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

Abstract

Cassava mosaic disease (CMD) is caused by whitefly-transmitted geminiviruses and is a major constraint to cassava production in Africa. Field surveys were conducted in three (Bushbuckridge, Mariti and Tonga) cassava growing areas of Limpopo and Mpumalanga provinces in South Africa during two seasons (2004/2005 and 2005/2006). Results showed that a higher percentage (27.1%) of CMD infection was due to the use of infected planting materials compared to whitefly borne-infections (10.4%). Disease symptoms were generally mild. There was no change in disease incidence over the survey period. Molecular characterization of cassava mosaic geminiviruses (CMG’s), using differential primer PCR, restriction fragment length polymorphisms (RFLP’s), phylogenetic and recombination analysis and screening for satellite DNA’s. Differential primer PCR and RFLP’s showed that African cassava mosaic virus (ACMV) was the most prevalent virus in South Africa and that mixed infections were a common occurrence. Phylogenetic analysis and RFLP’s showed the presence of a ‘new’ strain of ACMV in South Africa. EACMV isolates from this study showed more frequent recombination compared to ACMV isolates. None of the samples tested positive for satellite DNA’s. Phylogenetic analysis of Bemisia tabaci using the mitochondrial cytochrome oxidase gene sequences revealed a ‘new’ sister clade of B. tabaci that is closely related to the previously identified southern African clade and the presence of the Q biotype that groups with Q biotypes of North African/Mediterranean origin. Good cultural practices, introduction of resistant cultivars and continuous monitoring are required to reduce the impact of CMD in South Africa.