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

Incidence, distribution and characteristics of major tomato leaf curl and mosaic virus diseases
2006 personal communication). In addition to the above techniques, immunoelectron
microscopy can be used to identify individual viruses occurring in a single plant sap
extract at the same time, and also to study virus particle size and shape (Duncan and
Torrance, 1992). Furthermore, there is the Double-Antigen-Sandwich Enzyme-Linked
Immunosorbent Assay (DAS-ELISA) used for immediate serological identification of
viruses in a sample, based on viral protein differences (Clark and Adams, 1977). DAS-
ELISA is widely used. The reagents and chemicals required are readily available, and it
gives adequate identification of viruses. Nono-Womdim and Atibalentja (1993) used
DAS-ELISA to identify PVMV in sweet pepper (Capsicum annuum) in Cameroon. Like
all other ELISAs, it is fairly cheap, especially if antisera can be produced locally and do
not have to be bought from commercial companies.
Triple-Antibody-Sandwich (TAS-ELISA) is another form of ELISA. It uses monoclonal
antibodies to detect viruses such as tomato geminiviruses (Credi et al., 1989; Roberts et
al., 1984; Thomas et al., 1986). TAS is efficient and easy to conduct in conditions of
limited time and space. Macintosh et al. (1992) used TAS to study tomato geminiviruses
in Europe. It is more specific than DAS, because of the monoclonal antibodies used.
Like DAS, it is fairly cheap (Matthews, 1991), though not as accurate as molecular
techniques.
2.1.2.3 Molecular Tests
Recently, molecular techniques have been developed to identify viruses. Polymerase
Chain Reaction (PCR) is one of these techniques (Duncan and Torrance, 1992). It is
based on differences between viral nucleic acid, and is very efficient, as well as accurate
depending on the type of primers used (specific or general) (Lewin, 1997). However, it is
an expensive technique using high cost equipment and reagents (Maniatis et al., 1982).
Similarly, the nucleic acid hybridisation test is another convenient molecular technique
for identifying viruses that cannot be identified by using serology (Czosnek et al., 1988).
It is efficient, but uses expensive reagents and chemicals. It requires special expensive
laboratory facilities, especially if it uses radioactive probes (P³²) and not non-radioactive
30