Survey of cowpea viral disease symptoms and detection of associated viruses in selected cowpea growing areas in Ghana

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JUNE, 2016
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A survey was conducted in 2014/2015 growing seasons covering 100 fields within four locations in Ghana. Cowpea (Vigna unguiculata (L.) Walp) plants showing leaf mosaic and other virus-like symptoms, were noticed during the 2014 growing season in fields located at Mampong, Ejura- Sekyeredumasi, Nkoranza (Humid forest zone) and Amantin-Atebubu (Derived savannah zone) in the Ashanti and Brong Ahafo Regions of Ghana. Incidence and severity of some viral symptoms as well as farmers’ perception of viral diseases in the surveyed areas were obtained with the aid of survey sheets and questionnaires. Symptomatic leaf and mature seed samples were collected from each location for virus identification in the laboratory. Seed and Aphid transmission tests, Mechanical (sap) transmission, ACP-ELISA and RT-PCR were used for virus detection. The survey showed that farmers cultivate virus-infected cowpea seeds season after season, thus causing high incidence and severity of viral diseases. High incidence and severity of viral diseases were observed in the Ejura-Sekyeredumasi District where most farmers in the other districts obtained seeds for cultivation. Mosaic and mottling were the commonest symptoms observed. The highest incidence (81.6%) and mean severity (3.01) values of virus symptoms were observed in Ejura. Percent incidences (72.5 and 70.7%) and severities (2.7 each) recorded at Atebubu and Mampong, respectively were not significantly different (P>0.01). Nkoranza recorded the lowest incidence (46.7%) and severity (2.4) of cowpea virus symptoms. Viruses detected in the leaf and seed samples serologically were: Cowpea Aphid Borne Mosaic Virus (CABMV), Bean Common Mosaic Virus strain Blackeye Cowpea Mosaic Virus (BCMV-BICM), Cowpea Mottle Virus (CPMoV), Cowpea Mild Mottle Virus (CPMMV), Southern Bean Mosaic Virus (SBMV), Cowpea Yellow Mosaic Virus (CYMV) and Cucumber Mosaic Virus (CMV). The viruses were detected infecting cowpea plants in all the four districts surveyed. BCMV-BICM was detected to be seed borne with transmission rates between 0.8 and 27%. Aphis craccivora Koch. also transmitted BCMV-BICM in a non-persistent manner. In all cases, only symptomatic seedlings were found infected with the virus. Also, systemic infections were observed on mechanically inoculated ‘Ife brown’ cowpea plants. The study identified seven (7) cowpea viruses in the country of which six (6) are reported to be seed-borne. This, therefore, necessitates the need for the production and use of virus-free seeds, development of virus resistant genotypes and adoption of efficient seed certification systems.
A thesis submitted to The Department of Crop and Soil Sciences, Faculty of Agriculture, Kwame Nkrumah University of Science and Technology, Kumasi, in partial fulfilment of the requirements for the degree of Master of Philosophy in Crop Protection (Plant Virology).