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

Incidence, distribution and characteristics of major tomato leaf curl and mosaic virus diseases
(biotin) ones. Furthermore, DNA hybridisation is very useful in that it can be used to test
samples sent from normally poorly equipped laboratories in developing countries to
advanced laboratories.
One way of quickly differentiating geminiviruses is by sequence pairwise comparison
and phylogenies (Padidam et al., 1995; Brown, 1997; Fauquet et al., 2003). However,
there are different opinions. Some scientists feel that sequence comparisons could be
based on the intergenic region and coat protein gene (Brown, 1997) in the absence of a
complete DNA sequence, while others feel that full sequence comparisons (Fauquet et
al., 2003) are necessary. In the absence of adequate resources, the former approach to
geminivirus identification would be most appropriate for developing countries, whereas
complete sequence comparisons would be encouraged, where possible.
Based on results obtained by using the above techniques, and under the coordination of
the International Committee for the Taxonomy of Viruses (ICTV), tomato viruses have
been characterized and grouped in orders, families, genera and species (Padidam et al.,
1997; Fauquet and Mayo, 1999; Pringle, 1999; Mayo and Brunt, 2005).
2.1.3 Virus Epidemiology and Vector Relationships
2.1.3.1 Virus Transmission
Viruses can be transmitted by mechanical means, and by pollen, seeds, fungi, nematodes,
mites, as well as insects (Green and Kim, 1991; Green, 1991; Jones et al., 1991),
although there is considerable specificity and any one virus is often transmitted by only
some of these routes. Mechanical transmission is mostly restricted to non-persistent and
semi-persistent aphid-transmitted, leafhopper-transmitted and some whitefly-transmitted
viruses (Brunt et al., 1990; Green 1991; Jones et al., 1991). Known insect vectors of
tomato viruses are beetles, aphids, leafhoppers, thrips and whiteflies (Lyons et al, 1985;
Duffus, 1987; Black et al., 1991; Green and Kim, 1991 and Green, 1991; Jones et al.,
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