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

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