Identification of white yam viruses in Ghana and their effects on yield

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An experiment was conducted at Kwame Nkrumah University of Science and Technology and Crops Research Institute, both in Kumasi to identify white yam viruses in important white yam growing districts in Ghana and also assess their effects on yield of the crop. Ten yam-growing districts in the Brong Ahafo and Ashanti regions namely; Kumasi, Ejura/Mampong, Atebubu, Nkoranza, Kintampo, Wenchi, Techiman, Sunyani and Duayaw Nkwanta were surveyed. One hundred and seventy-six diseases leaf samples were randomly collected in equal representation from each district and then characterized using the Enzyme linked immunosorbent assay technique (TAS and PAS ELISA) with four already characterized yam virus antibodies supplied by the Virology unit of the International Institute of Tropical Agriculture, (IITA) namely: Yam mosaic potyvirus (YMV), genus potyvirus, Dioscorea alata potyvirus, (DaV or YVI-yam virus I) genus potyvirus, Dioscorea alata badnavirus, genus badnavirus and cucumber mosaic virus (CMV), genus cucumovirus. None of the samples tested positive to antibodies raised against cucumber mosaic virus and Dioscorea alata badnavirus. About 39% of the samples tested positive to YMV while 20.5% tested positive to DaV. Eleven percent (11.4%) of the samples reacted to both YMV and DaV. Fourty one percent (41%) of the samples did not react to any of the antibodies tested even though they had virus-like symptoms. Mixed infection of two viruses, namely; YMV and DaV or YV1 were found on inoculated and non inoculated experimental yam test plants when detected with antibodies homologous to them. These two viruses, YMV and DaV together, depending on the time of infection were found to reduce the yields of white yam under screen house conditions by as much as 54% and 28% respectively. Symptom severity also had a direct significant effect on yield. Plants with higher disease severity score produced lower yields when compared with symptomless plant. However, it is suggested that further studies be conducted to come out with possible ways to alleviate the effect of virus infection through the production of virus-free white yam planting materials or to apply a treatment that can reduce their effects.
A thesis submitted to the Department of Crop Science, Kwame Nkrumah University of Science and Technology in partial fulfilment of the requirements for the award of Master of Science, 2004