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

Incidence, distribution and characteristics of major tomato leaf curl and mosaic virus diseases
In general, major tomato viruses so far encountered in tropical Africa form two groups
based on symptoms (Figure 2.1). The two groups are leaf curl-causing viruses, and
mosaic/mottling symptom-causing viruses (Brunt et al., 1990).
2.1.1.1.1 Tomato Leaf Curl Symptom Causing Viruses
Viruses that cause tomato leaf curl, yellowing and chlorosis, and that are transmitted by
whiteflies, belong to families Geminiviridae and Closteroviridae. Viruses of the family
Geminiviridae cause mostly leaf curl, small round leaflets and marginal yellowing
(Czosnek and Laterrot, 1997; Cohen and Nitzany, 1966), whereas those of the
Closteroviridae induce infectious chlorosis (Wisler et al., 1998).
2.1.1.1.1.1 Family Geminiviridae
According to ICTV, this family is divided into four genera: Mastrevirus, Curtovirus,
Begomovirus and Topocuvirus (Pringle, 1999). The first two genera include viruses
infecting maize (Maize streak virus) and beet (Beet curly top virus), respectively.
Members of genera Begomovirus and Topocuvirus infect tomato. Genus Begomovirus, to
which Bean golden mosaic bigeminivirus belongs as the type species, is the only genus of
family Geminiviridae that has viruses infecting tomato in both the New and Old Worlds.
Genus Topocuvirus has only one member, Tomato pseudo curly top virus, which is the
type species. It has a monopartite genome and also infects dicotyledonous plants.
Geminiviruses have either a monopartite genome or a bipartite genome. According to
Brown (1997), monopartite genomes have sizes of 2.7 to 2.8 kb and contain at least six
genes. On the other hand, bipartite genomes are 5.2 to 5.4 kb in size, and have two
genomic components, named A and B or DNA 1 and DNA 2. Geminiviruses with the
bipartite genome were especially reported in the New World (Davies et al., 1989;
Rochester et al., 1994).
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