No abstract

Cassava is a major factor in food security across sub-Saharan Africa. However, the crop is susceptible to losses due to biotic stresses, in particular to viruses of the genus Begomovirus (family Geminiviridae) that cause cassava mosaic disease (CMD). During the 1990s, an epidemic of CMD severely hindered cassava production across eastern and central Africa. A significant influence on the appearance of virus epidemics is virus diversity. Here, a survey of the genetic diversity of CMD-associated begomoviruses across the major cassava-growing areas of Kenya is described. Because an initial PCR-restriction fragment-length polymorphism analysis identified a much greater diversity of viruses than assumed previously, representative members of the population were characterized by sequence analysis. The full-length sequences of 109 components (68 DNA-A and 41 DNA-B) were determined, representing isolates of East African cassava mosaic virus and East African cassava mosaic Zanzibar virus, as well as a novel begomovirus species for which the name East African cassava mosaic Kenya virus is proposed. The DNA-B components were much less diverse than their corresponding DNA-A components, but nonetheless segregated into western and eastern (coastal) groups. All virus species and strains encountered showed distinct geographical distributions, highlighting the importance of preventing both the movement of viruses between these regions and the importation of the disease from adjacent countries and islands in the Indian Ocean that would undoubtedly encourage further diversification.

Cloned DNA-A and DNA-B components of Kenyan isolates of East African cassava mosaic virus (EACMV, EACMV-UG and EACMV-KE2), East African cassava mosaic Kenya virus (EACMKV) and East African cassava mosaic Zanzibar virus (EACMZV) are shown to be infectious in cassava. EACMV and EACMKV genomic components have the same iteron sequence (GGGGG) and can form viable pseudorecombinants, while EACMZV components have a different sequence (GGAGA) and are incompatible with EACMV and EACMKV. Mutagenesis of EACMZV has demonstrated that open reading frames (ORFs) AV1 (encoding the coat protein), AV2 and AC4 are not essential for a symptomatic infection of cassava, although mutants of both ORF AV1 and AV2 produce attenuated symptoms in this host. Furthermore, ORF AV1 and AV2 mutants were compromised for coat protein production, suggesting a close structural and/or functional relationship between these coding regions or their protein products.

Progress curves of cassava mosaic virus disease (CMD) and populations of the whitefly vector (Bemisia tabaci) were assessed using four cassava varieties grown alone and as a random mixture in two experiments established under epidemic conditions at a site near Kampala in southern Uganda. There were significant differences in final CMD incidence and in the areas under the disease progress curves between varieties when grown alone and as a mixture in both experiments. Variety Ebwanateraka had the highest incidence and SS4 the lowest, even though it supported the largest populations of adult whiteflies. The overall incidence of CMD in the mixture was similar to that in pure stands of the partially resistant Nase 2 and greater than in the resistant Migyera and SS4. Compared to pure stands, incidence of CMD in each component of the mixture was reduced significantly only in Ebwanateraka, whereas vector populations were less only in SS4 and Nase 2. On several observation dates the actual incidence of CMD and populations of adult whiteflies in the mixture were significantly less than expected values estimated from the results for the four varieties when each was grown alone. A highly significant positive relationship was established for each variety between peak populations of adult whitefly and leaf area index at the time. The implications of the findings and the scope for future research on the use of varietal mixtures for the management of CMD are discussed.