Bibliography: pages 155-179. / Ornithogalum 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.

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Double infection by cowpea strains of southern bean mosaic virus (CP-SBMV) and of tobacco mosaic virus (CP-TMV) caused additive growth reductions in California blackeye cowpea. Plant height, weight and numbers of seed and pods were significantly reduced by double infection and by CP-TMV single infection compared to healthy and CP-SBMV-sing 1 y infected plants. Singly and doubly inoculated California blackeye cowpea plants developed CP-SBMV symptoms on the primary leaves, but CP-SBMV symptoms in doubly infected trifoliates were masked by CP-TMV symptoms. CP-TMV symptoms did not mask CP-SBMV symptoms in systemically infected trifoliate leaves of another cowpea variety, V45-Bots. CP-TMV infection conditioned systemic infection of V45~Bots by CP-SBMV, as indicated by infectivity,serology and analytical sucrose density gradient centrifugation. CP-TMV also induced susceptibility of Pinto to infection by CP-SBMV, as ascertained by infectivity, immunodiffusion and electron microscopy. Analytical sucrose density gradient centrifugation measurements demonstrated that in doubly inoculated primary leaves of California blackeye cowpea CP-SBMV and CP-TMV were synthesized less than in the same leaves singly inoculated. CP-SBMV synthesis in trifoliate leaves, following simultaneous inoculations of primary leaves, was enhanced 5 times that in singly infected trifoliate leaves, whereas CP-TMV synthesis was not greatly affected. When CP-TMV preceded CP-SBMV in the primary leaves by 2k and 72 hr CP-SBMV synthesis was enhanced more in trifoliate leaves that were undifferentiated at the time of inoculation than in those of plants simultaneously inoculated. When CP-TMV preceded CP-SBMV into preformed 3rd trifoliate leaves by 22 hr, the ratio of CP-SBMV concentration in doubly infected tissue to that in singly infected tissue was 2.7 versus 1.9 when both viruses arrived simultaneously at these leaves. When either virus preceded the other by 72 hr into preformed 3rd trifoliate leaves the synthesis of the challenging virus was greatly retarded. CP-SBMV synthesis was also enhanced by CP-TMV infection under differential temperature synchronous system of infection. Although virions of both viruses were detected in the same cell no genomic masking was detected by infectivity neutralization test. It is theorized that CP-TMV infection predisposes the host cells to infection by CP-SBMV and thus the enhanced synthesis of CP-SBMV. The effect of CP-TMV infection on CP-SBMV synthesis in cowpea seems to be a physiological one. CP-SBMV, but not CP-TMV, was transmitted through planted seed and decontaminated embryos of California blackeye cowpea. Buffer extracts made from decontaminated embryos also were infectious for CP-SBMV. Seed coats contained both viruses. Double infection of California blackeye cowpea decreased seed transmission of CP-SBMV from 13-5 to 7.6%. Buffer extracts of healthy seed were inhibitory to infectivity of both viruses. Germination of seed reduced infectivity of CP-SBMV in the seed coats, but not of CP-TMV. It is also concluded that seed transmission of CP-SBMV is a result of embryo infection rather than contamination with virus in the seed coats. / Land and Food Systems, Faculty of / Graduate

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