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.     

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.     

The isolation and identification of parsley viruses occurring in Britain*

Severe stunting of parsley plants, with leaf chlorosis and reddening was reported from four localities in Britain in 1968-70. Affected plants were collected from thirteen sites in Bedfordshire, Buckinghamshire, Cheshire and Bristol, and five viruses (designated PV1-PV5) were isolated from them. The viruses were distinguished by electron microscopy, host range and type of aphid transmission. From diagnostic reactions in a range of host species and its transmission by Cavariella aegopodii Scop., the most frequently isolated virus (PV4) and the principal cause of the parsley disease was identified as carrot mottle virus (CMotV). The other four viruses were infrequently isolated. PV1, PV2 and PV3 were transmitted in the non-persistent manner by Myzus persicae Sulz. Each was purified and identified serologically as western celery mosaic virus, cucumber mosaic virus and broad bean wilt respectively. PV5 was not fully identified, but was transmitted by C. aegopodii in the presence of CMotV and had particles ca. 500 nm in length. Each of these viruses was re-transmitted to parsley, but induced slight symptoms or none.     

Transmission characteristics and some other properties of bean yellow vein-banding virus, and its association with pea enation

Bean yellow vein-banding virus (BYVBV) has been found occasionally in mixed infection with pea enation mosaic virus (PEMV) in spring-sown field beans (Vicia faba minor) in southern England. Glasshouse tests confirmed that, like PEMV, BYVBV is transmissible by manual inoculation and by aphids in the persistent manner. However, BYVBV can be transmitted by aphids only from plants that are also infected with a helper virus, usually PEMV. Thus after separation from PEMV by passage through Phaseolus vulgaris it was no longer aphid-transmissible. It became aphid-transmissible again only after re-mixing in plants with PEMV or with a substitute helper, bean leaf roll virus (BLRV). It was not transmitted by aphids that fed sequentially on plants singly infected with PEMV and BYVBV. Thus the interaction between BYVBV and PEMV (or BLRV) that enables BYVBV to be transmitted by aphids seems to occur only in doubly infected plants. However, it was not transmitted by aphids from plants doubly infected with BYVBV and broad bean wilt virus (BBWV). BYVBV and PEMV were transmitted more readily by Acyrthosiphon pisum than by Myzus persicae; neither virus was transmitted by Aphis fabae. Phenol extracts of BYVBV-infected leaves were more infective than phosphate buffer or bentonite-clarified extracts and were sometimes infective when diluted to 1/1000. The infectivity of BYVBV in phosphate buffer extracts of leaves singly infected with BYVBV, unlike that in extracts of leaves doubly infected with BYVBV and PEMV (or BLRV), was destroyed by treatment with organic solvents. BYVBV infected 11 of 28 plant species that were inoculated with phenol extracts; seven of the infected species were legumes. No transmission of BYVBV was detected through seed harvested from infected field bean plants. Isometric particles c. 30 nm in diameter were seen in extracts of plants doubly infected with BYVBV and PEMV but not in extracts of plants infected with BYVBV alone. Leaves of plants infected with BYVBV, alone or with PEMV, contained membrane-bound structures c. 50–90 nm in diameter associated with the tonoplast in cell vacuoles. These structures were not found in healthy leaves. BYVBV has several properties in common with other known aphid-borne viruses that are helper-dependent and transmitted in a persistent manner. Possibly, as suggested for some of them, aphid transmission of BYVBV depends on the coating of its nucleic acid with helper virus coat protein.     

Purification of carrot red leaf virus and evidence from four serological tests for its relationship to luteoviruses

Carrot red leaf virus (CRLV) was purified from infected chervil by centrifuging whole plant extracts at low speed and incubating the resuspended pellets with Driselase; the digest was then treated with 1% (v/v) Triton X-100 and the virus concentrated by centrifugation twice at high speed through a layer of 20% sucrose. The preparations (about 1 μg virus/g tissue) contained isometric particles c. 25 nm in diameter which formed a single u.v.-absorbing component in sucrose density gradients. Chervil seedlings exposed to aphids (Cavariella aegopodii) that had been injected with or had fed on fractions from the u.v.-absorbing zone developed typical symptoms of infection with CRLV. CRLV particles had a sedimentation coefficient (s_20,w) of 104 S, buoyant density in CsCl of 1.403 g/cm³ and A₂₆₀/A₂₈₀ of 1.62. Antiserum with a gel-diffusion titre of 1/512 was obtained from a rabbit injected intradermally with 100 μg purified virus. CRLV was detected by immunosorbent electron microscopy and enzyme-linked immunosorbent assay in extracts of the petioles and leaf midribs of infected chervil and in groups of five to 20 viruliferous C. aegopodii. Analysis of antiserum/virus reactions by density gradient centrifugation showed that CRLV is distantly related to all luteoviruses tested; its relationships were closest to barley yellow dwarf virus (RPV strain), and perhaps also to beet western yellows virus, more distant to tobacco necrotic dwarf, potato leafroll and bean leafroll viruses, and very distant to barley yellow dwarf (MAV strain) and soybean dwarf viruses. Some of these relationships were detected by double diffusion in agarose gels and by electron microscopy of antiserum/virus mixtures. Immunosorbent electron microscopy detected all these relationships but suggested that CRLV was more closely related to tobacco necrotic dwarf and potato leafroll viruses than to barley yellow dwarf virus (RPV strain). The results show that CRLV should be considered a definitive member of the luteovirus group, and provide confirmation of recent evidence that potato leafroll virus is a luteovirus.     

Purification, properties and transmission of parsnip yellow fleck, a semi-persistent, aphid-borne virus

Parsnip yellow fleck virus (PYFV) is the commonest cause of virus-like symptoms in parsnip plants in Britain: it is sap-transmissible but systemically infects few species outside the Umbelliferae. It has isometric particles 29–31 mμ in diameter, a sedimentation coefficient of 167s, and loses infectivity in sap after dilution to 10^-3-10^-4, heating for 10 min at 57·5–65°C, or storage at room temperature for 4–7 days. Two isolates, from parsnip and Anthriscus sylvestris respectively, are only distantly serologically related. The aphid Cavariella aegopodii transmits PYFV in a semi-persistent manner from A. sylvestris but not from parsnip. Transmission by aphids apparently depends on the presence in A. sylvestris or other source plants of a second virus, anthriscus yellows (AYV), which is persistent in the vector and not manually transmissible. PYFV was therefore not transmitted by aphids from manually inoculated plants or from parsnip or other plants immune to AYV. In controlled experiments, C. aegopodii transmitted PYFV (both A. sylvestris and parsnip isolates) from chervil plants inoculated separately with PYFV and AYV, but not from plants inoculated only with PYFV.     

Effect of mixed viral infections (potato virus Y-potato leafroll virus) on biology and preference of vectors Myzus persicae and Macrosiphum euphorbiae (Hemiptera: Aphididae)

Mixed viral infections of heterologous viruses such as Potato virus Y (family Potyviridae, genus Potyvirus, PVY) and Potato leafroll virus (family Luteoviridae, genus Polerovirus, PLRV) are a regular occurrence in Idaho's potato, Solanum tuberosum (L.), cropping systems. An increased number of plant samples from Idaho's potato fields over the past 2 yr has serologically tested positive for both PVY and PLRV via double antibody sandwich enzyme-linked immunosorbent assay (DAS-ELISA) and exhibited more severe symptoms than singly-infected plants (PVY or PLRV). Several studies have extensively examined the mixed infection phenomenon but to the best of our knowledge, none have examined the effect of such infections on vector biology and preference. Laboratory studies were conducted to examine the effect of mixed viral (PVY-PLRV) infection on the fecundity and preference of two of the most efficient PVY and PLRV vectors, the green peach aphid, Myzus persicae (Sulzer), and the potato aphid, Macrosiphum euphorbiae (Thomas) (Hemiptera: Aphididae). M. persicae and M. euphorbiae adults were clip-caged (one adult per cage) to leaflets of PVY, PLRV, PVY-PLRV-infected, and noninfected potato plants. The number of nymphs produced in all four treatments was recorded after 96 h. M. persicae and M. euphorbiae fecundity was significantly higher on mixed infected plants than on singly infected plants or noninfected plants. Preference of alatae and apterae of M. persicae and M. euphorbiae was determined with the use of settling bioassays. Both alatae and apterae of M. persicae and M. euphorbiae preferentially settled on PVY-PLRV-infected plants than on singly infected plants (PVY or PLRV) or noninfected plants.     

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.     

Specificity of the effect of sap-transmissible viruses in increasing the accumulation of luteoviruses in co-infected plants

The concentration of potato leafroll luteovirus (PLRV) (c. 1300 ng/g leaf) in singly infected Nicotiana clevelandii plants was increased up to 10-fold in plants co-infected with each of several potyviruses, or with narcissus mosaic potexvirus, carrot mottle virus or each of three tobravirus isolates. With the tobraviruses, PLRV concentration was increased equally by co-infection with either NM-type isolates (coat protein-free cultures containing RNA-1) or M-type isolates (particle-producing cultures containing RNA-1 and RNA-2). In contrast, the accumulation of PLRV was not substantially affected by co-infection with either of two nepoviruses, cucumber mosaic cucumovirus, broad bean mottle bromovirus, alfalfa mosaic virus, pea enation mosaic virus or parsnip yellow fleck virus. The specificity of these interactions between PLRV and sap-transmissible viruses was retained in tests made in Nicotiana benthamiana and when beet western yellows luteovirus was used instead of PLRV.     

Quantitative studies on the uptake and retention of potato leafroll virus by aphids in laboratory and field conditions

Enzyme-linked immunosorbent assay (ELISA) was adapted for the efficient detection and assay of potato leafroll virus (PLRV) in aphids. Best results were obtained when aphids were extracted in 0.05 M phosphate buffer, pH 7.0, and the extracts incubated at 37 °C for 1 h before starting the assay. Using batches of 20 green peach aphids (Myzus persicae), about 0.01 ng PLRV/aphid could be detected. The virus could also be detected in single aphids allowed a 1-day acquisition access period on infected potato leaves. The PLRV content of aphids depended on the age of potato source-plants and the position of source leaves on them. It increased with increase in acquisition access period up to 7 days but differed considerably between individual aphids. A maximum of 7 ng PLRV/aphid was recorded but aphids more usually accumulated about 0.2 ng PLRV per day. When aphids were allowed acquisition access periods of 1–3 days, and then caged singly on Physalis floridana seedlings for 3 days, the PLRV content of each aphid, measured subsequently, was not strongly correlated with the infection of P. floridana. The concentration of PLRV in leaf extracts differed only slightly when potato plants were kept at 15, 20, 25 or 30 °C for 1 or 2 wk, but the virus content of aphids kept on leaves at the different temperatures decreased with increase of temperature. PLRV was transmitted readily to P. floridana at all temperatures, but by a slightly smaller proportion of aphids, and after a longer latent period, at 15 °C than at 30 °C. The PLRV content of M. persicae fed on infected potato leaves decreased with increasing time after transfer to turnip (immune to PLRV). The decrease occurred in two phases, the first rapid and the second very slow. In the first phase the decrease was faster, briefer and greater at 25 and 30 °C than at 15 and 20 °C. No evidence was obtained that PLRV multiplies in M. persicae. These results are compatible with a model in which much of the PLRV in aphids during the second phase is in the haemocoele, and transmission is mainly limited by the rate of passage of virus particles from haemolymph to saliva. The potato aphid, Macrosiphum euphorbiae, transmitted PLRV much less efficiently than M. persicae. Its inefficiency as a vector could not be ascribed to failure to acquire or retain PLRV, or to the degradation of virus particles in the aphid. Probably only few PLRV particles pass from the haemolymph to saliva in this species. The virus content of M. euphorbiae collected from PLRV-infected potato plants in the field increased from early June to early July, and then decreased. PLRV was detected both in spring migrants collected from the plants and in summer migrants caught in yellow water-traps. PLRV was also detected in M. persicae collected from infected plants in July and August, and in trapped summer migrants, but their PLRV content was less than that of M. euphorbiae, and in some instances was too small for unequivocal detection.     

Infection of potato plants with potato leafroll virus changes attraction and feeding behaviour of Myzus persicae

Potato leafroll virus (PLRV; genus Polerovirus, family Luteoviridae) is a persistently transmitted circulative virus that depends on aphids for spreading. The primary vector of PLRV is the aphid Myzus persicae (Sulzer) (Homoptera: Aphididae). Solanum tuberosum L. potato cv. Kardal (Solanaceae) has a certain degree of resistance to M. persicae: young leaves seem to be resistant, whereas senescent leaves are susceptible. In this study, we investigated whether PLRV‐infection of potato plants affected aphid behaviour. We found that M. persicae's ability to differentiate headspace volatiles emitted from PLRV‐infected and non‐infected potato plants depends on the age of the leaf. In young apical leaves, no difference in aphid attraction was found between PLRV‐infected and non‐infected leaves. In fact, hardly any aphids were attracted. On the contrary, in mature leaves, headspace volatiles from virus infected leaves attracted the aphids. We also studied the effect of PLRV‐infection on probing and feeding behaviour (plant penetration) of M. persicae using the electrical penetration graph technique (DC system). Several differences were observed between plant penetration in PLRV‐infected and non‐infected plants, but only after infected plants showed visual symptoms of PLRV infection. The effects of PLRV‐infection in plants on the behaviour of M. persicae, the vector of the virus, and the implications of these effects on the transmission of the virus are thoroughly discussed.     

The role of the helper virus, anthriscus yellows, in the transmission of parsnip yellow fleck virus by the aphid Cavariella aegopodii

Transmission of parsnip yellow fleck virus (PYFV) by the aphid Cavariella aegopodii occurs only when the aphids are also carrying the helper virus, anthriscus yellows (AYV). None of five other viruses tested was able to act as helper. In experiments in which aphids were allowed to feed through membranes on crude or treated extracts from infected plants, aphids already carrying AYV acquired PYFV, but virus-free aphids failed to acquire either AYV or PYFV. PYFV was not transmitted by insects injected with haemolymph from aphids carrying both viruses, or with purified preparations of PYFV. PYFV was transmitted when AYV-carrying aphids, except those whose stylets had been removed, were contaminated externally with PYFV preparations. Ultraviolet irradiation of infected leaves did not prevent aphids from acquiring AYV, presumably because it is confined to deeply-lying tissues. AYV-carrying aphids could acquire PYFV from u.v.-irradiated leaves after acquisition access times of 2 h but not after feeds of only 2 or 15 min (which are adequate on unirradiated leaves), suggesting that PYFV is present in all parts of the leaf. No ‘helper agent’ distinct from AYV itself was detected in these experiments or in experiments on minimum acquisition feeding time or maximum period of persistence in the aphid. U.v.-inactivated PYFV competed with infective PYFV for retention sites in AYV-carrying aphids, whereas AYV apparently did not. It is suggested that there is no helper agent for PYFV, other than AYV particles. The possibility that there is one for AYV is not excluded.     

Tobacco yellow vein,a virus dependent on assistor viruses for its transmission by aphids*

Tobacco yellow vein, a disease found in Malawi, is caused by a combination of two viruses transmitted in the persistent manner by aphids. One component, tobacco yellow vein virus (TYVV) is manually transmissible, but aphids transmit it only from plants also containing the other (assistor) component, which is not manually transmissible. Aphids also transmit TYVV from plants containing either of two other assistor viruses - tobacco vein-distorting and groundnut rosette assistor. A virulent isolate of TYVV infected Soja max, Arachis hypogaea and several solanaceous species. It infected plants already containing tobacco mottle or groundnut rosette viruses but not those containing a mild isolate of TYVV.     

Viruses associated with chlorotic rosette and green rosette diseases of groundnut in Nigeria*

Groundnut (Arachis hypogaea) plants from Nigeria with chlorotic rosette disease contained a manually transmissible virus, considered to be a strain of groundnut rosette virus (GRV(C)). GRV(C) infected nine out of 32 species in three out of nine families. It caused local lesions without systemic infection in Chenopodium amaranticolor, C. murale and C. quinoa, and systemic symptoms in Glycine max, Nicotiana benthamiana, N. clevelandii and Phaseolus vulgaris as well as in groundnut. Some ‘rosette-resistant’ groundnut lines were also infected. GRV(C) was transmitted by Aphis craccivora, but only from groundnut plants that were also infected with an aphid-transmissible second virus, which was not manually transmissible and was considered to be groundnut rosette assistor virus (GRAV). Plants infected with GRAV contained isometric particles c. 25 nm in diameter which were detectable by immunosorbent electron microscopy on grids coated with antisera to several luteoviruses, especially with antisera to bean leaf roll, potato leafroll and beet western yellows viruses. No virus-like particles were observed in extracts from plants infected with GRV(C) alone. A single groundnut plant obtained from Nigeria with symptoms of green rosette contained luteovirus particles, presumed to be of GRAV, and yielded a manually transmissible virus that induced symptoms similar to those of GRV(C) in C. amaranticolor but gave only mild or symptomless infection of N. benthamiana and N. clevelandii. It was considered to be a strain of GRV and designated GRV(G).     

Restricted distribution of potato leafroll virus antigen in resistant potato genotypes and its effect on transmission of the virus by aphids

Enzyme-linked immunosorbent assay was used to measure the concentration of potato leafroll virus (PLRV) antigen in different parts of field-grown secondarily infected plants of three potato genotypes known to differ in resistance to infection. The antigen concentration in leaves of cv. Maris Piper (susceptible) was 10–30 times greater than that in cv. Pentland Crown or G 7445(1), a breeder's line (both resistant). Differences between genotypes in antigen concentration were smaller in petioles and tubers (5–10-fold) and in above-ground stems (about 4-fold), and were least in below-ground stems, stolons and roots (about 2-fold). PLRV antigen, detected by fluorescent antibody staining of tissue sections, was confined to phloem companion cells. In Pentland Crown, the decrease in PLRV antigen concentration in leaf mid-veins and petioles, relative to that in Maris Piper, was proportional to the decrease in number of PLRV-containing companion cells; this decrease was greater in the external phloem than in the internal phloem. The spread of PLRV infection within the phloem system seems to be impaired in the resistant genotypes. Green peach aphids (Myzuspersicae) acquired < 2800 pg PLRV/aphid when fed for 4 days on infected field-grown Maris Piper plants and < 58% of such aphids transmitted the virus to Physalis floridana test plants. In contrast, aphids fed on infected Pentland Crown plants acquired <120 pg PLRV/aphid and <3% transmitted the virus to P. floridana. The ease with which M. persicae acquired and transmitted PLRV from field-grown Maris Piper plants decreased greatly after the end of June without a proportionate drop in PLRV concentration. Spread of PLRV in potato crops should be substantially decreased by growing cultivars in which the virus multiplies to only a limited extent.     

Virus-like particles in phloem tissue of chervil (Anthriscus cerefolium) infected with carrot red leaf virus

Isometric virus-like particles c. 22–25 nm in diameter were found in ultrathin sections of chervil leaves infected with carrot red leaf virus (CRLV). The particles were confined to the phloem and occurred in less than 5% of the cells in the vascular bundles. They were commonest in companion cells, occurred frequently in sieve elements and were also found in phloem parenchyma cells. The observations support other evidence that CRLV should be classified in the luteovirus group.     

Properties and partial purification of infective material from plants containing groundnut rosette virus*

Groundnut plants with chlorotic rosette disease contain a manually transmissible virus, groundnut rosette (GRV), which is also transmitted in the persistent (circulative) manner by aphids (Aphis craccivora), but only from plants that are co-infected with a manually non-transmissible luteovirus, groundnut rosette assistor virus (GRAV). Strains of GRV from plants with chlorotic or green forms of rosette are called GRV(C) and GRV(G) respectively. An isolate of GRV(C) from Nigeria remained infective in Nicotiana clevelandii leaf extracts for 1 day at room temperature and for 15 days at 4d?C, but lost infectivity after 1 day at -20d?C or after dilution to 10^--⁴. Its infectivity and longevity in vitro were not altered by addition of 1 mg/litre bentonite to the extraction buffer. Infectivity in leaf extracts was abolished by treatment with 50% (v/v) ether, 10% (v/v) chloroform or 8% (v/v) n-butanol, but not by treatment for 30 min with RNase A at up to 100 ng/ml. In attempts to purify GRV(C), nearly all the infectivity from N. clevelandii extracts was found in the pellets from centrifugation at 65 000 g for 1. 5 h; infectivity also occurred in a cell membrane fraction that collected at the top of a 30% sucrose ‘cushion’ containing 4% polyethylene glycol and 0.2 M NaCI. However, no virus-like particles were found in either type of preparation by electron microscopy. Nucleic acid preparations made directly from GRV(C)-infected N. clevelandii leaves were very infective; this infectivity was totally inactivated by treatment for 30 min with RNase A at 10 ng/ml in buffers of both low and high ionic strength and was therefore attributed to ssRNA. When nucleic acid preparations were electrophoresed in gels no virus-specific bands were visible but the position of the infectivity indicated that the infective ssRNA has an apparent mol. wt of c. 1.55 × 10⁶. A similar mol. wt was indicated by the rate of sedimentation of the infective ssRNA in sucrose gradients. Preparations of dsRNA made from GRV(C)-infected N. clevelandii leaves contained a species of mol. wt c. 3.0 × 10⁶; in addition some dsRNA preparations contained an abundant component of mol. wt c. 0.6 × 106 together with several other components of intermediate mol. wt. Similar patterns of bands were observed in dsRNA preparations made from Nigerian-grown groundnut material infected with GRV(C) alone, or with GRV(C) + GRAV, or with GRV(G) + GRAV. The properties of GRV closely resemble those of two other viruses that depend on luteoviruses for transmission by aphids, carrot mottle virus and lettuce speckles mottle virus.     

The prevalence and spatial distribution of viruses in natural populations of Brassica oleracea

We report a survey of four viruses (beet western yellows luteovirus (BWYV), cauliflower mosaic caulimovirus (CaMV), turnip mosaic potyvirus (TuMV), turnip yellow mosaic tymovirus (TYMV)) in five natural populations of Brassica oleracea in Dorset (UK). All four viruses were common; 43% of plants were infected with BWYV, 60% with CaMV, 43% with TuMV and 18% with TYMV. For each virus there were significant differences in the proportion of infected plants among populations, which were not completely explained by differences in the age of plants. Multiple virus infections were prevalent, with 54% of plants having two or more virus types. There were statistically significant associations between pairs of viruses. The CaMV was positively associated with the other three viruses, and BWYV was also positively associated with TuMV. There was no detectable association between BWYV and TYMV, whereas TuMV and TYMV were negatively associated. We suggest these associations result from BWYV, CaMV and TuMV having aphid vectors in common, as aphids are attracted to plants that already have a virus infection. Infected plants were distributed randomly or were very weakly aggregated within populations. The implications of widespread multiple virus infections in natural plant populations are discussed with respect to the release of transgenic plants expressing virus-derived genes.     

Patterns of spread of Carrot virus Y in carrot plantings and validation of control measures

Patterns of spread of Carrot virus Y (CarVY) were examined in carrot plantings in Western Australia into which naturally occurring aphid vectors spread the virus from external infection sources. Within three field trials, CarVY ‘infector’ plants were introduced between or at different distances from carrot plantings. There was a marked decline in CarVY incidence over distance from adjacent introduced infection sources. Clusters of infected plants that enlarged and coalesced were concentrated next to such sources but, later, isolated, expanding clusters formed further away. With a small external virus source, initial spread into the edge of a planting was less extensive than with a larger source. When 15‐m‐wide fallow areas separated a CarVY source from carrot plots, spread was much slower than when the separation was only 1 m; it was also slower upwind than downwind of this source. The data collected help validate the inclusion of isolation and ‘safe’ planting distances, intervening fallow, planting upwind, prompt removal of virus sources, avoidance of side‐by‐side plantings and manipulation of planting date within an integrated disease management strategy for CarVY in carrots.     

Accumulation of potato virus Y is enhanced in Solatium brevidens also infected with tobacco mosaic virus or potato spindle tuber viroid

The accumulation of potato virus Y?(PVY?) and potato leaf roll virus (PLRV) was studied in plants of Solanum brevidens co-infected with each of six viruses or a viroid. Virus could not be detected by ELISA in plants of S. brevidens infected solely with PVY. However, accumulation of PVY was increased c. 1000-fold in plants doubly infected with tobacco mosaic virus or potato spindle tuber viroid (PSTVd). PVY titres in doubly infected plants of S. brevidens were between 1% and 0.1% of those found in the PVY-susceptible interspecific Solanum hybrid DTO-33. Double infections of 5. brevidens by PVY and alfalfa mosaic virus or potato viruses M, S, T or X did not significantly enhance PVY accumulation. Accumulation of PLRV was not enhanced in plants co-infected with any of the six viruses or PSTVd.