Abstract
PhD
In South Africa, tomato (Solanum /ycopersicum) is an important vegetable crop with
considerable nutritional and economic value. Over the last decade, begomovirus
(family Geminiviridae) infections associated with an upsurge of the whitefly vector,
Bemisia tabaci, on tomato crops has become a serious threat to sustainable tomato
production in South Africa. Begomovirus disease control in tomato is challenging and
requires an integrated "pest" and "vector" management strategy, primarily based on the
use of chemical and cultural practices aimed at reducing the virus vector as well as the
use of resistant cultivars. Development of effective disease management practices for
South Africa therefore requires detailed information on the complex vector-virus-host
cropping system interactions. The aim of the study presented in this thesis was to
investigate the South African whitefly vector/begomovirus/tomato-host pathosystem,
with emphasis on the virus and vector diversity and distribution, and the identification of
possible resistance sources.
A survey of tomato-infecting begomoviruses was conducted during a six-year period
(2006-2011 ). Techniques used to determine begomoviruses diversity included whole
genome amplification using PCR, RCA (rolling circle amplification), conventional as
well as next generation sequencing and development of a RCA-RFLP (restriction
fragment length polymorphism) for rapid assessment of diversity. Sequence
comparisons and phylogenetic analyses revealed the presence of three new
monopartite begomovirus species, in addition to ToCSV, all of which belong to the
African/South West Indian Ocean (SWIO) begomovirus clade. Recombination analysis
indicated that all four tomato-infecting begomovirus species appear to be complex
recombinants and suggests that they have evolved within the sub-Saharan Africa
region, along with other African begomoviruses and that they are most likely
indigenous to the region. Several weed species were also confirmed as symptomless
begomovirus reservoirs, supporting their role in the emerging begomovirus epidemics
in South Africa.