Prevalence and identification of yam viruses responsible for seed yam degeneration in the Ejurasekyedumase and Atebubu-Amantin Districts of Ghana
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Date
November, 2018
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Abstract
Yam viruses are reported to be widespread in all areas where yam is grown. In Ghana, viral
diseases are known to cause about 50% of yield reduction on yam. It was for this reason that this
research was carried out to manage yam viruses in Ghana. Surveys were conducted in the 2015
and 2016 cropping seasons in two major growing districts, Ejura-Sekyedumase and AtebubuAmantin of Ghana to determine the prevalence of yam virus. There was the need to identify the
specific viruses producing the symptoms that were observed during the survey, as such
symptomatic leaf samples were taken for analysis at the laboratory using RT-PCR. Out of the 40
leaf samples collected from each district, Ejura-Sekyedumase District had six mixed infections
for both Yam mosaic virus (YMV) and Yam mild mosaic virus (YMMV) while eight single
infections were recorded for YMMV. Atebubu-Amantin District had 10 mixed infections for
both viruses and eight single infections for YMV. Molecular-based diagnostics techniques were
also employed to monitor the health status of seven plants (Diosecorea rotundata Poir)
established from positive selection, of which five of them did not amplify for any of the two
viruses tested while two amplified for both viruses that were tested. Seed yams (‘Dente’, ‘Pona’
and ‘Laribako’) selected in 2015 from symptomless or mildly infected plants (positive selection),
as well as those purchased from the Ejura market (farmer practice) and those selected from field
diseased plants were established in field experiments in 2016 and 2017 cropping seasons at Ejura
and Fumesua using a 3 x 3 factorial in randomized complete block design. The performances of
the three seed yam sources were compared for their reaction to yam mosaic virus infection and
tuber yield. The three white yam (D. rotundata) varieties used were; ‘Dente’, ‘Pona’ and
‘Laribako’. Plants raised from positive selection performed significantly (P<0.05) better with
least virus percentage infection and disease severity scores irrespective of the variety. Positiveiii
selection Dente out-yielded farmer practice Dente and diseased Dente by 35 and 66.7%
respectively in the 2016 cropping season at Ejura. Similar result was obtained at Fumesua with
positive selection Dente out-yielding farmer practice and diseased seed yams by 32.6 and 60.7%
respectively. In the 2017 cropping season, even though there was general yield reduction
indicating loss of seed yam quality with time, similar trend occurred with positive selection
plants performing significantly (P<0.05) better with least virus incidence and severity scores at
both locations. Positive selection Laribako produced the highest yield at both locations. With
farmers’ current practice of recycling seed yams from one season to another, this study showed
that positive selection was a good approach to reducing virus load in farmers’ farms as well as
reducing seed yam degeneration while maintaining fairly good yields.
Description
A thesis submitted to the Department of Crop and Soil Sciences, Kwame Nkrumah University of Science and Technology, Kumasi, Ghana, in partial fulfillment of the requirement for the Degree of Master of Philosophy in Crop Protection (Plant Virology).
Keywords
Prevalence, yam viruses, Ejurasekyedumase, Atebubu-Amantin Districts of Ghana