Epidemiological and molecular characterisation of cassava mosaic geminiviruses and the their effect on yield of cassava in Zambia
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An epidemiological and molecular characterization study of cassava mosaic disease was carried out in four cassava growing Provinces of Zambia in 2013. The objectives of the study were to determine the incidence and severity of Cassava mosaic disease (CMD) in the study areas; to determine molecular diversity of Cassava mosaic geminiviruses (CMGs); to evaluate the effect of CMD on yield of selected cassava genotypes. The study involved survey of CMD in four cassava producing provinces, virus isolate characterization using the polymerase chain reaction (PCR) and sequencing of purified PCR products and evaluation of nine landrace and improved cassava genotypes to CMD. The results from the survey showed CMD incidence and severity was 61.5 % and 2.7, respectively. The results also showed that common transmission of the disease was due to cutting infection (57.7 %) compared to whitefly infection (3.8 %). The polymerase chain reaction showed single infections of African cassava mosaic virus (ACMV) in 34.7 % of the positive samples while East African cassava mosaic virus (EACMV) occurred in 4.5 % of the positive samples. The dual infections of ACMV and EACMV were detected in 60.8 % of the positive samples. Restriction fragment length polymorphisms (RFLP‟s), complimented by sequence analysis done by multiple sequence analysis established the existence of different CMG isolates in Zambia. The sequence identity analysis and phylogenetic analysis displayed high nucleotide (nt) sequence identities amongst one another with the isolate sequences showing identities of 89 % to 99 % within the coat protein (CP) gene of ACMV. The Zambian isolates in this study showed substantial homology with sequences of ACMV-UGMild Uganda (AF126800.1), ACMV-UGSvr Uganda (AF126802.1), ACMV-[MG:MG310A1] Madagascar, and ACMV-CM39 Cameroon (AY211462.1) with sequence identities of 97 %, 97 %, 97 % and 98 %, respectively. However, the Zambian isolates showed greater variability within the EACMV species nucleotide sequence divergence ranging between 77 % to 99 %. The isolates showed similarity to the Kenyan (EACMV-KE), Malawian (EACMMV) and Tanzanian (EACMV-TZT) with sequence identities of 96 %, 90 % and 96 %, respectively. Among the genotypes evaluated for CMD response, Manyopola and Bangweulu were found to be susceptible (3.5) whilst Kampolombo was resistant (1.7). Mweru had the highest root yield (1.6 kg plant-1) while Kapumba, the second most susceptible genotype, had the lowest root yield (0.2 kg plant-1). The current study showed slight epidemiological variation from previous results in same areas. This underlines the importance of continuous evaluation of the disease in order to develop effective control measures.
The University of Zambia