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

Incidence, distribution and characteristics of major tomato leaf curl and mosaic virus diseases
Dole (2003) reported that aluminium foil and mulches are effective in repelling insect
pests, and that black plastic mulches improve crop yield better than bare ground.
Mutwiwa et al. (2005) found that ultra-violet light absorbing plastics used as mulches or
green house roofing material repel whitefly pests when low UV-intensity plastics are
used. Other cultural options include timely planting, intercropping, and use of barrier
hedges (Simons et al., 1980). These methods effectively reduce tomato yield losses, virus
incidence and spread, even though they are simple and common.
2.1.4.2 Cross-protection
Cross-protection is the method of controlling virus disease by using a mild virus strain to
immunize otherwise healthy plants against a severe virus strain (Matthews, 1991). The
method has been used against ToMV and TSWV. Avgelis (1987) used a mild strain (MX
lV – l) of ToMV to cross-protect ToMV-susceptible tomato cultivars Earlypak and
Dombo C2 VF2 in Crete, leading to 14% yield improvement. He expected higher yields,
and as such attributed low increase in yield to higher than expected temperatures, which
could have reduced the effectiveness of mild strains. Another example of cross-protection
is when Min-Wang and Gonsalves (1992) and Cho et al. (1989) used nitrous acid to
induce mutation of TSWV to mutant R27G, which was put in tomato in order to control
severe TSWV strains from Hawai and Oklahoma.
This method requires more sophisticated facilities and careful management of the
outcome, which could otherwise result into more dangerous strains. With limited
resources in the south, one would to be very careful to recommend use of such a
technology, even though it could be required in some instances.
2.1.4.3 Host Plant Resistance
Use of resistant crop varieties is the most convenient and cost-effective control measure
of all (Hall, 1980; Gajos, 1981; Kumar and Irulappan, 1992; Nono-Womdim, 1993;
Rubio et al., 2003; Yang et al., 2004; de Castro et al., 2005). For example, planting of
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