Abstract
Tomatoes (Solanum lycopersicum) are produced in all South African provinces and are
considered to be the second most important (economically) and popular crop. South Africa
produces an average of four hundred and fifty thousand (450 000) tons of tomatoes each year,
grossing approximately R1,6 billion (ZAR) in 2011. The tomato industry supplies jobs to 22
500 people, with approximately 135 000 dependents. A large portion of these workers are
minimum wage, low skilled labourers. Tomatoes are low in fat, free of cholesterol, a source of
fibre, carbohydrates, vitamins C and A, potassium and antioxidants. Like all plants, tomato is
subjected to attack by many potential pathogens, such as nematodes, bacteria, fungi and
viruses. Viral diseases of tomato can have a large impact on tomato production and can cause
large financial losses.
Viruses of the genus Begomoviridae (family: Geminiviridea) constitute an important group of
tomato pathogens and is currently the largest genus of all viral taxonomy (with 288 accepted
species). Infection often causes devastating symptoms of leaf chlorosis, leaf cupping, stunted
growth and reduced fruit set leading to large economic losses. Begomoviruses can be either
monopartite or bipartite and are transmitted by the whitefly vector Bemisia tabaci. Whiteflies
are polyphagous, plant-sucking insects with a very wide host range and as such are important
invasive agricultural pests. Within the B. tabaci cryptic species complex, some members are
extremely invasive, for instance Middle East - Asia minor 1 (MEAM1, also called B. argentifolii
and biotype B) and Mediterranean (MED, also called biotype Q), are known worldwide as
invasive. Some factors contributing to Begomoviridae currently being the largest viral
taxonomy group includes the invasiveness of some B. tabaci species, as well as their ability
to adapt to new host and new geographical regions. Another factor is the begomoviruses' well
known propensity for recombination, which is the driving force of begomovirus evolution. In
1997 a new monopartite begomovirus was found to be infecting tomatoes in South Africa,
causing devastating damage, that virus was named Tomato curly stunt virus (ToCSV).
Although ToCSV currently remains the predominant begomovirus species infecting crops and
weeds in tomato production areas in South Africa and Mozambique, many new species have
been detected. In 2008 a new begomovirus, named Tomato curly stunt Mooketsi virus
(ToCSMV), was detected in tomato plants in Mooketsi. In 2009 a second new begomovirus
was detected in Noordoewer, it was named Tomato curly stunt Noordoewer virus (ToCSNV).
And in 2010 another begomovirus was detected in Klawer, this viral isolate had previously
been detected in Lanseria and thus was named Tomato curly stunt Lanseria virus (ToCSLV).
In Chapter 2 these three new begomoviruses are characterized molecularly as well as
phylogenetically. This chapter also describes the construction of infectious clones of these
three viruses, which were used to study their symptom phenotype as well as fulfil Koch's...
M.Sc. (Biochemistry)