Studies on sweet potato viruses from the major sweet potato growing agro-ecologies of Ghana

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Sweetpotato virus disease complex (SPVD) is the most destructive viral disease in Africa. It can cause yield loss up to 50%. In Ghana, not much work has been done on the identification and detection of sweetpotato viruses from the major sweetpotato growing agro-ecologies. A study was conducted to ascertain the incidence of sweetpotato viruses from the major sweetpotato producing areas and to ascertain the effects of sweetpotato virus diseases (SPVD) on the crop in Ghana. Sweetpotato viral disease samples were collected from all agro-ecologies in Ghana where the crop is grown and then preserved in the screenhouse for diagnostic purposes. Nitrocellulose membrane (NCM) enzyme-linked immunosorbent assay (ELISA), using specific virus antibodies and PCR techniques involving the use of specific and degenerate primers were used for the diagnostics. Virus diagnostics were done directly on virus-infected sweetpotato samples collected from the field and also on Ipomoea setosa indicator plants after they have been grafted with virus-infected sweetpotato collected from the various locations. In all, 127 samples were assayed. Effects of SPVD were assessed on three sweetpotato varieties, namely; ‘Dadanuei’, ‘Ligri’ and ‘Bohye,’ which are all varieties released by the CSIR-Crops Research Institute, Fumesua, Ghana. These were put under four levels of disease regimes; tissue culture cleaned and virus indexed planting materials, apparently healthy planting material collected from the field, virus infected planting material collected from field and artificially (using whiteflies) infected planting materials. There were four treatments and each treatment was repeated three times in a randomized complete block design (RCBD). Virus diagnostics, using NCM-ELISA, detected the following viruses; SPFMV (85.71%), SPCSV (16.67%), SPCaLV (6.35%), SPVG (4.76%), SPMSV (4.76%), SPCFV (1.57%) and CMV (3.97%). RT-PCR and PCR confirmed the detection of SPFMV and SPCSV as well as Begomoviruses in some of the samples. Several mixed infections were also detected in samples collected mostly from local varieties whilst the released varieties had mainly single virus infections. The study has also optimized serological detection (NCM-ELISA) and RT-PCR protocols for the effective diagnosis of sweetpotato virus isolates in Ghana. Across board, tissue culture cleaned virus-indexed planting materials of the three varieties produced the largest yield with a mean of 12.00 tons/ha, whilst artificially infected (whitefly inoculated) planting materials produced the least yield of 0.78 tons/ha. The study revealed planting tissue culture cleaned virus indexed planting materials can affect yield of the crop positively whilst it showed the usefulness in planting improved varieties, compared to the local varieties in term of virus infections.
A thesis submitted to The Department of Crop and Soil Sciences, Kwame Nkrumah University of Science and Technology, Kumasi, Ghana, in partial fulfilment of the requirement for the degree of Master of Philosophy in Crop Protection (Plant Virology), 2016