus capsid protein1

Transgenic Nicotiana benthamiana plants expressing the coat protein of an aphid-transmissible strain of plum pox potyvirus (PPV-D) were infected with an aphid non-transmissible strain of another potyvirus, zucchini yellow mosaic potyvirus (ZYMV-NAT). Non-viruliferous Myzus persicae could acquire and transmit ZYMV-NAT from these plants but not from infected N. benthamiana control plants (not transformed, or transformed by the vector alone). Immunosorbent electron microscopy experiments using the decoration technique revealed that ZYMV-NAT virus particles in the infected transgenic plants expressing the PPV coat protein could be coated not only with ZYMV antibodies but also, on segments of the particles, with PPV antibodies. This suggests that aphid transmission of ZYMV-NAT occurred through heterologous encapsidation, and reveals a potential risk of releasing genetically engineered plants expressing viral coat proteins into the environment.     

Citrus leprosis virus: properties, diagnosis, agro-ecology and phytosanitary importance

Citrus leprosis disease, caused by citrus leprosis virus (CiLV), had severe effects on sweet oranges in Florida (US) until the 1920s, after which it became rare. In South America, it appeared in the 1930s, first in Argentina and then in Brazil, where it is now widespread and very dangerous. It has also been reported in Paraguay, Peru, Uruguay and Venezuela. CiLV is transmitted by three species of Brevipalpus , mainly Brevipalpus phoenicis. The virus mainly attacks sweet orange, but also citrange, citron, Cleopatra mandarin, grapefruit, lemon, mandarin, sour orange and tangor. CiLV is a non-enveloped rhabdovirus characterized by bacilliform particles measuring 120–130 × 50–55 nm, present in mesophyll and vascular parenchyma within cisternae of the endoplasm reticulum. Viroplasm structures are present in the infected cells. CiLV has been partially purified and its dsRNA as been investigated. It is mechanical transmissible to 13 test plant species belonging to the genera Atriplex, Beta and Chenopodium (Chenopodiaceae), Gomphrena (Amaranthaceae) , and Tetragonia (Tetragoniaceae). Using some of these herbaceous test plants, grown at a suitable temperature, it is possible to diagnose CiLV in 3–4 days. CiLV is covered by lists and requirements in phytosanitary regulations, but the information given is often misleading. For phytosanitary purposes, it is important to consider the following main points: (a) both CiLV and its vectors need to be considered; (b) sweet orange fruits can be infected even more than propagation material; and (c) CiLV does not infect susceptible citrus systemically, or any of its known hosts.     

Phytosanitary status of almond and apricot in Tunisia

Surveys were carried out in the traditional areas of almond and apricot cultivation in Tunisia to assess the phytosanitary status of these species in varietal collections, mother block stands and commercial orchards. The presence of virus and virus-like diseases was ascertained through field surveys, sap transmission to herbaceous hosts, and ELISA and IEM tests. The mean infection level in almond was 34%. compared with only 4.7% in apricot. The following viruses were identified: apple chlorotic leaf spot trichovirus (ACLSV), apple mosaic (ApMV), prune dwarf (PDV) and prunus necrotic ringspot (PNRSV) ilarviruses. Plum pox potyvirus (PPV) was not detected. Likewise, none of the six nepoviruses tested for (SLRV, TBRV, RRV. CLRV. ArMV and ToRSV) was encountered.     

Potential host range of four Phytophthora interspecific hybrids from Clade 8a

In recent years several interspecific hybrids have been reported in the plant pathogenic oomycete genus Phytophthora. Due to the large genotypic and phenotypic changes, these hybrids might have broader or more limited host ranges compared with their parental species. It is crucial to understand the host range of Phytophthora hybrids to minimize the economic losses caused by their infection. The potential host range of four hybrids belonging to Clade 8a of the Phytophthora phylogenetic tree was investigated in this study. Thirty species of herbaceous plants as well as eight species of woody plants were inoculated and monitored for any symptom of infection. In addition, the detached twigs of 32 tree species, fruits of six plant species, tubers of potato, and roots of carrot and sugar beet were investigated for susceptibility to these hybrids. Almost all hybrids caused severe rot on all tested fruits, tubers, and roots, although different isolates showed different pathogenicity on detached tree twigs. All hybrids tested had a different host range compared with their parental species: they were able to infect plants outside the host range of their parents, infect hosts of both parental species, although these parents did not have overlapping hosts, or, in some cases, they were not able to infect hosts infected by the parents.     

Control and monitoring: Plum pox virus quarantine situation in France

Sharka disease caused by Plum pox virus (PPV) is present in several areas in France, and its total eradication from the territory is not considered feasible. Although sharka falls under mandatory control measures, the objective is, in the infected areas, to contain the disease in the orchards at sufficiently low levels so that the production remains economically viable. In parallel, official controls are carried out to guarantee that plants for planting, including seedlings, which are moved in trade are free from PPV.     

Breeding for resistance: breeding for Plum pox virus resistant plums (Prunus domestica L.) in Germany

In German plum breeding programmes new varieties tolerant to Plum pox virus (PPV) were initially obtained by selecting hybrids originating from crossings between tolerant old varieties while, later on, sharka tolerant varieties bred in Čačak, Serbia were used as crossing partners. Varieties released from these programmes replaced the sensitive ones in all sharka infected regions in Germany. While all the new tolerant varieties can be infected by the virus, PPV symptoms on the fruits are acceptable. However, environmental factors can weaken the plant, causing them to suffer more from PPV infections and to display worse symptoms on fruits, as did occur during some recent very dry and warm years. A breeding programme at the University of Hohenheim is tackling this problem with genotypes that show a hypersensitive response after infection. These genotypes are completely field resistant to PPV, remaining virus free in the field since they cannot be infected via aphid transmission. They are able to isolate the virus after infection. The first hypersensitive variety was 'Jojo'. Many seedlings originating from crossings with at least one hypersensitive parent are under evaluation. Since the inheritance of the trait 'hypersensitivity against PPV' is very good, combining hypersensitivity with excellent fruit quality and good cropping capacity will be possible soon. In 2005, a breeding programme for hypersensitive Prunus genotypes began at the Technical University of Munich.     

Viruses and viroids of stone fruits in Syria

Field surveys were carried out in the main stone fruit-growing areas of Syria to evaluate the sanitary status of mother blocks, varietal collections and commercial orchards. The presence of virus and virus-like diseases was checked by enzyme-linked immunosorbent assay (ELISA), sap transmission to herbaceous hosts, testing on the woody indicators Prunus persica cv. GF 305 and Prunus serrulata cv. Kwanzan and dot-blot hybridization tests. A total of 1337 samples was tested by ELISA (444 apricot, 283 peach, 246 cherry, 222 almond and 142 plum). The overall mean infection rate was 13%, and the percentage infection level of single species was: peach 24%, cherry 16%, almond 13.5%, apricot 6%, plum 5%. The following viruses and viroids were detected: PNRSV, PDV, ACLSV, PPV, ApMV, PLMVd and HSVd ¹ .     

Viruses and virus diseases of grapevine in Tunisia

Mature canes were collected from vines in the main grapevine-growing areas in Tunisia (Cape Bon, Bizerte, Ben Arous), from commercial vineyards and mother-plant plots, to assess the presence of virus and virus-like diseases. Biological (mechanical transmission onto herbaceous hosts and grafting onto indicator woody plants) and serological detection (ELISA) methods were applied. ELISA showed that 96.4% of 669 vines tested were infected, most of them (88.1%) by at least two viruses. Grapevine leafroll-associated 3 closterovirus (GLRaV-3) was the most widespread virus (87.9%), followed by grapevine A vitiviras (GVA, 69.4%), grapevine fleck virus (GFkV, 51.9%), grapevine leafroll-associated 1 closterovirus (GLRaV-1, 36.8%), grapevine leafroll-associated 2 closterovirus (GLRaV-2, 19.1%), grapevine fan leaf nepovirus (GFLV, 18.2%) and grapevine B vitiviras (GVB, 14.8%). ELISA tests yielded negative results for grapevine leafroll-associated 7 closterovirus (GLRaV-7) and potato X potexvirus (PVX). The highest infections were found in Bizerte and Cape Bon regions (100 and 99.2%), and in vineyards aged over 20 years (98.5%) as compared with the younger ones (81.1%). Rootstocks in mother-plant plots were practically free from all the viruses tested (1 plant infected out of 81), whereas severe infections were found in Vitis vinifera mother plants (67.4% of 341 samples), in particular table grapes (92.6%) compared with wine grapes (47.9%). In these mother-plant plots, the prevailing viruses were GLRaV-3 (41.3%), followed by GFkV (36.7%), GVA (27.9%), GLRaV-1 (17%) and GLRaV-2 (15.2%). GFLV and GVB were far more limited (1.5 and 0.6%, respectively). The presence of vein necrosis and vein mosaic was ascertained by transmission onto 110R and Vitis riparia indicators, whereas only GFLV was mechanically transmitted onto herbaceous hosts (from about 20% of the samples).     

Some properties of alfalfa mosaic virus isolated from Buddleia davidii in the UK

Alfalfa mosaic virus (AMV) and cucumber mosaic virus (CMV) were isolated from clones of Buddleia davidii collected at Long Ashton Research Station for assessment of horticultural characteristics. The AMV isolate investigated in this study (AMV-B) infected 18 species and cultivars of herbaceous test plants. The in vitro properties of AMV-B were determined. The virus was purified by permeation chromatography on controlled-pore glass beads and an antiserum prepared. AMV-B appeared to be serologically indistinguishable from AMV isolate 15/64. The sizes and shapes of the virus particles were those of classical AMV components. The capsid protein was a single polypeptide of Mr 24 375. Buddleia seedlings inoculated with AMV-B showed only mild chlorosis and slight distortion of leaves.     

Characterization of a strain of Apple stem grooving virus in Actinidia chinensis from China

A new strain of Apple stem grooving virus (ASGV) has been identified in Actinidia chinensis imported from China. The leaves of these plants exhibited a variety of symptoms including interveinal mottling, chlorotic mosaics and ringspots. Capillovirus-like particles were observed under the electron microscope, and the virus could be mechanically transmitted to a range of herbaceous indicators. The virus was detected using ELISA with antisera raised against ASGV. Sequencing of the virus revealed that it had more than 95% amino acid identity with ASGV in the putative coat and movement proteins. From the morphological, transmission, serological and molecular evidence, it was concluded that the virus is a strain of ASGV. It is not known how this strain of ASGV is transmitted, other than by grafting, nor is it known what effect the virus has on the growth of infected vines. The Actinidia -infecting strain of ASGV does not occur in New Zealand, and infected plants will not be released from quarantine. The detection methods used during the research will assist quarantine and the safe movement of breeding material.