Thesis (M.Sc.)--University of the Witwatersrand, Faculty of Science, School of Molecular and Cell Biology, 2001.Cassava mosaic disease is prevalent in Africa and significantly affects the
growth and yield of cassava. This disease is caused by a number of whiteflytransmitted
begomoviruses. The aims of this study were to establish the
identity of cassava begomoviruses in southern Africa and to develop assays
for their differentiation.
Using primers that target the highly conserved core region of the coat protein
gene it was possible to identify and establish the geographical distribution
and relatedness of cassava begomoviruses in 6 countries within southern
Africa. It was found that African cassava mosaic virus occurred in five
countries (except Angola), East African cassava mosaic virus was present in
all countries (except Zambia) and South African cassava mosaic virus was
present in South Africa and Swaziland. In addition, this study reports for the
first time in southern Africa, the Ugandan variant virus (UgV) which occurs
frequently in mixed infections with other cassava-infecting begomoviruses.
Bemisia tabaci (Gennadius) is the vector of begomoviruses that cause
cassava mosaic disease (CMD) in Africa and India. The taxonomy of the B.
tabaci complex is problematic, making it unclear whether more than one
variant or 'type' of the vector is involved in the transmission of cassavainfecting
begomoviruses. Phylogenetic analysis of mitochondrial COl gene
sequences revealed that B. tabaci colonising cassava in Africa form 3 distinct
clades (clade 1: Mozambique, South Africa, Swaziland, Zambia; clade 2:
Cameroon; clade 3 : Zimbabwe). These results indicate that topotypes within
B. tabaci vector populations from cassava exist and suggest a geographic
basis for the separation of cassava-colonising B. tabaci in Africa.
New cassava viruses and viral strains continue to be discovered and simple,
rapid and sensitive techniques are needed for screening of cassava
plantations. Here we report on the development of a heteroduplex mobility
assay (HMA) for differentiating cassava-infecting begomoviruses. The HMA
profiles were able to differentiate four different viral species and eleven
different virus strains, and showed a good correlation with sequencing results
and phylogenetic comparisons with other sequenced cassava viruses. This
technique was found to be sensitive and rapid and had the added advantage
of being able to detect mixtures of viruses in field-grown cassava.
Current serological methods and antibodies are limited in their usefulness
and specificity and new antibodies need to be developed to detect all the
possible viral species. The viability of using phage antibodies to detect
begomoviruses proved promising as a number of phage clones were isolated
and characterised. These clones, when used in combination, were able to
differentiate between several cassava-infecting begomoviruses. However a
number of improvements on this technique would need to be implemented
before it became an acceptable method for producing antibodies to identify
and distinguish between cassava begomovirus species and strains
