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

Incidence, distribution and characteristics of major tomato leaf curl and mosaic virus diseases
c-TYLCV, suspected to be a problem in Uganda, is not different from tomato yellow leaf
curl viruses (sensu lato) occurring elsewhere in the world, and these relationships can be
established using partial sequences (Brown, 1997).
d-Tomato leaf curl viruses are among the major viruses on tomato and their incidence and
spread are influenced by the occurrence and population dynamics of the whitefly vector
(Bemisia tabaci) (Moriones and Navas-Castillo, 2000), as well as weather conditions in
the agro-ecosystem.
e- TYLCV control is possible through whitefly vector management using knowledge of
its population dynamics throughout the year, cultural practices and chemical control in an
integrated package (Chan and Jeger , 1994; Perring et al., 1999).
1.1.5 Description of the Thesis
This thesis has six chapters. Chapter one is the general introduction, which gives the
background, research problems, objectives and hypothesis. Chapter two deals with an
overview of tomato viruses with specific attention to geminiviruses, their transmission
and management. In chapter three, we focus on solving identified problem(s) by
generating information on causal organisms of observed viral disease symptoms. Based
on the literature review in chapter two, a report on virus-symptom diversity, identification
and single or mixed incidence of causal tomato viruses in Uganda, is given. Furthermore,
attention is drawn to some new viruses and alternative hosts identified in Uganda. With
leaf curl as the most prevalent symptom, and seemingly causing more loss, chapter four
goes deeper into characterization of tomato leaf curl and associated mosaic or mottling
diseases, which could not be identified using serological techniques. Two viruses,
TYLCV-UG and ToLCV-UG, which were identified using molecular techniques, are
analyzed and discussed here. Comparison with other geminiviruses whose sequences
exist in the European Molecular Biology Laboratory (EMBL), UK and in the National
Center for Biotechnology Information (NCBI) GenBank, USA is expounded on, and
ToLCV-UG is presented as a new begomovirus. Given that geminivirus-associated leaf
curl symptoms are widespread and their known vector Bemisia tabaci is common,
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