The research seeks to understand the virus-host plant interactions for cassava brown streak disease (CBSD) caused by two viruses, Cassava brown streak virus (CBSV) and Ugandan Cassava brown streak virus (UCBSV) of the genus Ipomovirus, family Potyviridae. The diversity of six CBSD isolates from the endemic (Kenya, Malawi, Mozambique and Tanzania) and the recently developed epidemic areas (Uganda) of the disease in eastern Africa was studied. Five cassava varieties differing in virus resistance levels; Albert, Columbian, Ebwanateraka, TMS60444 (all susceptible) and Kiroba (tolerant) were graft-inoculated with the UCBSV and CBSV isolates. Based on a number of parameters, the isolates can be grouped into two main categories; severe and milder forms.
Transmission of viruses using non-vector modes confirmed that CBSV was sap transmissible from cassava to cassava. Graft-inoculation of infected scions onto CBSD-free cassava plants was the most efficient mode of transmission which resulted in 80 and 100% rate for UCBSV and CBSV respectively. The two virus isolates were not transmitted through contaminated tools and hands. The effect of host-tolerance on virus was investigated in a long-term experiment where three cassava varieties Albert, Kiroba and Kaleso (field-resistant to CBSD) were graft-inoculated with UCBSV and CBSV. The three cassava varieties showed differences in virus movement, symptom development, severity and relative virus titres.
The mechanisms of resistance to CBSD were investigated by making cuttings, from various parts of the plants, and a greater number of disease-free plants were generated from cuttings made from Kaleso than Kiroba and Albert. The fecundity of B. tabaci and its ability to transmit the virus were determined and results indicated no significant differences in the ability of the three cassava varieties to support whitefly development.
Finally, thermal and chemical treatments of tissue cultured plants were conducted and the combinations of both treatments produced the greatest number of disease-free plants in all three varieties; Kaleso (50%), Kiroba (44%) and Albert (35%). The information generated in this thesis has greatly improved our understanding of the interactions between the three biotic factors; the host, virus and vector in the CBSD-pathosystem, which would be highly useful in designing effective disease management strategies
