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

Incidence, distribution and characteristics of major tomato leaf curl and mosaic virus diseases
In some circumstances, the incidence and rate of spread of TYLCV are directly
proportional to the whitefly population present in the environment (Mansour et al., 1992;
Mehta et al., 1994). Both adults and larvae can acquire the virus by feeding on infected
plants with a minimum access and acquisition period (AAP) of 15 minutes. The virus has
a latent period of 21-24 hours, and persists for 10 to 20 days in viruliferous B. tabaci
adults (Cohen et al., 1966; Zeidan and Czosnek, 1994). For the whitefly to transmit the
virus persistently, it must have adequate inoculation access periods (IAP) following
acquisition access periods (AAP) (Cohen et al., 1964).
According to Zeidan and Czosnek (1994), TYLCV multiplies inside B. tabaci. Cohen and
Antignus (1994) demonstrated that TYLCV could be found as double-stranded DNAs in
viruliferous whiteflies, which implies replication of viral DNA in the vector. Once the
whitefly has acquired the virus, it can continue transmission through out its life (Brown,
1997). These observations are opposed to earlier findings (Cohen and Nitzany 1966),
which stated that TYLCV triggers an antiviral mechanism in B. tabaci, hence preventing
multiplication of the virus in the vector and necessitating the need for repeated
acquisition (Cohen and Harpaz, 1964). In another study, Marco et al. (1975) found anti-
TMV factors that influenced periodic acquisition of the virus, and reduce whitefly ability
to acquire and transmit viruses. According to Briddon et al. (1990) and Hiebert et al.
(1995), factors involved in virus transmission include presence of viral coat protein and
genes on the complementary DNA strand with open reading frames ACI, AC2, AC3, and
AC4.
The presence of viral DNA in the vector is proof of vector transmission of that particular
virus (Navot et al. (1992). It can be determined by using modern techniques like
Polymerase Chain Reaction (PCR). Navot et al. (1992) used specific primers to achieve
high level DNA amplification in PCR experiments. However, the best approach for use
in developing countries with limited research facilities would be the use of DNA
hybridisation to check for viral DNA in plants and whiteflies (Czosnek et al., 1988;
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