Cassava Mosaic Virus Disease in Ghana: Distribution and Spread
Cassava is an important staple crop in most of the tropics, including Ghana. The productivity of the crop is beset with pest and disease attacks. With the emergence of virulent strains of the cassava mosaic virus (CMV), regular surveys are necessary to ascertain the prevalence of CMV and their whitefly vectors in farmers’ fields to help manage CMV disease affecting the crop. Field surveys were conducted in September and October of 2015 and December 2016 to January 2017 using a harmonized sampling protocol developed by the West African Virus Epidemiology (WAVE) for root and tuber projects. Three hundred and ninety-three fields were visited throughout Ghana and 11,760 cassava leaf samples examined. Whiteflies were counted on 5 plants/field. Diseased samples with varying symptoms collected were assayed using PCR and genomic sequencing. Cassava mosaic disease (CMD) symptoms were recorded in about ninety-six percent (96.4%) of fields surveyed with varying severity. These symptoms included leaf mosaic, leaf distortion/twisting, malformation, filiform leaves, stunting and chlorosis. Cultivars with red petiole colour were the most prevalent while those with green petiole colour were the least. No whitefly was found on cultivars with purple and Green petioles while cultivars with reddish-green petioles had highest count of whiteflies/plant. The Upper West and Upper East regions had the least amount of whiteflies/plant. The Ghanaian isolates clustered with the East African cassava mosaic Cameroon virus (EACMCV) isolates in a cluster analysis. BLASTn analysis of 513 bp fragment of the DNA-B of Ghanaian isolate GH07216 showed 89.9% similarity with EACMCVGhana isolates and 90.54% identity with the EACMCV-Ivory Coast. Similarly, the Ghanaian isolate GH07216 showed 95.8% nucleotide sequence identity to the EACMCV-Ghana isolate and 94.22% to the EACMV-Ivory Coast isolate. Nucleotide sequences of DNA-A of the Ghanaian isolates were less variable: between 95.90-96.73% when compared to already published sequences to a range of CMG sequences available on the GenBank. The study has updated the existing literature on CMD incidence which can contribute to the development CMD-resistant cassava varieties in Ghana.