Epidemiology of Cassava mosaic disease and molecular characterization of Cassava mosaic viruses and their associated whitefly (Bemisia Tabaci) vector in South Africa

摘要

Cassava mosaic disease (CMD) is caused by whitefly-transmitted geminiviruses and is audmajor constraint to cassava production in Africa. Field surveys were conducted in threeud(Bushbuckridge, Mariti and Tonga) cassava growing areas of Limpopo and Mpumalangaudprovinces in South Africa during two seasons (2004/2005 and 2005/2006). Resultsudshowed that a higher percentage (27.1%) of CMD infection was due to the use of infectedudplanting materials compared to whitefly borne-infections (10.4%). Disease symptomsudwere generally mild. There was no change in disease incidence over the surveyudperiod. Molecular characterization of cassava mosaic geminiviruses (CMG’s), usinguddifferential primer PCR, restriction fragment length polymorphisms (RFLP’s),udphylogenetic and recombination analysis and screening for satellite DNA’s.udDifferential primer PCR and RFLP’s showed that African cassava mosaic virusud(ACMV) was the most prevalent virus in South Africa and that mixed infections wereuda common occurrence. Phylogenetic analysis and RFLP’s showed the presence of aud‘new’ strain of ACMV in South Africa. EACMV isolates from this study showedudmore frequent recombination compared to ACMV isolates. None of the samplesudtested positive for satellite DNA’s. Phylogenetic analysis of Bemisia tabaci using theudmitochondrial cytochrome oxidase gene sequences revealed a ‘new’ sister clade of B.udtabaci that is closely related to the previously identified southern African clade andudthe presence of the Q biotype that groups with Q biotypes of NorthudAfrican/Mediterranean origin. Good cultural practices, introduction of resistantudcultivars and continuous monitoring are required to reduce the impact of CMD inudSouth Africa.