Nematode transmissibility of pseudo-recombinant isolates of tomato black ring virus

Pseudo-recombinant isolates of tomato black ring virus (TBRV), containing RNA-i of the potato bouquet serotype and RNA-2 of the beet ring-spot serotype, were transmitted by the nematode Longidorus elongatus, which also transmits the beet ringspot serotype but not the potato bouquet serotype. Transmissibility by L. elongatus was correlated with antigenic specificity of the virus particles, providing further evidence that nematode transmissibility depends on the structure of the virus coat protein. The distribution of genetic determinants for biological properties between the RNA-1 and RNA-2 of TBRV resembles that for raspberry ringspot virus.     

Seed-transmission of nematode-borne viruses

Transmission through seed of crop and weed plants seems to be characteristic of nematode-borne viruses. It occurred with tomato black ring virus (TBRV) in nineteen species (thirteen botanical families), with arabis mosaic virus (AMV) in thirteen species (eleven families), with raspberry ringspot virus (RRV) in six species (five families), and also, in more limited tests, with tomato ringspot, cherry leaf roll and tobacco rattle viruses. A remarkable feature was that infected seedlings, except those containing tobacco rattle virus, often appeared healthy. The occurrence and extent of seed-transmission depended on both the virus and the host plant. In many progenies more than 10%, and in some 100%, of seedlings were infected. The viruses were transmitted through at least two or three generations of seed of those host species tested. After 6 years' storage, TBRV- and RRV-containing seed of Capsella bursa-pastoris and Stellaria media germinated to give infected seedlings. In controlled crossing experiments with strawberry and raspberry, virus was transmitted to seed from both male and female parents but, at least in raspberry, the presence of competing virus-free pollen much decreased the ability of pollen from infected plants to set seed. There was no evidence that healthy mother plants became infected when their flowers were pollinated with infected pollen.     

Seed-transmission in the ecology of nematode-borne viruses

Virus-free populations of vector nematodes can acquire tomato black ring (TBRV), raspberry ringspot (RRV) and arabis mosaic (AMV) viruses from weed seedlings grown from virus-carrying seed. When soils from fields where nematode-borne viruses occurred naturally were air-dried to kill vector nematodes and then moistened, TBRV and RRV occurred commonly in the weed seedlings that grew, but AMV occurred only rarely. Similar tests did not detect tobacco ringspot, grapevine fanleaf or tobacco rattle viruses in weed seeds in the single soil studied in each instance, although these three viruses are also seed-borne in some of their hosts. Many weed species, when infected experimentally, readily transmit TBRV and RRV to their seed, but the viruses were much commoner in naturally occurring seed of some of these species than of others. These discrepancies between the frequency of seed-transmission of viruses from experimentally infected plants and the extent of natural occurrence of infected seed seem largely to reflect the host preferences of the vectors. Infective Longidorus elongatus kept in fallow soil retained TBRV and RRV only up to 9 weeks. When weed seeds in the soil were then allowed to germinate, the nematodes reacquired virus from the infected seedlings. Some weed species were better than others as sources of virus. Persistence of these viruses in fields through periods of fallow or fasting of the vector therefore depends on a continuing supply of infected seedlings produced by virus-containing weed seeds. This is probably less true of viruses like AMV and grapevine fanleaf, which persist for 8 months or more in their vectors (Xiphinema spp.). A few seeds containing TBRV and RRV were found in soils free of vector nematodes, suggesting that the viruses are disseminated in weed seed. This probably explains how TBRV and RRV have reached a large proportion of L. elongatus populations in eastern Scotland.     

A comparison of the effectiveness of the four British virus vector species of Longidorus and Xiphinema

The frequency with which the four virus-vector species of longidoroid nematodes occurring in Britain transmitted their associated plant viruses were compared in a series of experiments using a standard procedure. In these tests Xiphinema diversicaudatum proved an effective vector of British isolates of arabis mosaic virus and strawberry latent ringspot virus and Longidorus attenuatus of an isolate of tomato black ring virus from England. In comparison, isolates of raspberry ringspot virus and tomato blackring virus from Scotland and of raspberry ringspot virus from England were transmitted much less readily by their respective vectors, L. elongatus and L. macrosoma. These differences in ability to transmit virus were not related to differences in feeding access on the virus source- or bait-plants, in the extent to which virus was retained within the nematode feeding apparatus or in the frequency with which virus was recovered from Longidorus in concurrent slash tests. Three Scottish isolates of raspberry ringspot and tomato black ring viruses were transmitted equally infrequently by two populations of L. elongatus and the frequency with which virus was transmitted was not greatly increased when the species of source or bait plants was changed.     

Infectibility and sensitivity of UK raspberry, blackberry and hybrid berry cultivars to Rubus viruses

Six blackberry or hybrid berry cultivars and 19 raspberry cultivars were assessed for their infectibility with, and sensitivity to, graft inoculation with 10 distinct viruses found infecting Rubus in the UK. Cultivars were grafted with each of, two isolates of the pollen borne raspberry bushy dwarf virus (RBDV), five aphid borne viruses: black raspberry necrosis, raspberry leaf mottle (RLMV), raspberry leaf spot (RLSV), rubus yellow net and raspberry vein chlorosis (RVCV); and isolates of the nematode transmitted nepoviruses, arabis mosaic, raspberry ringspot, strawberry latent ringspot and tomato black ring. All tested cultivars were infectible with a resistance breaking isolate of RBDV but only about half of that number with the Scottish type isolate of the virus. The raspberry cvs Autumn Bliss, and occasionally Glen Garry and Glen Prosen, developed leaf yellowing symptoms following infection with RBDV, but none of the other infected cultivars showed obvious leaf symptoms when kept in a heated glasshouse during the growing season. All tested cultivars were infectible with each of the four viruses transmitted in nature by the aphid, Amphorophora idaei. Most were infected symptomlessly, but seven cultivars developed severe leaf spotting symptoms due to infection with RLMV or RLSV. All but one of the raspberry cultivars were infectible with RVCV, which is transmitted in nature by the aphid Aphis idaei, and almost all infected plants developed leaf symptoms; only one of the hybrid berry or blackberry cultivars tested was infected with RVCV. In tests with the four nepoviruses, all tested cultivars, except Tummelberry, were infectible with at least one or more of these viruses. However, cultivars responded differently to challenge inoculation with different isolates of individual nepoviruses. Several cultivars developed chlorotic leaf mottling following infection with some nepovirus isolates. The implications of these results for virus control are discussed in the light of the changing pattern of virus and virus vector incidence in the UK.     

Purification and properties of lucerne Australian symptomless virus, a new virus infecting lucerne in Australia

A virus with isometric particles c. 26–28 nm in diameter isolated from naturally infected lucerne (Medicago sativa) in Australia and reported there to be a strain of lucerne Australian latent virus (LALV), is shown to be a distinct virus. The virus, called lucerne Australian symptomless (LASV), was mechanically transmitted to 10 of 22 plant species inoculated, but only induced symptoms in three Chenopodium species and Gomphrena globosa. Virus particles occurred in relatively low concentrations in plant sap, and the virus could not be reliably maintained in culture by serial transmission to plants during winter (October-April). During the summer, sap of infected C. quinoa remained infective after diluting 10^-2 but not 10^-3, after heating for 10 min at 50 but not 55 ^oC and after storage for 24 days (the longest period tested) at 20, 4 and -15 ^oC. LASV was seed-borne to 6% of C. quinoa seedlings. Partially purified preparations of virus particles contained one nucleoprotein component with a sedimentation coefficient of c. BOS. Particles contained two polypeptide species of estimated mol. wts 26 000 and 40 000, and two ssRNA species which, when denatured in glyoxal, had apparent mol. wts of 2–5 times 10⁶ and 1–4 times 10⁶. The infectivity of virus RNA was abolished by incubation with proteinase K. Purified particles of LASV reacted with homologous antiserum (gel diffusion titre 1:256) but not with antiserum to LALV or to 13 other plant viruses with isometric particles including arracacha B (AVB), broad bean wilt, rubus Chinese seed-borne (RCSV) and strawberry latent ringspot (SLRV) viruses, and five comoviruses. These properties distinguish LASV from LALV and from all recognised nepoviruses and comoviruses. Its closest affinities are with SLRV, RCSV and possibly AVB; these viruses may comprise a distinct virus group or nepovirus subgroup.     

Zur Beurteilung der Feldresistenz von Wintergerste (Hordeum vulgare L.) gegenüber Zwergrost (Puccinia hordei Otth)

Eine anorganische Phosphatase aus Escherichia coli wurde als neuer Enzym‐Marker zum ELISA‐Test bei vier Nepoviren‐raspberry ringspot virus, strawberry latent ringspot virus, tomato black ring virus und arabis mosaic virus‐benutzt. Ein Vergleich des ELISA bei Nutzung von Peroxydase aus Meerrettich und von anorganischer Pyrophosphatase als Marker ergab, daß die Nutzung von Pyrophosphatase einen empfindlicheren Nachweis der Viren in gereinigten Präparaten, in Extrakten aus krautigen Testpflanzen sowie in Rohsäften von Himbeerpflanzen erlaubte.     

The location of raspberry ringspot and tomato black ring viruses in the nematode vector, Longidorus elongatus (de Man)

Electron microscopy of thin sections of Longidorus elongatus (de Man) fed on plants infected with raspberry ringspot and tomato black ring viruses showed virus-like particles in the lumen of the buccal capsule and in the space between the stylet and the guiding sheath. In sections of L. elongatus fed on plants infected with arabis mosaic virus, which it does not normally transmit, a few virus particles were seen in the buccal capsule but none was associated with the stylet guiding sheath. It is suggested that the association of virus particles with the cuticular guiding sheath is an explanation of the specificity of virus transmission evident in L. elongatus.     

Arabis mosaic and Prunus necrotic ringspot viruses in hop (Humulus lupulus L.)

Purified virus preparations made from nettlehead-diseased hop plants, or from Chenopodium quinoa, to which the virus was transmitted by inoculation of sap, contained polyhedral virus particles of 30 mμ diameter which were identified serologically as arabis mosaic virus (AMV). There were serological differences between AMV isolates from hop and from strawberry, and also differences in host range and in symptoms caused in C. quinoa and C. amaranticolor. AMV was always associated with nettlehead disease. The nematode Xiphinema diversicaudatum occurred in small numbers in most hop gardens, but was numerous where nettlehead disease was spreading rapidly. Preparations from nettlehead-affected hops also contained a second virus, serologically related to Prunus necrotic ringspot virus (NRSV), in mild and virulent forms which infected the same range of test plants but showed some serological differences. Mild isolates did not protect C. quinoa plants against infection by virulent isolates. Hop seedlings inoculated with virulent isolates of NRSV developed symptoms indistinguishable from those of split leaf blotch disease. Latent infection with NRSV was prevalent in symptomless hop plants. Nettlehead disease is apparently associated with dual infection of AMV and virulent isolates of NRSV. An unnamed virus with rod-shaped particles 650 mμ long was common in hop and was transmitted by inoculation of sap to herbaceous plants. Cucumber mosaic virus was obtained from a single plant of Humulus scandens Merr.     

Purification and properties of elderberry latent virus, one of four sap-transmissible viruses obtained from American elder (Sambucus canadensis L.)

Cherry leaf roll, tomato black ring and two previously unrecorded viruses were transmitted to Chenopodium quinoa from Sambucus canadensis plants imported from the U.S.A. Of the two newly recognized viruses one, code-named elderberry virus A, has filamentous particles about 650 times 15 nm; the other, named elderberry latent virus (ELV), was transmitted to several herbaceous species but remained symptomless in elder and most other hosts. In C. quinoa sap ELV lost infectivity after dilution to 10^--⁵ to 10^-- ⁶ , 10 min at 85–90°C, and 7 days at 18°C. Infectivity of nucleic acid extracts was abolished by ribonuclease in 0.2 m sodium chloride. ELV was purified from C. quinoa leaf extracts that were clarified with chloroform, by precipitation at pH 5 and differential centrifugation. Purified preparations contained numerous isometric particles c. 30 nm in diameter and a few particles c. 17 nm in diameter. In 0.06 M phosphate buffer ELV sedimented as a major 112 S (calculated for infinite dilution) component and a 48 S minor component. ELV showed no serological relationship to twenty-seven other isometric plant viruses. Its present cryptogram is R/I: *I*:S/S:S/*.     

Host range and properties of potato black ringspot virus

Potato black ringspot virus (PBRV), obtained from cultivated potato in Peru, was found to have a very wide host range resembling that of tobacco ringspot virus (TRSV-B), to which PBRV is distantly related serologically. However, PBRV caused the more severe symptoms in many species and, unlike TRSV B, infected Lycopersicon esculentum and Cyamopsis tetragonoloba. In Solanum tuberosum, PBRV caused necrotic spots and ringspots in systemically infected leaves in the year of infection and was readily transmitted through tubers to progeny plants, most of which developed no obvious symptoms although systemically infected. TRSV-B infected non-inoculated S. tuberosum leaves only sporadically, did not induce symptoms in them and was not transmitted through tubers to progeny plants. PBRV was cultured in Nicotiana clevelandii and infectivity was assayed in Cheno-podium amaranticolor or C. quinoa. Virus particles were purified from leaf extracts, after clarification using chloroform, by precipitation with 6% polyethylene glycol and differential centrifugation. Purified preparations contained 25 nm diameter isometric particles with somewhat angular outlines, sedimenting as three components (T, M and B) at 49, 84 and 117 S, and containing a single protein species of mol. wt 59 000. Preparations of PBRV nucleic acid contained two species, estimated by polyacrylamide gel electrophoresis in non-denaturing conditions to have mol. wt of about 25 10⁶ (RNA-1) and 15 10⁶ (RNA-2). Infectivity was associated with B particles, preparations of which contained RNA-1 and RNA-2, presumably in different particles. M particles contained RNA-2, were not infective and enhanced infectivity only slightly when added to B particles. Similar relative amounts of RNA-1 and RNA-2 were extracted from unfractionated virus using phenol or Pronase, but preparations obtained using phenol were much the more infective. PBRV has properties typical of nepoviruses; its present cryptogram is (R/1):2–5/41 + 15/28 or 2 1 5/46:S/S:S/*, nepovirus group.     

A damaging outbreak of arabis mosaic nepovirus in blackcurrant, the occurrence of other nepoviruses in Ribes species, and the demonstration that alfalfa mosaic virus is the cause of interveinal white mosaic in blackcurrant

In a crop of blackcurrant (Ribes nigrum), cv. Baldwin in Eire, chlorotic mottling and ringspot symptoms in leaves on plants and severe crop loss was associated with infection with arabis mosaic nepovirus (ArMV) and the presence in the soil of its nematode vector, Xiphinema diversicaudatum. This is only the second report of ArMV damaging a crop of blackcurrant. Tomato black ring (TBRV) and raspberry ringspot nepoviruses were detected in single plants of redcurrant (R. rubrum) in England and flowering currant (R. sanguineum) in Scotland respectively; each of these infected plants showed foliar chlorotic line-pattern symptoms. This is the first record of TBRV in redcurrant. A single blackcurrant plant in New Zealand showing symptoms typical of those described for interveinal white mosaic disease, contained alfalfa mosaic virus (AMV). When AMV particles were purified and concentrated from herbaceous test plants and mechanically inoculated to young blackcurrant plants, several became infected with AMV and most infected plants developed systemic symptoms typical of the original disease. This provides the strongest evidence to date that AMV is the causal agent of interveinal white mosaic disease.     

Purification and properties of Australian lucerne latent virus, a seed-borne virus having affinities with nepoviruses

An isolate of Australian lucerne latent virus (ALLV) from lucerne in New Zealand was mechanically transmitted to a few herbaceous hosts. It induced diagnostic symptoms in several species of the Chenopodiaceae, but was symptomless in most other hosts including lucerne and Trifolium subterraneum. It was seed transmitted in lucerne. When assayed to Chenopodium quinoa, infective C. quinoa sap lost infectivity after diluting to 10^-4, heating for 10 min at 55°C and storage for 4 days at 4°C. ALLV was purified from infected C. quinoa or pea plants by extracting sap in 0.1 m borate buffer (pH 7) containing 0.2% 2-mercaptoethanol and clarifying with 15% bentonite suspension, high and low speed centrifugation and sucrose density gradient centrifugation. Purified virus preparations contained isometric particles about 25 nm in diameter and sedimented as three virus components with sedimentation coefficients (s_20-w⁰) of 56 S, 128 S and 133 S. The 56 S component appeared to consist of nucleic acid-free protein shells. Polyacrylamide gel electrophoresis of virus preparations showed that ALLV contained a single protein species of mol. wt 55 000 and two RNA species of mol. wt 2.1 × 10⁶ and 2.4 × 10⁶. An antiserum to ALLV had an homologous titre of 1/256 to purified virus but failed to detect ALLV in infective sap of C. quinoa, pea or lucerne. Purified ALLV failed to react to antisera to 28 distinct isometric plant viruses including those to 10 nepoviruses.     

Further Studies on a Tomato Black Ring Virus Isolate from Artichoke

An isolate of tomato black ring virus from artichoke (TBRV-A) was compared biologically, physico-chemically and serologically with three strains of the virus, i.e. TBRV-potato bouquet (TBRV-BU), TBRV-beet ringspot (TBRV-W), and TBRV-celery yellow vein (TBRV-Ce). Cytopathic effects of TBRV-A infection in C. quinoa and its relationships with two strains of artichoke Italian latent virus (AILV-S and AILV-G) were also investigated. Physical properties in vitro, sedimentation coefficients and molecular weight of protein subunits and nucleic acid species of TBRV-A were very similar to those known for TBRV. In serological tests, TBRV-A appeared more closely related to TBRV-W (SDI = 1) than to TBRV-Ce and TBRV-BU (SDI = 2–3). Finally, TBRV-A was very, distantly related to AILV-S and AILV-G (SDI = 11–12).     

The relationship of potato black ringspot virus to tobacco ringspot and allied viruses

The relationship between potato black ringspot virus (PBRV), isolates of tobacco ringspot virus from blueberry (TRSV-B), cherry (TRSV-C) and calico-diseased potato (TRSV-P), and eucharis mottle virus (EuMV) was examined in tests of three types. In gel-diffusion precipitin tests, the reaction end-points of antisera, and spur formation, indicated that PBRV and TRSV-P are very closely related but not identical antigenically, as are TRSV-B and TRSV-C, and that these two pairs are more distantly related to each other and to EuMV. In plant-protection tests in Nicotiana angustifolia, PBRV, TRSV-B and EuMV conferred protection against the homologous virus but not against one another. PBRV, but not TRSV-B, conferred protection against TRSV-P. In tests with the two RNA species of PBRV, infectivity increased greatly when preparations of RNA-1 and RNA-2 were mixed, and both species are probably needed for infection. Infectivity did not increase when RNA-1 or RNA-2 of PBRV was mixed with RNA-2 or RNA-1, respectively, of TRSV-B; the two viruses seem too distantly related to form pseudo-recombinants. It is concluded that PBRV and tobacco ringspot virus should be considered separate viruses, and that TRSV-P should be considered a strain of PBRV. EuMV should perhaps be recognised as a virus distinct from, but related to, PBRV and tobacco ringspot virus.     

Studies on the inheritance in the red raspberry of immunities from three nematode-borne viruses

Lethal or chronic diseases of the raspberry caused by the nematodeborne viruses raspberry ringspot, arabis mosaic and tomato black ring can cause serious reductions in the productivity of raspberry plantations, but the existence of clear-cut immunities from these diseases provides a basis for control through plant breeding. The inheritance of these immunities was studied by means of graft tests on families of raspberry seedlings. Immunity from each virus was found to be dominant to susceptibility, but there was evidence that more than one gene was concerned in each case: while it was not possible to decide whether the second gene was a dominant complementary or a linked recessive affecting the viability of the immune segregates, the frequent occurrence in the raspberry of aberrant segregation ratios due to such lethal genes makes the latter explanation the more probable. There was also evidence of linkage between the genes for the three immunities. The experiment confirmed the practicability of breeding to incorporate genes for immunities from these three viruses into new raspberry varieties.     

RASPBERRY YELLOW DWARF, A SOIL-BORNE VIRUS

An apparently undescribed virus, provisionally named raspberry yellow dwarf virus (RYDV), was isolated from naturally infected raspberry, strawberry, blackberry and several weed species by mechanical inoculation of sap to Chenopodium amaranticolor. The severe disease it caused in Malling Exploit raspberry usually occurred patchily in otherwise normal plantations: these patches increased in size from year to year. RYDV was differentiated from raspberry ringspot and tomato black ring viruses by the symptoms produced in C. amaranticolor , tobacco and Petunia hybrida. RYDV lost infectivity when sap was heated for 10 min. at 61° C., diluted 10^-5or kept for 15 days at 18° C. RYDV was precipitated without inactivation by acetone and by ammonium sulphate.
Isolates of RYDV from different plants and localities, and of different virulence, were identified by plant-protection and serological tests. Such tests gave no evidence that RYDV was related to raspberry ringspot, tobacco ringspot, tomato black ring or cucumber mosaic viruses.
Raspberry and sugar-beet plants became systemically infected with RYDV when grown under glass in soil from a field where the disease had occurred in raspberry plants, and where the virus persisted in the soil for 3 years after the raspberry plants were removed. RYDV seems to be widely disseminated in England but recently introduced and rare in eastern Scotland.
Like raspberry ringspot and tomato black ring viruses, RYDV causes symptoms of the ringspot type in tobacco, has a wide natural and experimental host range, is soil-borne and of local importance. Such features seem characteristic of ringspot viruses as a group.     

Purification,electron microscopy and serology of virus isolated fromEuonymus europaea

“Euonymus mosaic virus”, purified from cucumber cotyledons by the differential and density-gradient centrifugation, shows typical nucleoprotein absorption spectrum. Electron microscopy reveals isometric virus particles of about 37 nm diameter. No reaction of purified “Euonymus mosaic virus” was observed with antisera against a raspberry ringspot virus, tobacco ringspot virus, cherry leaf roll virus, strawberry latent ringspot virus, tomato ringspot virus, elm mosaic virus, arabis mosaic virus, tomato bushy stunt virus and watermelon mosaic virus.     

Hydrangea mosaic virus, a new ilarvirus from Hydrangea macrophylla (Saxifragaceae)

A previously unrecognised virus isolated from Hydrangea macrophylla with chlorotic mosaic leaf symptoms in West Sussex was named hydrangea mosaic virus (HydMV). HydMV was mechanically transmitted without difficulty to four of 16 species from three of five families, and was seed transmitted in Chenopodium quinoa, but was not aphid transmitted. Although relatively unstable in vitro, HydMV was purified by clarifying leaf extracts by emulsification with chloroform and acidification with citric acid, followed by differential centrifugation and sucrose density gradient centrifugation. Purified virus incompletely separated on sucrose density gradients into three components (T. M and B) with sedimentation coefficients (s^o_20w) of 86, 97, and 105 S respectively, but all particles had buoyant densities in caesium chloride of 1.37 g/cm³. Virus contained a single polypeptide species (mol. wt 26.4 times 10³), appeared quasiisometric to ovoid or elliptical, and measured c. 28 times 30 (T), 30 times 30 (M) or 30 times 32–38 nm (B). Single-stranded RNA species or mol. wt 1–25, 1–08, 0–83, 0–36 and 0–27 (RNA-1 to 5 respectively) were obtained from virus preparations but mixtures containing RNA-1 to 3 plus either RNA-4 + 5 or the coat protein, were infective. These properties suggest that HydMV has affinities with ilarviruses, but it showed no serological relationship to any of six ilarviruses or 42 other viruses.     

Investigations of carnation viruses

Carnation vein mottle virus (CarVMV) is rare in glasshouse carnations in Britain, although locally common in Dianthus barbatus in private gardens. In Sim carnations free from other viruses, CarVMV caused slight diffuse chlorotic mottling in the younger leaves, decreased flower yield by c. 22%, and caused flower breaking in cvs William Sim and Dusty. In non-Sim cultivars Pink Shibiuya, Orchid Beauty and Vesta, leaf symptoms and flower breaking were more pronounced. In mixed infections with carnation mottle virus, symptoms were much more severe. CarVMV was not eliminated from carnation or D. barbatus plants grown for 4 wk at 37^oC, and only rarely from cuttings then taken from them, but it was readily eliminated by meristem-tip culture. Myzus persicae adults or nymphs acquired and transmitted the virus within a total time of 4 min, and remained infective for 30–60 min if feeding, or for 75 min if starved. The carnation aphid, M. persicae f. dianthi, transmitted the virus much less efficiently. The virus was not transmitted by dodder (Cuscuta campestris), or through seed of D. barbatus or Chenopodium quinoa. The maximum infective dilution in sap of D. barbatus, carnation and C. quinoa ranged from 10^-2 to 10^-5. The virus withstood 10 min at 60 but not 65^oC, up to 9 days at c. 18^oC or 3–4 wk at c. 2^oC. CarVMV infected twenty-two of 107 plant species in six of thirty-seven families; suscepts were confined to the Chenopodiaceae, Caryophyllaceae and closely allied families. C. quinoa was the best local lesion assay host. Seedling clones of D. barbatus, selected as resistant to carnation mottle virus, proved the best indicator and propagation species. Up to 50 mg virus/kg tissue were obtained by butanol clarification followed by differential and density gradient centrifugation. The preparations contained a single sedimenting component, s_20w= 144S, and had flexuous filamentous particles, c. 790 times 12 run; the particles contained a single polypeptide, mol. wt 34800, and 5% of a single-stranded ribonucleic acid (RNA) with nucleotide base ratios of G21: A25: C25: U29. Serologically CarVMV was related distantly to turnip mosaic (cabbage black ring strain), pea mosaic, watermelon mosaic (Strain 2) and bean yellow mosaic viruses, more closely to pepper veinal mottle virus, but unrelated to twelve other potyviruses. CarVMV is not at present a danger to carnation crops in Britain, but the recent trend of sending carnation plants to overwinter outdoors in warmer countries involves potential risks of more rapid spread by effective vector races of M. persicae.