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

SUMMARY
Tomato is economically important in Uganda whereby each of about 3 million
households consume tomato in their sauce every meal. However, yields per hectare are
still very low, i.e. 10 ton/ha (Ministry of Agriculture, Animal Industry and Fisheries
(MAAIF) estimate for 1998 to 2000 unpublished). Low tomato yields in Uganda are
mostly due to poor agronomic practices, lack of improved varieties, which are well suited
for high yield and resistance to diseases, and also due to damage caused by pests and
diseases (Varela, 1995; Hansen, 1990; Defrancq, 1989). As such, continued prevalence of
viral diseases is considered to be the third major cause of low tomato productivity in
Uganda after Phytophthora infestans blight and Ralstonia solanacearum bacterial wilt.
As it is, this thesis presents the first research study on major leaf curl and mosaic virus
diseases incidence, distribution, and characteristics in Uganda.
The study addressed these issues by firstly screening tomato viruses occurring in tomato
fields and providing detailed information on genetic identity and vector relationship of
one main virus disease, i.e. tomato yellow leaf curl virus (sensu lato) disease. To generate
this information, three major activities were implemented. First, a survey was conducted
in eight major tomato growing districts of Uganda, representing five agro-climatic zones.
Virus-like symptoms were found on tomato plants in all eight districts surveyed. Two
major categories of symptoms were encountered in surveyed tomato fields, i.e. leaf curl,
and mosaic or mottling. Secondly, leaf samples taken from suspected virus-infected
tomato plants were analysed to get unequivocal identification of viruses that occur in
tomato in areas surveyed. A number of preliminary bioassays, such as indicator plant
inoculation, and mechanical transmission as well as grafting, were conducted to confirm
that leaf curl, mosaic and mottling symptoms were indeed due to virus infection.
Serological and molecular virus identification tests were conducted using (1) antisera
against ribonucleic acid (RNA) tomato viruses reported elsewhere in Africa; (2) general
probes against deoxyribonucleic acid (DNA) geminiviruses; (3) specific probes against
tomato yellow leaf curl viruses; and (4) general and specific primers in polymerase chain
reaction (PCR). Viruses identified included Tomato mosaic virus (ToMV), Cucumber
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