This report covers two projects, one on sweet potato viruses and one on cassava brown streak disease. They were linked through both being whitefly-transmitted viruses of root crops. The sweet potato virus disease (SPVD) project identified the whitefly-borne component of this disease in East Africa to be a serologically and genomically distinct strain of sweet potato chlorotic stunt closterovirus (SPCSV). Characteristic symptoms of the two main diseases (SPCSD & SPVD), caused by SPCSV in sweet potato in either the absence or presence of sweet potato feathery mottle virus, have been described and an ELISA-based method for detecting SPCSV has been developed and proven in Africa. Polyclonal antiserum has been developed using bacterially expressed coat protein. The strain of SPCSV found in Uganda has been shown to occur also in Kenya, Tanzania, Zambia and Madagascar. Two serotypes possessing small differences in their coat protein and HSP70 homologue genes were detected and their symptomatologies and distributions in Uganda described. Differences in the incidence of SPVD in Uganda and Tanzania have been demonstrated and mapped in districts located around the perimeter of Lake Victoria. Areas where SPVD was rare tended to have few whiteflies throughout the year or a very SPVD-resistant cultivar predominated. Given the importance of sweet potato to rural livelihoods and the major yield losses incurred annually in sweet potato in many areas of East Africa, the project will contribute substantially to increasing crop yields in East Africa as superior resistant varieties are developed. The causal agent of cassava brown streak disease was found to be a single virus (CBSV) and a member of the Potyviridae family. CBSV is most closely related to the whitefly-transmitted virus, sweet potato mild mottle virus (SPMMV). These two viruses now form the only two sequenced members of a genus called the Ipomoviruses. CBSV was shown to not react with antiserum raised to members of the Potyviridae family, including SPMMV. The first reliable test for this disease, a PCR detection method, has been developed and is described here. Three distinct isolates of CBSV have been found in Tanzania and one in Mozambique. These isolates explain the different symptoms reported here and previously with secondary host plants. A construct for expressing the coat protein of the virus has been built and the coat protein of the virus purified. The subsequent polyclonal antiserum proved to be of poor quality and unsuitable for use with infected cassava. A MAP peptide has been made to 15 amino acids of the virus coat protein and polyclonal antiserum to this peptide is being made. Two binary vectors have been built suitable for plant transformation. These constructs deliver identical T-DNA regions to the plant, which express the coat protein of the virus in the antisense orientation under the control of the 35S promoter. Tobacco plants containing these constructs have been generated but further testing for resistance to the virus was not possible due to lack of time.