Browsing by Author "Von Wechmar, M Barbara"
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- ItemOpen AccessThe characterisation of Ornithogalum mosaic virus(1991) Burger, Johan Theodorus; Von Wechmar, M BarbaraOrnithogalum mosaic virus (OMV) is the most serious pathogen of commercially grown Ornithogalum and Lachenalia species in South Africa. Although omithogalum mosaic disease was first reported as early as 1940, attempts to purify or characterise the virus(es) were not successful. The extremely mucilaginous nature of omithogalum and lachenalia plant extracts severely hampered virus purification from these hosts. No alternative propagation host for OMV is known: a virus purification protocol for systemically infected ornithogalum and lachenalia was therefore developed. This method eliminated the mucilage in leaf extracts by hemicellulase digestion. Physicochemical characterisation of purified particles suggested that a single virus was present: it had elongated, filamentous particles with a modal length in the range 720- 760 nm; a single major coat protein of Mᵣ30 000, and a single genomic ssRNA of Mᵣ2.90 x 10⁶ daltons. Oligo(dT)-cellulose chromatography confirmed that the genomic RNA was polyadenylated.
- ItemOpen AccessCharacterisation of two aphid picorna-like viruses(1988) Williamson, Carolyn; Rybicki, Edward P; Von Wechmar, M BarbaraA new aphid virus, aphid lethal paralysis virus (ALPV), was isolated from laboratory-propagated Rhopalosiphum padi aphids co-infected with R. padi virus (RhPV). ALPV and RhPV were separated and ALPV was characterised in detail. Virions are isometric with a diameter of 26 nm, a sedimentation coefficient of 164 Sand a density in CsCl of 1.34 g/ml. Virions contain a 9.7 kb polyadenylated, singlestranded RNA and three major proteins with molecular weights of approximately 30 kilodaltons. By characterising RhPV further, two additional putative capsid proteins were found, an RNA poly(A) tract was detected and an RNA size of 10 kb was determined. A South African isolate of RhPV (RhPVoFs) was found to be serologically identical but physically distinct from a USA isolate. Complementary DNA was synthesized from RhPVOFS RNA and cloned into the plasmid vector, pBR322. This clone was used for the detettion of virus in aphids. ALPV and RhPV are serologically unrelated. ALPV is serologically distantly related to two insect picornaviruses, cricket paralysis virus (CrPV) and Drosophila C virus. No nucleic acid homology was detected between ALPV cDNA and CrPV by dot-blot hybridization. ALPV is serologically unrelated to seven other insect picornalike viruses. RhPV is serologically unrelated to any of the above mentioned viruses. ALPV and RhPV RNAs were efficiently translated in rabbit reticulocyte lysate into high molecular weight polypeptides, the sum of which exceeded the coding capacity of the genomes. Putative capsid precursor proteins of ALPV and RhPV were identified by immunoprecipitation. ALPV translation products were post-translationally cleaved as demonstrated in pulse-chase experiments and in experiments using a translation inhibitor. The efficiency of cleavage was concentration-dependent indicating the action of a protease. In parallel experiments with RhPV RNA, no evidence of post-translational cleavage was observed. In a survey of aphids collected in South Africa, ALPV and RhPV were detected in aphids from two major small-grain producing areas. Both viruses were found to naturally infect most of the cereal aphid species found in this country. ALPV and RhPV infections of R. padi resulted in a marked reduction in longevity and fecundity relative to uninfected aphids. Both viruses were found to be horizontally and vertically transmitted through aphid populations, and aphid host plants and aphid predators could be implicated in virus dissemination. ALPV and RhPV have many properties in common with each other as well as with insect and mammalian picornaviruses. Based on this data, it is proposed that ALPV and RhPV be classified into the picornavirus group (family Picornaviridae).
- ItemOpen AccessThe effect of aphid lethal paralysis virus (ALPV) on the biology of grain aphids in the Western Cape(1992) Laubscher, Jacobus Martin; Von Wechmar, M BarbaraAphid lethal paralysis virus (ALPV) could be detected by indirect immunofluorescent technique in dissected aphids. This technique was found to be more sensitive when compared to DAS-ELISA. The choice of a sensitive, low cost .detection method was of importance to test for low levels of virus in infected aphid body tissues where inapparent infection could cause detection problems. ALPV was visualized in ultrathin sections of diseased aphid body tissues by immunocytochemistry utilizing immunogold label. ALPV antigen was detected in the ovariole tissue, tracheocytes, symbionts of the mycetocytes, fat body cells, brain tissue, nerve tissue and stomach epithelial tissue. Virions were detected predominantly in the cytoplasm but were also found in the nucleus. ALPV antigen was not detected in muscle fibres or mitochondria. ALPV and Rhopalosiphum padi virus (RhPV) are transmitted transovarially. Different incidences of transmissions of ALPV were obtained for R. padi (2996) and Sitobion avenae (1696) and ALPV infections dramatically reduced the longevity and fecundity of these aphids. Infected apterous R. padi aphids were more fecund than alate aphids of the same clone. The percentage of viral infections in different aphid species (R. padi, S. avenae and Diuraphis noxia) was positively associated with temperature; higher temperatures dramatically increased the incidence of ALPV and RhPV and vice versa. The influence of ALPV on a natural R. padi aphid population was found to reduce the population size by 4996. This reduction coincided with a high death factor (70) of aphids per plant. A dramatic decline in R. padi aphid numbers and a high incidence of ALPV present in this aphid population was experienced. Parasitic fungal infections peaked at a later stage than ALPV, and a level of 21 parasitized aphids per plant was reached during this period. This appears to indicate that the presence of ALPV contributes to limit population development in R. padi aphids. Similar results were obtained with S. avenae aphids. Based on this data, ALPV could be considered as a major growth limiting factor in the development of small grain aphid populations in the western Cape. If the presence of the virus is taken into consideration, it could influence pest management strategies directly.
- ItemOpen AccessIdentification and characterisation of South African strains of cucumber mosaic virus(1985) Lupuwana, Pumezo; Von Wechmar, M BarbaraThis project was then aimed at finding naturally occurring isolates of CMV, characterising them, producing much needed antisera and to use such antisera in a comparison with other well characterised strains by the use of new contemporary sensitive serological techniques.
- ItemOpen AccessMixed infections of maize dwarf mosaic virus and cucumber mosaic virus in maize(1986) Knox, Elizabeth; Von Wechmar, M BarbaraMaize plants collected in three geographically distinct regions of South Africa were found to be doubly infected with maize dwarf mosaic (MDMV) and cucumber mosaic virus (CMV). A mixed infection of these two viruses could be maintained in maize plants grown under laboratory conditions. The possibility of synergism or of an interference mechanism between MDMV and CMV in dual infections was investigated and it was found that prior infection with CMV interfered with subsequent infection by MDMV. MDMV and CMV were shown to be non-persistently transmitted by Myzus persicae, Rhopalosiphum padi and Rhopalosipbum maidis aphids. Protoplasts were isolated from maize seedlings and could be viably maintained for up to 66 hours. The maize protoplasts were infected with CMV and MDMV either singly, or together as a mixed inoculum. Infection curves for each virus were plotted. The presence of CMV in a mixed inoculum appeared to prevent infection of the protoplasts by MDMV. Protoplasts were isolated from plants systemically infected with CMV and/or MDMV. Superinfection of protoplasts prepared from CMV infected seedlings with MDMV was not possible. As a possible vehicle for virus infection of protoplasts liposomes were produced. Initially fluorescent dyes were incorporated in them. These were fused to the maize protoplasts. Attempts were made to encapsulate virus particles in the liposomes and fuse them to maize protoplasts but this was not successful.
- ItemOpen AccessA serological study of some cauliflower mosaic virus isolates(1981) Du Plessis, Dion Henri; Von Wechmar, M BarbaraEnzyme-linked immunosorbent assay (ELISA) was used successfully to detect cauliflower mosaic virus (CaMV) in crude leaf extracts. Small serological differences between CaMV isolates could be shown by ELISA and serum cross-absorption. Serological reactivity of CaMV was found to depend on the proteolytic degradation state of the virus coat protein so making it impossible to establish definite serological relationships among the virus isolates tested. Proteolysis during purification of CaMV could not be entirely eliminated. The coat protein of CaMV was shown to be glycosylated by the specific binding of labelled Concanavalin A. The role of carbohydrate residues in CaMV serological reactivity was evaluated.
- ItemOpen AccessA study of filamentous viruses in maize and smallgrains(1985) Chauhan, Ramola; Von Wechmar, M BarbaraThe occurrence of maize dwarf mosaic virus (MDMV) in field grown maize was investigated. For this purpose, maize showing mosiac symptoms was collected from different maize growing areas in South Africa by Prof. M.B. von Wechmar. These samples from Transvaal, Orange Free State and Natal were then investigated for the presence of MDMV and possible strains of this virus. Three virus isolates were purified and partially characterised. These isolates were serologically compared together with a fourth isolate SCMV 4975, obtained from the U.S., to establish strain relationships.
- ItemOpen AccessA study of the molecular variability of some South African isolates of tobacco necrosis virus(1996) Freeborough, Michael-John; Von Wechmar, M Barbara; Purves, MThe relationship of various tobacco necrosis viruses, isolated from various crops and other sources in South Africa was determined. An isolate from avocado was chosen for partial characterisation to confirm that it was a member of the Necrovirus group of plant viruses. TNV was detected in potatoes that exhibited abnormal necrotic foliar symptoms and a D-type TNV was isolated and identified from the Up-to-Date potato variety. Immunoelectroblotting assay grouped the TNV isolates studied into two serotypes (A and D). This result was confirmed by NA hybridization with probes derived to the coat protein of an A- (TNVWheat) and a D-type (TNV-Papaya green lesion) isolate. RT-PCR with A and D specific primers did not amplify the coat protein of three A- and D-type TNV isolates which appears to indicate that the detection by PCR with specific primers is too selective to be used for a general test, unless degenerate primers to a more conserved region of the coat protein gene are used. Sequence analysis of the coat protein gene was used to determine the phylogenetic relationship amongst nine TNV isolates examined and also by comparison to three isolates for which the coat protein gene had already been sequenced. Sequence analysis showed high degree of homology amongst the A-type isolates and the D-type isolates, with approximately 45 % homology between the two TNV types.
- ItemOpen AccessViruses implicated in the woodiness disease of South African passionfruit, and the molecular characterization of a new potyvirus(1992) Brand, Reon J; Von Wechmar, M Barbara; Rybicki, Edward PWoodiness disease caused by virus infection is the most serious virus disease of passionfruit and affects economic production of this crop worldwide. A preliminary survey of diseased Passiflora material collected from various regions in South Africa revealed the presence of at least three different viruses. A diseased P. caerulea rootstock specimen from a woodiness diseased vineyard in Natal was selected as a source for isolation and further characterization of viruses. Two viruses that were present in a mixed infection were isolated and purified from this material: a spherical virus which appeared to be cucumber mosaic virus (CMV) and a filamentous virus which was initially presumed to be an isolate of passionfruit woodiness virus (PWV). The host range, transmission and prevalence of these viruses were studied by employing techniques such as electron microscopy (negative staining and immunosorbent), electroblot immunoassay, double antibody sandwich enzyme-linked imunoassay and nucleic acid hybridization. In transmission studies, the CMV-isolate and the potyvirus were found to be sap, aphid and graft transmissible. Separation of the two viruses was achieved by passage through a selective host range.