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

Incidence, distribution and characteristics of major tomato leaf curl and mosaic virus diseases 111
4.4 Discussion
Using intergenic region and coat protein sequences (Brown, 1997), it was possible to
generate meaningful pairwise comparisons and phylogenies, as well as deeper
understanding of the identities of the two geminiviruses, which we documented before in
Uganda. Our study findings show that the 482 bp sequence of the C1gene of ToLCV-UG
was less than 90% identical to other viruses used in the comparison and had 89% identity
to ATLCV-TZ. Furthermore, ToLCV-UG (521) bp segment for the coat protein gene and
parts of the intergenic region (IR) showed less than 90% identity to other viruses used in
the comparison, and 85% to the EACMV-MW. Since similar geminivirus strains have
identical coat protein gene, replicative gene and intergenic region (Padidam et al, 1995;
Brown, 1997), ToLCV-UG is very closely related to ATLCV-TZ and EACMV-MW, but
a different virus.
By contrast, the 277 bp sequence of the IR of TYLCV-UG was 96%-99% identical to
eight genbank yellow leaf curl viruses. TYLCV-EG was 99% similar to TYLCV-UG,
which is therefore taken to be a strain of the latter. Based on findings of Padidam et al.
(1995), a break-off percentage of 90% identity, and the fact that all related geminiviruses
have identical nucleotide sequences for the intergenic region, TYLCV-UG, TYLCV-EG,
TYLCV-Cu, TYLCV-JM, TYLCV-Mx, TYLCV-US, and TYLCV-LB are considered
similar strains of TYLCV-Is. Consequently, we can conclude that two identified
begomoviruses, i.e. TYLCV-UG and ToLCV-UG, are responsible for leaf curl symptoms
on tomato in Uganda. TYLCV-UG is a strain of TYLCV-Is, while ToLCV-UG can
tentatively be called a begomovirus related to ATLCV-TZ, which is a yellow leaf curl
virus of tomato in Tanzania, and EACMV-MW, which is a cassava mosaic virus reported
from Malawi and parts of western Kenya.
The International Committee for the Taxonomy of Viruses (Mayo, 2000) considered 36
members of family Geminiviridae and classified them into two vector-transmission