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

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mosaic virus (CMV), Alfalfa mosaic virus (AMV), Pepper veinal mottle virus (PVMV),
Potato virus Y (PVY), Potato virus X (PVX), and Tomato spotted wilt virus (TSWV).
Furthermore, three other viruses were identified, i.e. Chili veinal mottle virus (ChiVMV),
Tomato yellow leaf curl virus (TYLCV) and Tomato leaf curl virus (ToLCV). ChiVMV
(Brunt et al., 1990) is known to belong to genus Potyvirus, and causes moderate damage
to tomato. The two tomato yellow leaf curl viruses, i.e. TYLCV-UG and ToLCV-UG,
occur at high incidence in areas surveyed, and are causing important economic losses to
tomato producers. TYLCV–UG and ToLCV-UG belong to genus Begomovirus, whose
members are transmitted by a whitefly vector (Bemisia tabaci). TYLCV-UG is a strain of
TYLCV-Israel (Is) (Russo et al., 1980; Czosnek et al., 1988). ToLCV-UG was identified
as a begomovirus by using intergenic region and coat protein sequences (Brown, 1997). It
had a DNA sequence homology of 89% < 90% with African Tomato Leaf Curl Virus-
Tanzanian isolate (ATLCV-TZ; Chiang et al., 1996) for the 521 bp strand of the coat
protein (CP) gene and part of the intergenic region (IR). Still with the same virus
(ATLCV-TZ), but for the 482 bp sequence of the replication gene (Rep), ToLCV-UG had
a low 4% < 90% homology. This begomovirus had fairly high homology (85% < 90%)
with the East African Cassava Mosaic Virus-Malawi (EACMV-MW) isolate (Pita et al.,
2001) for the CP gene. However, both CP gene homology percentages were considered to
be below the required 90% nucleotide sequence identity (Padidam et al., 1995) for the
two viruses to be similar. TYLCV-UG was closely related to TYLCV-EG (Nakhla et al.,
1993) with a DNA sequence homology of 99% > 90% for the 277 bp of the intergenic
region. TYLCV-EG is a strain of TYLCV-Is. As such, it was the first time the
occurrence of these tomato viruses was established in Uganda. Based on its replication
gene and coat protein sequences, ToLCV-UG is different from other tomato leaf curl
viruses and is considered to be new in Uganda. Recent findings on tomato leaf curl
viruses in Uganda, which were based on complete sequence comparisons (Shih, et al.,
2006), confirmed our finding that tomato leaf curl virus (ToLCV-UG) is new.
Thirdly, field studies of the virus-vector relationship established that virus occurrence
varied in space and time, and with management practices, crop development stage, and