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

Incidence, distribution and characteristics of major tomato leaf curl and mosaic virus diseases 151
These viral disease symptoms were not specific to a particular virus. For example,
combinations of two to five viruses were encountered in individual tomato plants. In one
experiment, it was observed that PVMV alone caused no visible significant damage to the
crop, but co-existence with ChiVMV in the same plant showed severe and synergistic effects
on tomato crop performance.
Viruses responsible for those symptoms observed on tomato were reported for the first time,
in Uganda, i.e. RNA viruses ChiVMV, PVMV, AMV, CMV, TSWV, PVY and PVX, as well
as begomoviruses ToLCV-UG and TYLCV-UG (Chapter 3). However, tomato plants infected
with the latter two DNA viruses had characteristic small leaves, with marginal yellowing and
leaf curl symptoms. ToLCV-UG seems to be a new begomovirus different from tomato
yellow leaf curl viruses (sensu lato) found elsewhere in the New and Old Worlds. Whereas
this finding was based on intergenic sequence comparison results (Padidam et al., 1995), later
it was confirmed in our other research experiments conducted in collaboration with Dr. Green
of the World Vegetable Center, Taiwan (Shih et al., 2005), which were based on a complete
DNA sequence as was recently recommended by ICTV (Fauquet et al., 2003).
Field studies of the virus-vector relationship established that virus occurrence varied in space
and time, and with management practices, crop development stage, and weather conditions. A
negative relationship (R = -0.14, p = 0.04) was established between number of plants infected
with TYLCV (sensu lato) and percentage marketable tomato yield. On the other hand,
Bemisia tabaci, the vector of TYLCV (sensu lato) showed a variable population, which
depended on micro-climatic conditions in the agro-ecosystem, with high populations
prevailing during the dry season and decreasing with the onset of rain, and in turn influenced
tomato yellow leaf curl virus disease incidence. The more mature the tomato crop, the less it
was infested with whiteflies (R = -0.5, p < 0.0001), for whiteflies prefer tender leaves, which
are found on young tomato plants (Nono-Womdim et al., 1996). Therefore, variation in date
of planting could be used in management of both whiteflies and TYLCV (sensu lato).
Furthermore, an integrated package of uprooting TYLCV disease symptom bearing plants and
application of the insecticide dimethoate was found to be the most effective of the six