Cassava brown streak disease (CBSD) caused by two distinct ssRNA virus species (CBSV and UCBSV of genus Ipomovirus, family Potyviridae) and transmitted by whitefly (Bemisia tabaci), is a major constraint to cassava production in Africa, including Mozambique. In this research, two studies were conducted. First, in order to monitor the incidence, severity and geographical distribution of cassava brown streak disease and associated viruses in Mozambique, field surveys were performed in six cassava major growing provinces. A total of one hundred and fifteen fields and one hundred and forty six fields were surveyed in 2010 and 2012, respectively. The disease was only found in three of six provinces namely Zambezia, Nampula and Cabo Delgado. The CBSD incidence was highest (61.3% and 82.2% in 2010 and 2012, respectively) in Zambezia and lowest (23.6% and 35.1% in 2010 and 2012, respectively) in Cabo Delgado, with cultivars such as Cadri and Robero showing the highest susceptibility to CBSD, while Likonde and Amwalikampiche had relatively low CBSD incidence, illustrating some tolerance to the disease. The results, when compared to previous surveys conducted in 1999 and 2003, demonstrated that the disease is increasing, and replanting new fields with disease-affected cuttings could be responsible for the spread. The second aim of the study was to investigate the genetic diversity of Cassava brown streak viruses, based on analysis of partial sequences of the coat protein gene, in Mozambique. Collections of CBSD-symptomatic leaves were done between June 2010 and June 2012. Diagnostic RT-PCR, using specific primers to screen for the two species, revealed for the first time the presence of Uganda cassava brown streak virus (UCBSV) in Mozambique. UCBSV was found in mixed infections with CBSV, and only confined to a single province of Zambézia, while CBSV species were widely distributed. The phylogenetic analysis revealed two subgroups within CBSV, which were 6.7% divergent in nucleotide sequence. The heterogeneity observed among CBSV isolates in Mozambique suggests that in the future studies more sampling is needed to characterize strains and variants. Addtionally, sequencing of the full CP sequence of CBSaVs isolates is required, which may reveal even more diversity. Infectivity assays of cassava brown streak viruses (CBSV and UCBSV) were established using the host indicator plant Nicotiana benthamiana. Plant sap was extracted from infected cassava leaves and inoculated into N.benthamiana plants. CBSD-like symptoms were observed, and RT-PCR revealed the presence of CBSV in all samples, except for one which was co-infected with UCBSV and CBSV. This study provided further evidence that CBSaVs are efficiently transmitted to N.benthamiana. There is scanty information on alternative hosts, therefore more research is needed to identify other potential hosts of CBSaVs in order to develop an effective strategy to control CBSD.