Incidence, Distribution and Characteristics of Major Tomato Leaf ...

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Incidence, distribution and characteristics of major tomato leaf curl and mosaic virus diseases 2 CHAPTER 2 2.1 Literature Review 2.1.1 Virus Taxonomy In 1970, the International Committee on Taxonomy of Viruses (ICTV) approved sixteen groups of plant viruses. By 1990, Brunt et al. (1990) reported 36 plant infecting virus groups to occur worldwide. Matthews (1991) later reported 590 viruses to have been identified and ascribed to 35 families or groups of plant infecting viruses. Following considerable controversy regarding the taxonomy of viruses (Brown, 1997), ICTV later classified viruses into 233 genera (Pringle, 1999) using four criteria: (1) the general nature of the viral genome; (2) the stranded nature of the viral genome; (3) the facility for reserve transcription; and (4) the polarity of the virus genome. Of the 233 genera, 204 were classified into 64 families, leaving 29 genera that were not yet fully characterized (Pringle, 1999). Recently in 2005, Mayo and Brunt reported that ICTV had come up with new approvals for plant virus taxonomy, which include 18 virus families, 82 genera and 17 unassigned genera. Of the 18 families, seven families include viruses that infect tomato, i.e. families Geminiviridae, Bunyaviridae, Potyviridae, Flexiviridae, Bromoviridae, Closteroviridae, and Luteoviridae. By 2000, ICTV produced the orthography for all known plant viruses. The orthography for tomato viruses belonging to virus families reported above is shown in Table 2.1 (Mayo, 2000). Furthermore, the geminivirologist community and the family Geminiviridae study group under ICTV recognized the existence of the genus Topocuvirus (Fauquet et al., 2003), in addition to the previously distinguished genera Mastrevirus, Curtovirus and Begomovirus. Member viruses of genus Mastrevirus infect monocotyledonous plants like maize (Zea mays), and the type species is Maize streak virus. On the other hand, member viruses of genus Curtovirus infect dicotyledonous plants like beet (Beta vulgaris), with 10
Incidence, distribution and characteristics of major tomato leaf curl and mosaic virus diseases Beet curly top virus, which is the type species. Although members of genus Begomovirus infect dicotyledonous plants like tomato (Lycopersicon esculentum), they differ from other genera by having twin particles. The species type for this genus is Bean golden mosaic virus. Both begomoviruses and tocupoviruses infect tomato, but genus Tocupovirus has only one member virus, i.e. Tomato pseudo curly top virus, which has a monopartite genome different from that of members of genus Begomovirus (Fauquet et al., 2003). In the same paper guidelines for naming geminiviruses are outlined. This work (Chapters 3-6) follows 2005 ICTV recommendations (Mayo and Brunt, 2005). 2.1.1.1 Tomato Viruses About 146 viruses infect tomato worldwide (Green, 1991; http://image.fs.uidaho.edu/vide/descr002.htm). They are grouped into 33 genera, but 15 genera are of the most economic importance, i.e. Alfalfamovirus, Begomovirus, Carlavirus, Crinivirus, Cucumovirus, Ilarvirus, Luteovirus, Nepovirus, Potexvirus, Potyvirus, Tobamovirus, Tombusvirus, Topocuvirus, Tospovirus, and Tymovirus. As stated before (Section 2.1), these fifteen genera belong to families Bromoviridae, Bunyaviridae, Closteroviridae, Flexiviridae, Geminiviridae, Luteoviridae and Potyviridae (Pringle, 1999). Family Bunyaviridae has only one assigned plant-infecting genus (Tospovirus) to which Tomato spotted wilt virus (TSWV) belongs. Other genera of this family consist of virus species that infect animals only. Family Flexiviridae has been recently approved by ICTV (Mayo and Brunt, 2005). Its major tomato virus is Potato virus X, which belongs to genus Potexvirus. Nono-Womdim et al. (1994) reported that major tomato viruses in tropical Africa fall into five genera, i.e. Tobamovirus, Cucumovirus, Tospovirus, Begomovirus, and Potyvirus. No virus was evidenced to be of the genera Alfamovirus, Potexvirus, or Closterovirus, which have been reported to occur on tomato in Europe (Fauquet and Mayo, 1999: Wisler, 1998; Green, 1991; Brunt et al., 1990). 11
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Page 4 and 5: Charles SSEKYEWA Incidence, Distrib
Page 6 and 7: CONTENTS 1 CHAPTER 1...............
Page 8 and 9: ACKNOWLEDGEMENT The following perso
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Page 12 and 13: ix 9 Consequently, our findings con
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Page 20 and 21: LIST OF TABLES xvii 17 Table Page 1
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8 ANNEXES Incidence, distribution a
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