Introduction
Single-stranded DNA viruses (ssDNA viruses) are characterized by a single stranded DNA as the genetic material and using a DNA-dependent DNA polymerase during replication. Most of these viruses must enter the host nucleus before they can replicate. It is because they use the host cell’s polymerases when replicating their viral genome.
Geminiviridae is a family of plant viruses where virion possesses an unusual morphology consisting of a pair of isometric particles. Transmission occurs via leafhoppers or whitefly. Most important plant-pathogenic genera are Mastrevirus, Curtovirus, Topocuvirus, and Begomovirus.
Viruses – ssDNA viruses – Geminiviridae –
Mastrevirus
MSV and all related grass mastreviruses have single-component, circular, single-stranded DNA genomes of approximately 2,700 bases, encapsidated in 22-38-nm geminate particles comprising two incomplete T = 1 icosahedra, with 22 pentameric capsomers composed of a single 32-kDa capsid protein. Particles are generally stable in buffers of pH 4–8.
| Disease | Transmission | Management | Literatur |
|---|---|---|---|
| Mastrevirus – Maize streak virus Acronym: MSV |
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| MSV-A strain causes the most severe form of streak disease in maize, 10 other MSV strains (MSV-B to MSV-K) are known to infect other Potaceae. Occurs throughout Africa. Small, pale, circular spots appear on youngest leaves; ss disease progresses, newer emerging leaves have streaks up to several millimetres in length usually along the secondary or tertiary veins. The streaks are often fused appearing as narrow, broken, chlorotic stripes. Plants infected at early stage become stunted, producing undersized, misshapen cobs or giving no yield at all. Infected seedlings produce no yield or are killed, whereas plants infected at later times are proportionately less affected. | It is obligately transmitted by as many as six leafhopper species in the genus Cicadulina, but mainly by C. mbila Naudé and C. storeyi. | Disease avoidance can be practised by only planting maize during the early season when viral inoculum loads are lowest. Leafhopper vectors can also be controlled with insecticides such as carbofuran. However, the development and use of streak-resistant cultivars is probably the most effective and economically viable means of preventing streak epidemics. | Shepherd DN et al. Maize streak virus: an old and complex ’emerging’ pathogen. Mol Plant Pathol. 2010 Jan;11(1):1-12. |
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Begomovirus
Virions associated with CMD are 30 nm X 18 nm geminate particles, 30 kDa protein was shown to be the subunit making up the paired icosahedral coat structure and the genetic material comprises two components of single-stranded circular DNA (DNA-A snd DNA-B), both of which approx. 2,800 bp long. The first virus isolated from CMD was initially named cassava latent virus (CLV) because it was not possible to infect cassava and produce similar symptoms. The first sequence of DNA-A was published in 1983 (Stanley and Gay, 1983), and shortly after this, successful infection of cassava was achieved leading to the naming of the causal virus as African cassava mosaic virus (ACMV)
| Organism | Disease | Transmission | Literature |
|---|---|---|---|
| Begomovirus – African cassava mosaic virus Synonyms: cassava mosaic virus, Cassava latent virus Acronym: ACMV |
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| CMD pandemic has affected at least nine countries in East and Central Africa, causing famine and the death of thousands of people and an estimated annual economic loss of US$1.9–2.7 billion The symptoms are very variable in type, extent and severity. Plants affected by “green mosaic” have leaves with contrasting sectors of dark and light green tissue. Plants affected by “yellow mosaic” are much more conspicuous, as they have leaves with contrasting areas of normal green and yellow tissue. Moreover, the chlorotic areas may expand less than other parts of the leaf lamina, which can lead to distortion of the leaflets and rupturing of the tissues. Severe chlorosis is often associated with premature leaf abscission, a characteristic S-shaped curvature of the petioles. The most severely affected plants are so stunted that they produce virtually no yield of roots or stems for further propagation. | Transmitted by the whitefly Bemisia tabaci. The East and West African virus isolates are transmitted in a persistent manner, and the minimum (and optimum) acquisition access, inoculation access and latent periods for successful transmission are 3 h (5 h), 10 min and 3–4 h (6 h), respectively. Virus is retained by adult whitefly for at least 9 days. It persists during moulting, but is not transmitted transovarially. ACMD is spread in two ways: when the whitefly feeds first on diseased plants and then on healthy plants; or when diseased cuttings are used to establish a crop. | In general terms, there are three possible approaches to decreasing the losses due to a virus disease: (i) decrease the proportion of plants that become infected; (ii) delay infection to such a late stage of crop growth that losses become unimportant; (iii) decrease the severity of damage sustained after infection has occurred. These objectives can be achieved in diverse ways and the main possibilities for controlling CMD are phytosanitation, disease-resistant varieties, cultural practices, vector control and mild-strain protection. | Legg JP, Fauquet CM. Cassava mosaic geminiviruses in Africa. Plant Mol Biol. 2004 Nov;56(4):585-99./i> |