Infection of cereals by cocksfoot mottle and phleum mottle viruses

Cocksfoot mottle (CFMV) and Phleum mottle (PMV) viruses are both transmitted by the Chrystomelid beetles Ouletna melanopa and O. lichenis. Both viruses are readily inoculated into some cereals but only CFMV has ever been found causing a natural infection, in one wheat crop. The apparent absence of these viruses from cereals in the field results from inefficient transmission by the vectors and the existence of symptomless tolerance in cereals to both viruses, especially after tillering. In cereals symptom expression is related to virus concentration, and both may be influenced by light intensity and ambient temperature. O. melanopa, which is potentially the most important vector of the viruses to cereals, moves to winter-sown cereals only when the virus-tolerant growth stage has been reached, and later to spring-sown cereals when they too have commenced tillering. Whereas the spread of these viruses within grass crops is increased by cutting and grazing, cereal crops are neither cut nor commonly grazed.     

Relations of carrot red leaf and carrot mottle viruses with their aphid vector, Cavariella aegopodii

Studies with Scottish isolates of carrot red leaf (CRLV) and carrot mottle (CMotV) viruses confirmed the dependency of CMotV on CRLV for transmission by the aphid Cavariella aegopodii. CMotV was transmitted by aphids only when the two viruses were present in the same source plant, and its transmission was not assisted by anthriscus yellows virus, which acts as a helper for parsnip yellow fleck virus. Some test plants became infected with CRLV alone, and a few with CMotV alone. In winter, aphid transmission of CRLV and CMotV was greatly increased when the source plants received supplementary lighting whereas the CMotV infectivity of sap was not increased. C. aegopodii acquired CRLV and CMotV after minimum acquisition access times of 30 min and inoculated them after minimum inoculation access times of 2 min. There was a minimum latent period of 7–18 h. The viruses were retained by the aphid after moulting and are therefore circulative in the vector, but were not transmitted to progeny insects. Aphids allowed 24 h to acquire the viruses continued to transmit them for at least 12 days, but some aphids allowed 6 h or less for virus acquisition ceased to transmit after 3 or 4 days. CRLV is considered a tentative member of the luteovirus group.     

Studies on the transmission of velvet tobacco mottle virus by the mirid, Cyrtopeltis nicotianae

The minimum acquisition period of velvet tobacco mottle virus (VTMoV) by its mirid vector Cyrtopeltis nicotianae was about 1 min, with an increase in the rate of transmission (i.e. proportion of test plants infected) for acquisition periods up to 1000 min. Pre-acquisition starvation periods up to 18 h did not affect the rate of transmission. After an acquisition access period of 2 days, the minimum inoculation period was between 1 and 2 h and the rate of transmission increased with increasing inoculation time; when the acquisition access period was 1 h, or if vectors were fasted for 16 h after the 2 day acquisition, the rate of transmission was significantly lower. When mirids were transferred sequentially each day to a healthy plant after a 24 h acquisition feed, they transmitted intermittently for up to 10 days. Up to 50% of mirids transmitted after a moult and this was not due to the mirids probing the shed cuticles or exudates of infective insects. Mirids transmitted after a moult, following acquisition periods of 10, 100 or 1000 min. C. nicotianae transmitted solanum nodiflorum mottle virus (SNMV), sowbane mosaic virus (SoMV) and southern bean mosaic virus (SBMV), but not subterranean clover mottle virus (SCMoV), lucerne transient streak virus (LTSV), tobacco ringspot virus (TRSV), galinsoga mosaic virus (GMV), nor nicotiana velutina mosaic virus (NVMV). Tomato bushy stunt virus (TBSV) was transmitted to 1/58 test plants.     

Aphid-injection experiments with carrot mottle virus and its helper virus, carrot red leaf

Carrot mottle virus (CMotV) and its helper virus, carrot red leaf (CRLV), were not transmitted by aphids (Cavariella aegopodii) that had fed through membranes on, or had been injected with, sap from mixedly infected chervil plants or partially purified preparations of CMotV. However, the viruses were transmitted by recipient aphids injected with haemolymph from donor aphids that had fed on mixedly infected plants but not by a second series of recipients injected with haemolymph from the first series. Some of the first series of recipients transmitted both viruses for up to 11 days but others transmitted erratically and many lost ability to transmit after a few days. The results confirm that both viruses are circulative but provide no evidence for multiplication in the vector. Non-viruliferous aphids, or aphids that had acquired CRLV by feeding, did not transmit CMotV when they were injected with haemolymph from aphids that had fed on a source of CMotV alone, confirming that they can only transmit CMotV when they acquire it from a mixedly infected plant. When extracts from donor aphids were treated with ether before injection, recipient aphids transmitted both CRLV and CMotV, although the infectivity of CMotV grown in Nicotiana clevelandii in the absence of CRLV is destroyed by ether treatment. CMotV particles acquired by aphids from mixedly infected plants therefore differed in some way from those in singly infected plants. A plausible explanation of these results, and of the dependence of CMotV on CRLV for aphid transmission, is that doubly infected plants contain some particles that consist of CMotV nucleic acid coated with CRLV protein.     

Host-ranges of cocksfoot mottle and cynosurus mottle viruses

Cocksfoot mottle virus (CfMV) and cynosurus mottle virus (CyMV) had similar, but not identical, host-ranges within the Gramineae. Each infected fifteen out of forty-one species tested, considerably fewer than phleum mottle virus (PMV), but unlike PMV both infected Triticum aestivum (wheat), causing a lethal mottle. The species most useful for differentiating CfMV, CyMV and PMV are listed. Some species appear especially susceptible to virus infection.     

Transmission of Grapevine virus A and Grapevine leafroll‐associated viruses 1 and 3 by Planococcus ficus and Planococcus citri fed on mixed‐infected plants

The Grapevine virus A (GVA) and Grapevine leafroll‐associated viruses 1 and 3 (GLRaV‐1 and GLRaV‐3) are associated with grapevine diseases that induce severe reductions in yield and berry quality. These three viruses are known to coexist in both grapevine and insect vectors, but their cotransmission has been poorly characterised so far. This study investigates the acquisition and transmission of GLRaV‐1, GLRaV‐3 and GVA by Planococcus ficus and Planococcus citri (Hemiptera: Pseudococcidae) following feeding on multiple‐infected plants. The retention and load of the three viruses in the two insect species were analysed. After feeding onto GVA, GLRaV‐1 and GLRaV‐3 mixed‐infected grapevines, nymphs of P. ficus and P. citri showed similar virus acquisition rates and retained low quantities of viruses until the third post‐acquisition day. Despite the similar acquisition patterns, the two vectors differed in transmission efficiency: P. ficus showed a higher efficiency in transmitting GVA and GLRaV‐3, whereas P. citri transmitted GLRaV‐1 more efficiently. When focusing on the virus cotransmission, it appears that GVA could be transmitted to grapevine without GLRaV‐1 and/or GLRaV‐3 and that the transmission of both GLRaVs could take place in the absence of GVA. This comparative study involving different viruses and vector species improves the current knowledge of the semi‐persistent transmission of these three viruses and contributes to the understanding of grapevine virus epidemiology.     

Peanut mottle virus in East Africa

A very common and widespread virus pathogen of groundnut and soybean in East Africa was identified as peanut mottle virus (PnMV)* on the basis of particle morphology, serology, host range and reaction, transmission and physical properties. Virus concentration adequate for serological tests was obtained from cowpea (Vigna unguiculata) cultured at 27 °C but not at 23 °C. Purified preparations from this source gave a single, specific light-scattering zone in sucrose density gradients. PnMV was purified using 0.5 M sodium citrate buffer containing 1% mercapto-ethanol; an antiserum made against such preparations had a homologous titre of 1/8192. Groundnut and soya isolates from N., N.E., N.W. and S. districts of Uganda, N.W. Tanzania, and W. and E. (coastal) districts of Kenya were serologically similar and varied, within narrow limits, in symptoms induced in certain groundnut and soya varieties. A serologically related but distinct virus was isolated from Voandzeia subterranea. PnMV was not related serologically to any of ten viruses of the PVY group. Glasshouse experiments simulating groundkeeper conditions in the field indicated 20% seed transmission in groundnut; PnMV was transmitted by Aphis craccivora in the non-persistent manner. All twenty-one varieties and breeding lines of soybean tested were highly susceptible. The prevalence of PnMV in East Africa and the reduction in yield caused in groundnut indicates the virus to be economically important, and groundnut and soybean improvement programmes should include routine PnMV susceptibility tests.     

Further evidence on the nature of the dependence of carrot mottle virus on carrot red leaf virus for transmission by aphids

Preparations were made from chervil plants doubly infected with carrot mottle virus (CMotV) and its helper virus, carrot red leaf (CRLV), on which it depends for transmission by the aphid Cavariella aegopodii, by the procedure developed previously for CRLV. The preparations contained 25 nm isometric particles which were indistinguishable from those of CRLV but possessed aphid-transmissible infectivity of both viruses and manually transmissible infectivity of CMotV. Only one sedimenting and buoyant density component was detected. The manually transmissible CMotV infectivity was resistant to freezing and to organic solvents, treatments that destroyed the CMotV infectivity in extracts from singly infected plants. The aphid-transmissible CMotV infectivity in preparations from CRLV/ CMotV-infected plants, and that in extracts from CRLV/CMotV-carrying C. aegopodii, was abolished by treatment with CRLV antiserum but not with normal serum. These results show that transmission of CMotV by C. aegopodii is dependent on the packaging of its RNA in coats composed partially or entirely of CRLV particle protein. The aphid Myzus persicae does not transmit CRLV or CMotV from plants mixedly or singly infected with these viruses but it is a vector of beet western yellows virus (BWYV) and potato leafroll virus (PLRV) and it transmitted CMotV from plants that also contained either of these viruses. This suggests that the coat proteins of BWYV and PLRV can substitute for that of CRLV in packaging CMotV nucleic acid and thereby confer on it their own vector specificities.     

Production and properties of monoclonal antibodies to Solanum nodiflorum mottle sobemovirus

Black raspberry necrosis virus (BRNV) reaches only very low concentrations in herbaceous plants and is difficult to maintain in culture. However, in a mixed culture with an unrelated virus, Solanum nodiflorum mottle (SNMV), in the genus Sobemovirus, the concentration of BRNV particles increases about 1000‐fold. In attempts to produce monoclonal antibodies (MAbs) to BRNV for diagnostic use, purified virus particles from the mixed virus culture were used as immunogen and the resultant antibodies screened against cultures of SNMV alone, BRNV+SNMV and healthy plant extracts. None of the virus‐specific MAbs obtained in this way was specific to BRNV but six were specific to SNMV. Although the original objective was not achieved, the SNMV MAbs were characterised and used to study serological properties of SNMV and other Sobemoviruses. Characterisation of the six SNMV MAbs showed that four were IgG3, one IgG1 and the other IgG2b. SNMV was detected by all six MAbs in ELISA, by five in Western blotting, by three in agarose gel double diffusion tests, but only one was suitable for trapping virus particles in immuno‐electron microscopy (IEM). In Western blotting using virus in sap extracts of Nicotiana clevelandii, each of the five MAbs detected a single major band of Mc. 31 000 in sap containing SNMV, and additional bands of lower mass attributed to degradation of coat protein. In various serological tests, no cross‐reactions were detected between SNMV and seven other viruses from the genus Sobemovirus. However, in IEM but not in Western blotting, significant cross‐reactions were observed between SNMV and Velvet tobacco mottle virus, another species from the genus Sobemovirus. The significance of these different findings is discussed.     

Non-propagative translocation of velvet tobacco mottle virus in the mirid, Cyrtopeltis nicotianae

Nymphs of the mirid, Cyrtopeltis nicotianae became infective when injected with velvet tobacco mottle virus (VTMoV). Injections of amounts between 1 and 154 ng into the haemocoele induced 2/60 to infect test plants and these two nymphs contained 50 and 63 ng of virus respectively. Injection of amounts between 15 and 2400 ng rendered 11/47 nymphs infective. This observation is characteristic of a circulative association. However, there is no evidence that the salivary glands are involved in transmission and the virus is therefore defined as translocating, rather than circulating, in the mirid vector. Mirids which acquired infectivity by feeding lost it between 5 and 9 days after completion of acquisition, and the most rapid loss of infectivity occurred within 2 days. Nine days after acquisition none contained antigen detectable by ELISA, but detectable antigen decreased less rapidly than infectivity, and at all times more mirids contained antigen than were able to transmit. Mirids containing antigen carried between 150 and 3340 ng each. Thus, although VTMoV can be transmitted by its mirid vector following introduction of virus into the body cavity by injection, VTMoV is not propagative. Nor does the presence of virus within the mirid guarantee an ability to transmit.     

Distribution of velvet tobacco mottle virus in its mirid vector and its relationship to transmissibility

Following acquisition by feeding, velvet tobacco mottle virus (VTMoV) was detected in the gut, haemolymph and faeces of the mirid vector, Cyrtopeltis nicotianae, but not in the salivary glands. Virus antigen was detected in the gut and haemolymph for up to nine days following acquisition. Infective virus was detected in the secretions and excretions of the mirids immediately after acquisition and was also detected in the faeces of nymphs after six days. Insoluble nigrosin dye was eliminated intermittently from the gut up to six days after ingestion, in a manner similar to the loss of virus infectivity. Non-infective mirids were able to inoculate plants from infectious sap deposits on the upper epidermis. An ingestion-defecation model of insect transmission in which the salivary glands are not implicated is proposed as one explanation for the persistence of transmission in this mirid-virus association.     

Purification and properties of cassava green mottle, a previously undescribed virus from the Solomon Islands

A sap-transmissible virus obtained from cassava with a green mottle disease occurring at Choiseul, Solomon Islands, was transmitted to 30 species in 12 plant families and was readily seed-borne in Nicotiana clevelandii. In cassava plants infected by inoculation with sap, the first leaves to be infected systemically developed a mottle with some necrosis whereas leaves produced subsequently were symptomless but contained the virus. Most other species developed chlorotic or necrotic local lesions and systemic mottle or necrosis. This was followed, in several species, by production of small symptomless virus-containing leaves. The virus was cultured in N. clevelandii; Chenopodium quinoa was used for local-lesion assays. Leaf extracts from infected N. clevelandii were infective after dilution to 10–⁵ but usually not at 10–⁶, after heating for 10 min at 60°C but not at 65°C, and after storage at 20°C for at least 12 days. The virus has isometric particles of 26 nm diameter which sediment as three components, all containing a protein of mol. wt c. 53000. The two fastest sedimenting components respectively contain single-stranded RNA of mol. wt, estimated after glyoxylation, c. 2.9 × 10⁶ and 2.3 × 10⁶. Both RNA species are needed for infection of plants. In tests with antiserum prepared to purified virus particles, the virus was detected in cassava and N. clevelandii by gel-diffusion precipitin tests, by immunosorbent electron microscopy and by ELISA. Despite its similarity to nepoviruses, the virus did not react with antisera to 18 members of the group. It was named cassava green mottle virus and is considered to be a previously undescribed nepovirus.     

Leaf mottling of Spartina species caused by a newly recognised virus, spartina mottle virus

The cause of a previously undocumented leaf mottling of Spartina species was investigated. Negatively stained preparations of sap from mottled leaves revealed flexuous particles 725 × 12 nm. Pinwheels with associated laminar inclusion bodies were observed in thin sections of affected mesophyll cells. The virus was purified from infected Spartina anglica plants and had a sedimentation coefficient in 0·015 m borate of 150S. The virus was transmitted by inoculation of sap to healthy Spartina anglica, but not to a range of other graminaceous or dicotyledonous species tested. It was distantly serologically related to agropyron mosaic virus, but not to other viruses with similar morphology; the name spartina mottle virus is proposed.     

Relations of the semi-persistent viruses, parsnip yellow fleck and anthriscus yellows, with their vector, Cav arietta aegopodii

Studies were made of the relations of parsnip yellow fleck virus (PYFV) and its helper virus, anthriscus yellows (AYV), with their aphid vector, Cavariella aegopodii. Apterous insects were more efficient vectors than alates; apterous nymphs were as efficient as apterous adults. C. aegopodii never transmitted PYFV in the absence of AYV, but aphids carrying both viruses infected some test plants with one or other virus alone. C. aegopodii that fed first on a source of AYV and then on a source of PYFV transmitted both viruses to test plants, but aphids that fed on the sources in the reverse order transmitted only AYV. Test plants receiving some aphids from a source of AYV, and others from a source of PYFV, became infected only with AYV. C. aegopodii acquired AYV or the AYV/PYFV complex from plants in a minimum acquisition access time (AAT) of 10–15 mm and inoculated the viruses to test plants in a minimum inoculation access time (IAT) of 2 min. Increasing either AAT or IAT, or both, to 1 h or longer increased the frequency of transmission of each virus. Starving the insects before the acquisition feed on AYV or AYV/PFYV sources did not affect transmission. Aphids already carrying AYV acquired PYFV from plants in a minimum AAT of only 2 min; they acquired and inoculated PYFV in a minimum total time of 12 min. The data suggest that AYV is confined to deeply lying tissues whereas PYFV is distributed throughout the leaf. C. aegopodii transmitted both PYFV and AYV in a semi-persistent manner: the aphids retained both viruses for up to 4 days but lost them on moulting. Neither virus was passed to progeny of viruliferous adults. Earlier results suggesting that AYV is a persistent virus may have been caused by contamination of the AYV culture with carrot red leaf virus.     

Plant infection by two different viruses induce contrasting changes of vectors fitness and behavior

Insect-vectored plant viruses can induce changes in plant phenotypes,thus influencing plant-vector interactions in a way that may promote their dispersal according to their mode of transmission (i.e.,circulative vs.noncirculative).This indirect vector manipulation requires host-virus-vector coevolution and would thus be effective solely in very specific plant-virus-vector species associations.Some studies suggest this manipulation may depend on multiple factors relative to various intrinsic characteristics of vectors such as transmission efficiency.In anintegrative study,we tested the effects of infection of the Brassicaceae Camelina sativa with the noncirculative Cauliflower mosaic virus (CaMV)or the circulative Turnip yellows virus (TuYV)on the host-plant colonization of two aphid species differing in their virus transmission efficiency:the polyphagous Myzus persicae,efficient vector of both viruses,and the Brassicaceae specialist Brevicoryne brassicae,poor vector of TuYV and efficient vector of CaMV.Results confirmed the important role of virus mode of transmission as plant-mediated effects of CaMV on the two aphid species induced negative alterations of feeding behavior (i.e.,decreased phloem sap ingestion)and performance that were both conducive for virus fitness by promoting dispersion after a rapid acquisition.In addition,virus transmission efficiency may also play a role in vector manipulation by viruses as only the responses of the efficient vector to plant-mediated effects of TuYV,that is,enhanced feeding behavior and performances,were favorable to their acquisition and further dispersal.Altogether,this work demonstrated that vector transmission efficiency also has to be considered when studying the mechanisms underlying vector manipulation by viruses.Our results also re- inforce the idea that vector manipulation requires coevolution between plant,virus and vector.     

Comparative transmission of bean leaf roll and pea enation mosaic viruses by aphids

Acyrthosiphon pisum was a more efficient vector than Myzus persicae of bean leaf roll virus (BLRV), but the two species transmitted pea enation mosaic virus (PEMV) equally well and much more often than Megoura viciae. M. viciae did not transmit BLRV, and Aphis fabae did not transmit BLRV or PEMV. BLRV and PEMV were transmitted more often by nymphs of A. pisum than by adult apterae or alatae that fed on infected plants only as adults, but both viruses were readily transmitted by adults that had developed on infected plants. The shortest time in which nymphs acquired BLRV was 2 h, and 50 % transmitted after an acquisition period of 4 days. Some nymphs acquired PEMV in 30 min and 50% in 8 h. The shortest time for inoculation of BLRV by adults was 15 min, but some transmitted PEMV in probes lasting less than 1 min. The median latent periods of BLRV and PEMV in aphids fed for 12 h on infected plants were, respectively, 105 and 44 h. Clones of A. pisum differed in their ability to transmit BLRV and PEMV, and efficiency in transmitting the two viruses seemed to be unrelated. Some aphids that fed successively on plants infected with each virus transmitted both viruses, and infectivity with one virus did not seem to affect transmission of the other.     

Viruses occurring in East Africa that are related to peanut mottle virus

Viruses occurring in Cassia bicapsularis in Northern Tanzania, in Voandzeia subterranea in north western and eastern Tanzania, and in Phaseolus lunatus in the Kenya highlands, were all serologically related to peanut mottle virus. Their host ranges, and the symptoms they induced in test plants, were very similar, and they differed only in degree of virulence in some host species. The Voandzeia isolate did not infect groundnut, and only the Phaseolus isolate infected two species in the Cucurbitaceae. All the isolates infected Chenopodium amaranticolor, a species which formerly was reported as being immune to peanut mottle and thus considered of diagnostic value. In Africa, variation in peanut mottle virus isolates seems to be associated with host species and ecology, and there is at present no evidence for naturally occurring variants within a host species as occurs in groundnut in America. Three of the four isolates were purified by homogenising together infected leaf tissue, chloroform and 0.5 M sodium citrate buffer containing 1% 2-mercapto-ethanol at pH 8, in the proportion 1: 1:2 respectively, and precipitating the virus from the clarified homogenate with 5% w/v polyethylene glycol. When centrifuged in sucrose density gradients such preparations gave a single, bright specific light scat-tering zone with no haze.     

The elimination of four viruses from carnation and sweet william by meristem-tip culture

Carnation mottle virus (CarMV), carnation ringspot virus (CRSV), carnation vein mottle virus (CarVMV) and carnation latent virus (CarLV) were all eliminated from both carnation (Dianthus caryophyllus) and sweet william (D. barbatus) plants by meristem-tip culture. Carnation mottle virus was more readily eliminated from D. barbatus than from carnation. Carnation vein mottle and carnation latent viruses were more readily eliminated from carnation than from sweet william: they are rarely found in carnation but CarVMV is found frequently in sweet william. Carnation ringspot was eliminated equally readily from both hosts.     

Genetic control of the reactions of raspberry to black raspberry necrosis, raspberry leaf mottle and raspberry leaf spot viruses

Black raspberry necrosis virus (BRNV) induces a severe apical necrosis in black raspberry (Rubus occidentalis) but fails to induce diagnostic symptoms in red raspberry. However, BRNV infection of F₁, F₂ and F₃ hybrids from the cross black raspberry × red raspberry induced mosaic symptoms of varying intensity but no typical apical necrosis. In a survey of 28 red raspberry cultivars, a few developed severe angular chlorotic leaf spots when infected with raspberry leaf mottle virus and a few others did so when infected with raspberry leaf spot virus. These reactions were determined by single dominant genes designated Lm and Ls respectively. The value of the different host reactions for controlling the effects and spread of these viruses is discussed.     

Characterization of a Tymovirus Isolated from Anagyris foetida as a Strain of Scrophularia Mottle Virus

A virus isolated from Anagyris foetida and infecting species in the Leguminosae and Solanaceae had typical properties of a tymovirus. It sedimented as two components (49 S and 103 S) and induced the formation of double membrane-bounded invaginations in the chloroplasts of infected cells. Large numbers of ‘empty shells’ were found in the nuclei. The coat protein had a molecular weight of c. 20.000 and the RNA consisted of c. 5500 nucleotides. Crystallization of the virus in laminal crystals could be achieved by precipitation with 10 % polyethylenglycol 6000 and 1 % NaCl. Serologically, the virus was closely related to Scrophularia mottle, Ononis yellow mosaic and Plantago mottle viruses. The four viruses which all infect leguminous hosts are separated by serological differentiation indices which are mostly between 1 and 3. It is therefore suggested that they all should be considered as strains of the same virus which for reasons of priority should have the name Scrophularia mottle virus (ScrMV). The proposed Anagyris strain clearly differs from the Scrophularia mottle, Ononis yellow mosaic and Plantago mottle strains of ScrMV in host range, symptomatology, electrophoretic mobility serological properties and some cytopathogenic effects. It is not clear why the Anagyris strain infects A. foetida systemically in nature, but only locally under greenhouse conditions.