Beetle, contact and potato true seed transmission of Andean potato latent virus

A species of the flea beetle (Epitrix sp.) transmitted Andean potato latent virus (APLV) from Datura stramonium to D. stramonium, Nicandra physalodes and potato, and from N. physalodes to N. physalodes and D. stramonium. Although the acquisition and inoculation feeding times used were varied, this transmission always occurred at low efficiency. By contrast, APLV was very readily transmitted in tests in which infected plants of D. stramonium, N. physalodes, Nicotiana bigelovii and potato were brushed against healthy plants of one or more of these species. A low level of seed transmission of APLV occurred in potato.     

Use of Clq enzyme-linked immunosorbent assay to detect plant viruses and their serologically different strains

Clq was prepared from bovine serum using a simple method involving repeated dialysis at low ionic strength in the presence of chelating agents (yield c. 3 mg/100 ml serum). It was viable when stored at -18°C for up to 2 months, and at 4°C for at least 10 wk in a storage buffer containing 10% sucrose. When used in Clq ELISA this test was as sensitive as the direct double antibody sandwich form of ELISA (direct ELISA) in detecting purified potato virus Y (PVY), with a limit of detection in both methods of c. 15 ng/ml, and slightly more sensitive in detecting purified cocksfoot mild mosaic virus (CMMV), with limits of detection of c. 15 ng/ml and c. 15–60 ng/ml respectively. Using an antiserum to one strain of each virus, Clq ELISA readily detected strains of PVY, CMMV, Andean potato latent virus (APLV) and barley yellow dwarf virus (BYDV). This included detection of APLV-Hu by APLV-Caj antibodies and CMMV(G) by PMV(S) antibodies, neither of which system gives detection in direct ELISA. Clq ELISA was therefore less specific than direct ELISA in detecting serologically different virus strains. Virus detection by Clq ELISA was inhibited when sap of tobacco, Nicotiana clevelandii and Setaria italica was used at low dilution. Inhibition by N. clevelandii sap was alleviated by using increased concentrations of virus specific antibody to detect APLV and plum pox virus. Also, extracting APLV infective N. clevelandii or CMMV infective S. italica saps in a minimum of buffer, centrifuging at low speed and diluting the supernatant before testing, partially overcame the inhibition. The inhibitory substance(s) in sap may act by preventing the binding of Clq to virus-antibody aggregates. Sap of wheat, oat and barley did not appear to have an inhibitory effect and BYDV was readily detected in naturally infected field grown plants of these species.     

Tests for transmission of four potato viruses through potato true seed

The Andean potato calico strain of tobacco ringspot virus (TRSV-Ca) was detected in 2–9% of potato seedlings grown from true seed from plants of cv. Cara and clone G5998(6) infected with TRSV-Ca. Similarly, a potato isolate of the oca strain of arracacha virus B (AVB-O) was detected in 4–12% of progeny seedlings of cv. Cara and clone D42/8 infected with AVB-O. Potato virus T (PVT) passed through 33–59% of seed from PVT-infected cv. Cara, but only 0–2% infection was detected in seedlings from seed of PVT-infected clone D42/8. By contrast, no infection was detected in seedlings grown from seed from plants of G5998(6), D42/8 or cv. Cara infected with Andean potato latent virus strains Hu (APLV-Hu) or Caj (APLV-Caj), although both strains passed through seed of Nicotiana clevelandii. AVB-O, PVT and TRSV-Ca were detected in all tests of pollen from flowers of infected potato plants, but APLV-Hu and APLV-Caj were detected less frequently. AVB-O and PVT were transmitted through 2% and 8% respectively, of seed from healthy potato plants pollinated with pollen from infected plants. However, no transmission through seed was detected when pollen from TRSV-Ca infected plants was used. None of the four viruses were transmitted to healthy potato plants pollinated with pollen from infected plants. APLV-Hu caused exceptionally severe symptoms in the cv. Cara plants used for seed production, but the Bolivian strain of PVT induced only mild symptoms rather than the severe systemic necrosis previously reported for the type of strain of PVT in this cultivar. No symptoms developed in potato seedlings infected with TRSV-Ca, AVB-O or PVT through the seed.     

Pathology, soil transmission and characterization of cymbidium ringspot, a virus from cymbidium orchids and white clover (Trifolium repens)

Two strains of a virus, designated cymbidium ringspot virus (CyRSV), were isolated from cymbidium orchids and from Trifolium repens respectively in Britain. Experimentally infected cymbidiums developed slight chlorotic ring-mottle; T. repens developed flecks and mottling in the leaves, and slight stunting. Of 101 plant species tested, the cymbidium strain infected sixty-one (thirteen systemically) in twenty-three of thirty-five families; the clover strain infected sixty-four species (eighteen systemically) in twenty-two families. Both strains were propagated in Nicotiana clevelandii and assayed in Chenopodium quinoa. CyRSV was readily transmitted by inoculation of sap, and by foliage contact between plants, but not by the aphids Myzus persicae or Acyrtho-siphon pisum, nor through seed of T. incarnatum, Phaseolus vulgaris or N. clevelandii. Highly infective virus was released into soil from roots of infected N. clevelandii, and acquired by bait seedlings planted in such soil. Similar transmission occurred when purified virus was applied to the surface of sterilized soil containing bait plants; there was no evidence for any living soil vector. The virus was eliminated from 96 % of small cuttings taken from infected N. clevelandii plants grown at 35–37 °C for 9 wk. CyRSV was still infective in sap of N. clevelandii after dilution to 10?⁵-io–⁶ (only 2 × 10^_1 in cymbidium sap), or after 10min at 85–90 °C. It survived at least 10 months at c. 20 °C and more than 12 yr at 2 °C. Lyophilized sap was highly infective after over 13 yr at laboratory temperatures under high vacuum. Purified preparations made by clarification with n-butanol, followed by differential centrifugation and exclusion chromatography on controlled-pore glass beads, contained isometric particles c. 30 nm diam., with s°_20W= 137 S, and had a buoyant density in caesium chloride of 1–36 g/ml. The A 260/A 280 ratio was 1–55, and A max(26o)/A min(242) was 1–17. The virus contained c. 15 % of single-stranded RNA of mol. wt 1–7 × 10⁶; the nucleotide base ratios were: G27'8; A24/9; C2I-3; U26-I. There was one capsid polypeptide of mol. wt 43600. The virus was a good immunogen and a strongly reacting antigen in vitro; in Immunoelectrophoresis, each strain migrated as a single antigenic component towards the cathode. The cymbidium and clover strains were serologically closely related, although spurs were produced in immunodiffusion. No serological relationship was found to forty-three other isometric viruses, including eighteen tombusvirus isolates; CyRSV nevertheless shares many properties with tombusviruses, and we assign it provisionally to this group. The cryptogram is: R/r:1:7/15:S/S:S/O.     

Carnation Italian ringspot virus

Carnation Italian ringspot virus (CIRV) was obtained only twice in tests on several thousand carnations in Britain during 15 yr. The two isolates, from cultivars ‘Dusty Sim’ imported from Italy and ‘Orchid Beauty’ from the U.S.A., were indistinguishable serologically and in host reactions. CIRV was cultured in Nicotiana clevelandii and assayed in Chenopodium amaranti-color; it was readily transmitted by leaf-rubbing inoculation to 62 of 104 plant species tested. Virus-free carnations were infected only by injecting purified preparations into the stem, and developed chlorotic spots and oval rings in the younger leaves. CIRV was eliminated from Nicotiana clevelandii plants grown for 8 weeks at 36°C. CIRV presents no threat to carnation growing in Britain. In N. clevelandii sap, CIRV was infective at a dilution of 1/50000 to 1/100000, after heating 10 min at 85 °C (but not 90 °C), and after 16 weeks at 16 °C or 23 weeks at 2 °C. After freeze-drying, the virus survived at least 7 yr storage under vacuum at room temperature. CIRV was still infective and antigenic after treatment for 30 min at 18 °C with ultraviolet radiation (750 μW/cm²), ultrasound, 2% formaldehyde or 0.2% tri-sodium ortho-phosphate (TSP). Infectivity was not wholly abolished in 30 min by 2% TSP. The virus was readily purified by overnight maceration of N. clevelandii leaves extracted in phosphate buffer + butanol, followed by differential centri-fugation. Purified preparations contained abundant isometric particles c. 29 nm diameter, and like other serotypes of the tomato bushy stunt-pelargonium leaf curl group, gave three or four specific bands in density-gradient centri-fugation. The bands corresponded to four Schlieren peaks in analytical centrifugation. Virus from the lower bands was usually less invasive in N. clevelandii than from the upper bands, although the material in the different bands contained similar amounts of nucleic acid. Only one antigenic component was found by Immunoelectrophoresis; different serotypes of the TBSV-PLCV group differed widely in immunoelectrophoretic behaviour. The present cryptogram of CIRV is */*:*/*:S/S:S/*.     

A strain of cassava latent virus occurring in coastal districts of Kenya

A strain of cassava latent geminivirus (CLV) was isolated from mosaic-affected cassava plants from coastal districts of Kenya. This virus (CLV-C) did not infect Nicotiana clevelandii, a diagnostic host of the type strain (CLV-T); experimental host range was very restricted and CLV-C only infected N. benthamiana and N. rustica out of several solanaceous hosts readily infected by CLV-T. CLV-C was also isolated from naturally infected Jatropha multifida (Euphorbiaceae) and Hewittia sublobata (Convolvulaceae). CLV-C was propagated in N. benthamiana with difficulty and only those isolates derived from cassava plants infected with severe mosaic symptoms were maintained more or less successfully; these sources usually contained a higher concentration of CLV than plants with mild symptoms. Symptom variants generally remained unchanged when grafted into a highly susceptible South American cassava variety. CLV-C and CLV-T seemed to occur respectively only in coastal and western districts but their ranges overlapped in central Kenya where they could have been introduced in infected material. CLV-C could be purified satisfactorily with the method used for CLV-T but only after modifying the procedure by substituting phosphate for borate in the extraction buffer, n-butanol for n-butanol/chloroform in clarification of extracts, and phosphate for borate buffer when resuspending concentrated virus. A virus serologically indistinguishable from CLV-T was isolated from mosaic- affected material obtained from Nigeria; East African and Nigerian isolates were essentially similar in host range and symptomatology. In gel-diffusion serology tests, pronounced precipitation spurs developed between CLV-T and CLV-C indicating that the isolates were related but not identical serologically. Symptoms typical of cassava mosaic disease appeared in only three of 105 plants in experiments on transmission of CLV-C and CLV-T by whiteflies, when attempted acquisition of either clarified CLV-infective sap or purified CLV was made through ‘Parafilm’ membranes. Because it is possible that the three infections resulted from contamination, they cannot constitute proof of transmission. The presence of CLV in relation to the etiology of cassava mosaic thus remains unresolved.     

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.     

Narcissus mosaic virus

Narcissus mosaic virus (NMV) is widespread in British crops of trumpet, large-cupped and double daffodils, but was not found in Narcissus jonquilla or N. tazzeta. Many commercial daffodil cultivars seem totally infected, and roguing or selection is therefore impracticable. Strict precautions by breeders and raisers to prevent infection of new cultivars is recommended. Healthy daffodil seedlings were readily infected with NMV by mechanical inoculation, but the virus was not detected in them until 17 months after inoculation, when a mild mosaic appeared. NMV infected twenty-eight of fifty-three inoculated plant species; only five (Nicotiana clevelandii, Gomphrena globosa, Medicago sativa, Trifolium campestre and T. incarnatum) were infected systemically, and NMV was cultured in these and assayed in Chenopodium amaranticolor and Tetragonia expansa. The virus was not transmitted to and from G. globosa or N. clevelandii by three aphid species, or through the seeds of Narcissus, G. globosa and N. clevelandii but was transmitted by handling. G. globosa sap was infective at a dilution of 10 ^-5 but not at 10^-6, when heated for 10 min. at 70° C. but not at 75° C, and after 12 weeks at 18° C, or 36 weeks at 0–4° C. NMV withstood freezing in infected leaves and sap, and purified preparations and freeze-dried sap remained infective for over 2 years. NMV was precipitated without inactivation by ammonium sulphate (313 g./l.) but was better purified by differential centrifugation of phosphate-buffer extracts treated with n-butanol. Such virus preparations from G. globosa, N. clevelandii, C. amaranticolor and T. expansa were highly infective, serologically active, produced a specific light-scattering zone when centrifuged in density-gradients and contained numerous unaggregated particles with a commonest length of 548–568 mμ. Antisera prepared in rabbits had precipitin tube titres of 1/4096. NMV was detected in three experimental hosts but not in narcissus sap. Unlike some viruses with elongated particles, NMV precipitates with antiserum in agar-gel. Purified preparations reacted with antiserum to a Dutch isolate of NMV but not with antisera to seven other viruses having similar particles and in vitro properties, or to narcissus yellow stripe virus.     

Eggplant mosaic virus, and its relationship to Andean potato latent virus

Eggplant mosaic virus (EMV), obtained from Solanum melongena L. from Trinidad, is readily transmitted by inoculation of sap to several solanaceous and a few non-solanaceous plant species. Purified preparations of EMV contain isometric particles 30 nm in diameter, and with sedimentation co efficients of either 111 or 53 S. The particles have thirty-two major morphological subunits. EMV is closely serologically related to Andean potato latent virus and has a similar host range, but is more virulent. Also, whereas EMV accumulates fastest in Nicotiana clevelandii leaves at 20–24 °C, Andean potato latent virus accumulates fastest at 15 °C, and fails to attain a serologically detectable concentration at 24 °C. A few symptomatologically or serologically distinguishable strains of EMV were obtained. EMV has properties typical of viruses of the Andean potato latent subgroup of the turnip yellow mosaic group of viruses, and its present cryptogram is */*:*/*:S/S:S/Cl.     

Some hosts and properties of narcissus latent virus, a carlavirus commonly infecting narcissus and bulbous iris

Narcissus latent virus (NLV) is common in many cultivars of narcissus and bulbous iris, but was detected in only one of nineteen cultivars of nerine. It induced symptoms in some narcissus cultivars, but inconspicuous infection in bulbous iris and nerine. NLV was not seed-borne in narcissus or Nicotiana clevelandii but was transmitted readily by aphids (Acyrthosiphon pisum, Aphis gossypii and Myzus persicae) in the non-persistent manner and by sap-inoculation to twelve of fifty-three species from three of sixteen families. Sap from N. clevelandii was infective after dilution to io^-3 but not io*, after 10 min at 65 but not 70 ^oC or after 3–4 days at 20 ^oC or 16–32 days at 2 ^oC. Purified virus preparations were obtained from infected N. clevelandii by clarification of buffered leaf extracts with diethyl ether and carbon tetrachloride, followed by one or two cycles of differential centrifugation and molecular permeation chromatography. NLV has filamentous particles c. i3times65onm which sediment as a single component (i^o_{20, w}= 156S). They contain c. 5% nucleic acid and a single polypeptide of mol. wt 32·6 × 10³. The biological and physical properties of NLV place it in the carlavirus group; it is serologically related to lily symptomless virus, but not to fourteen other authentic carlaviruses. NLV has the cryptogram */*:*/(5):E/E:S/Ve/Ap.     

Host range and some properties of groundnut rosette virus

Two isolates of groundnut rosette virus from East Africa (GRVE₁ and GRVE₂) and from West Africa (GRVW₁ and GRVW₂) were transmitted by Aphis craccivora obtained from West Africa. A third isolate from West Africa (GRVW₃) was not transmitted by A. craccivora from three widely separated sources. GRVW₁, GRVW₂ and GRVW₃ caused leaf-symptoms in groundnut of a mosaic pattern in light and dark green. GRVE₁ and GRVE₂ caused chlorosis or chlorosis and leaf distortion as well as mosaic symptoms. Groundnut plants with GRVW₁ could not be infected by means of aphids with GRVE₁, and GRVE₁ gave similar protection against GRVW₁, which suggests that they are strains of the same virus. All isolates were transmissible manually from groundnut to groundnut (Arachis hypogea), Trifolium incarnatum and T. repens, and caused systemic infection. Inoculated Nicotiana clevelandii and N. rustica developed symptoms but virus could not be recovered from them. Chenopodium amaranticolor, C. hybridum and C. quinoa showed local lesions on inoculated leaves. Virus could be acquired by aphids from groundnut or Trifolium repens infected by means of aphids, but not from those infected by manual inoculation. Virus could not be recovered from T. incarnatum manually or by aphids, but was transmitted by cleft-grafting from clover to groundnut. Saps extracted in borax buffer plus zinc sulphate at pH 9 from plants infected with GRVW₁ and GRVE₁ remained infective at 18° C. for 1 week, and at — 20° C. for up to 4 weeks. Virus could be recovered from frozen leaves. Buffered saps lost infectivity when heated above 50° C. for 10 min.; most were still infective when diluted 1/10 and some at 1/100. Electron micrographs of partially purified preparations contained spherical particles 25–28 mμ in diameter. There were usually only about five per microscope field and they resembled those of some other viruses.     

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).     

Interactions of the Solanum spp. of the Etuberosa group and nine potato-infecting viruses and a viroid

All 26 accessions of Solanum brevidens, one accession of S. etuberosum and one accession of S. fernandezianum tested were all extremely resistant to potato leafroll virus (PLRV) and potato viruses Y (PVY) and A (PVA). S. brevidens and S. etuberosum were also resistant to Andean potato mottle virus (APMV) and moderately resistant to potato virus X (PVX), whereas S. fernandezianum was susceptible to these viruses. Additionally, S. brevidens was resistant to sap-inoculated potato viruses M (PVM) and S (PVS). All the Etuberosa accessions were susceptible by graft-inoculation to PVM, PVS, potato virus T (PVT) and Andean potato latent virus (APLV). Infections by the above mentioned viruses were symptomless in all of the Etuberosa spp. S. etuberosum and S. fernandezianum were infected by mechanical inoculation with potato spindle tuber viroid, S. etuberosum developing severe stunting and leaf-curl symptoms, but S. brevidens was infected only by graft-inoculation. The genes conferring resistance to PVY and PVX in S. brevidens and S. etuberosum appeared to be different from those currently utilised by plant breeders.     

Detection of strains of African cassava mosaic virus by nucleic acid hybridisation and some effects of temperature on their multiplication

DNA probes, made by cloning double-stranded forms of each of the genome parts (DNA-1 and DNA-2) of the Kenyan type isolate of African cassava mosaic virus (ACMV-T), reacted strongly with extracts from Nicotiana benthamiana plants infected with ACMV-T, or with Angolan or Nigerian isolates that are closely serologically related to the type isolate. However, only the DNA-1 probes reacted with extracts of TV. benthamiana infected with a Kenyan coast isolate (ACMV-C), which is serologically less closely related to ACMV-T. DNA-1 and DNA-2 probes also reacted with extracts of mosaic-affected Angolan cassava plants, including some which have not yielded ACMV particles detectable by immunosorbent electron microscopy and from which virus isolates have not been transmitted to TV. benthamiana. These anomalous plants, unlike other naturally infected cassava plants, showed mosaic symptoms on all their leaves which, however, contained only traces of virus particle antigen detectable by enzyme-linked immunosorbent assay. They contain isolates of ACMV that are probably defective for particle production. ACMV-T particles accumulated optimally in N. benthamiana at 20–25°C. At 30°C fewer particles, which apparently had a slightly greater specific infectivity, were produced. At 15°C, considerable quantities of virus particle antigen, virus DNA and virus particles were produced but the particles were poorly infective, and the few that could be purified contained an abnormally large proportion of polydisperse linear DNA molecules, and fewer circular molecules than usual. Angolan isolates, whether particle-producing or not, likewise replicated better in cassava plants at 23 °C than at 30 °C. In contrast, ACMV-C attained only very low concentrations in N. benthamiana, but these were greater at 30 °C than at 23°C.     

Tulip chlorotic blotch virus, a second potyvirus causing tulip flower break

Tulip chlorotic blotch virus (TCBV), an apparently undescribed potyvirus found in field grown tulips in Australia, causes symptoms in tulip leaves and flowers identical to those induced by tulip breaking virus (TBV). TCBV was transmitted mechanically to 14 of 34 species in four of 13 families. Nicotiana clevelandii is a suitable propagation host and Chenopodium amaranticolor a local-lesion assay host. TCBV was transmitted from tulip to tulip and TV. clevelandii by the aphid Myzus persicae. Unlike TBV it was not transmitted to Lilium formosanum either by M. persicae or by manual inoculation. Leaf extracts from TCBV-containing TV. clevelandii were infective after dilution to l0^-3 but not 10-⁴ and after heating for 10 min at 50°C but not 60°C; infectivity and particle recovery were adversely affected by freezing at -20°C. TCBV particles were purified (c. 1 mg/100g g N. clevelandii leaf) from tissue extracts in 0·3 M citrate buffer containing 10 mM EDTA and 0·2% (v/v) 2-mercaptoethanol at pH 7·4 by clarification with 8·5% (v/v) n-butanol followed by differential centrifugation and sucrose density gradient centrifugation. Purified particles measured c. 720 × 12 nm. Virus particle antigen was readily detected in leaf and tepal extracts of tulip by enzyme-linked immunosorbent assay. A distant serological relationship was found between particles of TCBV and those of bean yellow mosaic virus but no serological relationship was found to TBV or four other potyviruses.     

Tumor formation and morphogenesis on different Nicotiana sp and hybrids induced by Agrobacterium tumefaciens T-DNA mutants

A series of experiments are presented that have been performed to observe the interactions between Agrobacterium tumefaciens strains mutated in the T-DNA genes involved in indoleacetic acid and cytokinin biosynthesis and several Nicotiana species and hybrids. Infections were induced on leaf cuttings of Nicotiana debneyi, N. knightiana, N. clevelandii, N. bigelovii var bigelovii, N. bigelovii var quadrivalvis, N. glauca, N. langsdorffii, the amphidiploid tumorous hybrid N. glauca × N. langsdorffii, and a nontumorous mutant of it. The effect of deletions of the Ti plasmid varied according to plant genotype. Insertion mutants in iaaM and iaaH suppressed tumor formation in N. langsdorffii, reduced it in N. bigeloviivar quadrivalvis, had no effect in N. glauca and the two amphidiploid hybrids, and promoted tumorigenesis when compared to the wild-type Agrobacterium strain B6S3 in N. bigelovii N. debneyi, and N. knightiana. The same mutations induced shoot formation in N. glauca, increased it in N. debneyi, and suppressed root formation in N. knightiana. On the other hand, an insertion mutation of the isopentenyl transferase gene (ipt-) had no effect in N. bigelovii var quadrivalvis, N. debneyi, the tumorous hybrid, suppressed tumor formation in N. langsdorffii, and inhibited it in N. glauca, the nontumorous hybrid, N. bigelovii var bigelovii, and N. knightiana. Insertion in ipt suppressed shoot formation in the nontumorous hybrid and inhibited it in the nontumorous amphidiploid and N. debneyi, while promoting root formation in N. glauca and N. debneyi. The suggestion of the existence of specific hormone equilibria necessary for the shift to each morphogenetic pattern was supported by experiments with exogenous hormone treatments of three genotypes (N. glauca, N. langsdorffii, and the nontumorous N. glauca × N. langsdorffii).     

Cowpea mild mottle, a newly recognized virus infecting cowpeas (Vigna unguiculata) in Ghana

Cowpea mild mottle virus (CMMV), a previously undescribed virus widespread in cowpeas (Vigna unguiculata) in the Eastern Region of Ghana, was seed-borne in V. unguiculata, Phaseolus vulgaris and Glycine max, but was not transmitted by twelve aphid species including Aphis craccivora, A. fabae, Acyrthosiphon pisum and Myzus persicae. CMMV was transmitted by inoculation of sap to eleven of seventeen members of the Papilionaceae causing very severe diseases in G. max and Arachis hypogaea, and to ten of fifty-one species within five of nineteen other families; it was best propagated in G. max and Nicotiana clevelandii, and assayed in Chenopodium quinoa. Sap from systemically infected G. max was infective after dilution to 10^-3 but not 10^-4, after 10 min at 65 °C but not at 70 °C, or after 4 days at 18 °C or 16 days at 2 °C. Lyophilized sap was infective after 3 years in vacuo. CMMV has straight to slightly flexuous, fragile filamentous particles, c. 13 × 650 nm which, in sap, are occasionally surrounded by a loose external spiral. About 5 mg of purified virus was obtained from 1 kg of leaf tissue of G. max or N. clevelandii by clarifying leaf extracts in 0.02 m borate buffer (pH 9.5) with chloroform, followed by two or three cycles of differential centrifugation, and density gradient centrifugation. Virus preparations had ultraviolet absorption spectra typical of a nucleoprotein containing c. 5 % nucleic acid, contained numerous particles without external spirals, which sedimented as a single component with a sedimentation coefficient (s°_{20, w}) of 165 × 4S, and contained a single polypeptide species with a molecular weight of 32000–33000. CMMV showed a distant serological relationship to carnation latent virus, but not to ten other morphologically similar viruses; it thus seems to be a distinct member of the carlavirus group, and has the cryptogram: */*:*/(5):E/E:S/*.     

Properties of a resistance-breaking strain of potato virus X

During indexing of a potato germplasm collection from Bolivia, a strain of potato virus X (PVX), X_HB, which failed to cause local lesions in inoculated leaves of Gomphrena globosa was found in 7% of the clones. X_HB was transmitted by inoculation of sap to 56 species from 11 families out of 64 species from 12 families tested. It was best propagated in Nicotiana glutinosa or N. debneyi; Montia perfolia and Petunia hybrida were useful as local lesion hosts. Inoculated leaves of G. globosa plants kept at 10°, 14°, 18°, 22°, or 26 °C after inoculation were always infected symptomlessly. X_HB caused a mild mosaic, systemic chlorotic blotching or symptomless infection in 16 wild potato species and eight Andean potato cultivars, systemic necrotic symptoms in clone A6 and cultivar Mi Peru, and bright yellow leaf markings in cultivar Renacimiento. It caused necrotic local lesions in inoculated leaves of British potato cultivars with the PVX hypersensitivity gene Nb but then invaded the plants systemically without causing further necrosis; with gene Nx systemic invasion occurred but no necrotic symptoms developed. These reactions resemble those of PVX strain group four. X_HB differed from other known strains of PVX in readily infecting PVX-immune clones 44/1016/10, G. 4298.69 and USDA 41956, cultivars Saphir and Saco, and Solanum acaule PI 230554. X_HB had slightly flexuous filamentous particles with a normal length of 516 nm. It was transmitted readily by plant contact and it partially protected G. globosa leaves from infection with X_CP, a group two strain of PVX. Sap from infected N. glutinosa was infective after dilution to 10^--⁶ but not 10^--⁷ after 10 min at 75° but not 80 °C and after 1 yr at 20 °C. X_HB was readily purified from infected N. debneyi leaves by precipitation with polyethylene glycol followed by differential centrifugation. Microprecipitin tests showed that X_HB and X_CP are closely related serologically.     

Effects of virus genotype and temperature on seed transmission of nepoviruses

Transmission of different nepoviruses through chickweed (Stellaria media) seed was differently affected by ambient temperature during seed production. Raspberry ringspot and tomato black ring (Scottish isolate) viruses were similarly and frequently transmitted at 14 , 18 and 22 ^oC, whereas arabis mosaic virus was transmitted most frequently at 14 ^oC, and strawberry latent ringspot and tomato black ring (German isolate) viruses at 22 ^oC. When infected by seed-borne nepoviruses, seedlings of S. media and other species were symptomless at 15–25 ^oC, and the viruses were therefore detected by inoculating sap to Chenopodium quinoa indicator plants. However, typical symptoms of arabis mosaic and tomato black ring viruses were induced by growing Nicotiana tabacum, N. clevelandii and C. quinoa seedlings infected with seed-borne virus at 33–37 ^oC during the third and fourth weeks after sowing, preceded and followed by periods at 15–25 ^oC. The proportion of N. tabacum seedlings developing symptoms was the same as that of untreated seedlings yielding sap-transmissible virus. Seed transmissibility of pseudo-recombinant isolates of raspberry ringspot and tomato black ring viruses, containing RNA-i from one virus strain and RNA-2 from another strain, depended greatly on the transmissibility of the strain contributing RNA-i. The source of RNA-2 had an additional but smaller influence. The satellite RNA (RNA-3) of tomato black ring virus was seed-transmitted in S. media and its occurrence in cultures did not affect the frequency of transmission of the virus. Results of testing the infectivity of extracts of seed from infected mother plants suggested that failure of seed transmission reflected failure to become established in the seed, not subsequent inactivation. Whereas seed transmissibility of raspberry ringspot virus is primarily dependent on information carried in RNA-i, transmissibility by nematode vectors, another property of major ecological importance, is determined by RNA-2. In the field, selection pressures presumably can act independently on the two parts of the genome but evidence was also obtained of selection for mutual compatibility of RNA-i and RNA-2.     

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.