| Summary: | Yam (Dioscorea spp.) is one the most important food crops cultivated in the West
African yam zone comprising Cameroon, Côte d'Ivoire, Ghana, Nigeria, Benin and
Togo, which account for over 90% of the 4.59 million hectares of yam cultivation
worldwide. Viral pathogens are amongst the most important factors threatening yam
production and productivity and safe movement of germplasm. Cucumber mosaic
virus (CMV, genus Cucumovirus), Dioscorea bacilliform alata virus (DaBV, genus
Badnavirus), Yam mosaic virus (YMV, genus Potyvirus) and Yam mild mosaic virus
(YMMV, genus Potyvirus) have been reported to infect yam in West Africa. More
recently a second bacilliform virus, Dioscorea sansibarensis bacilliform virus
(DsBV), isolated from Dioscorea sansibarensis in Benin was characterized. DaBV
and DsBV are serologically related. Besides, occurrence of several uncharacterized
isometric, flexuous rod shaped and bacilliform viruses have been described in the
literature. Information on the occurrence and distribution of yam viruses is limited to
Nigeria and Côte d'Ivoire. In this study, surveys were conducted to document viruses
occurring in major yam producing zones in Benin during 2004 and 2005, and in
Ghana and Togo during 2005. Analysis of a total of 1632 yam leaves obtained from
these surveys by enzyme-linked immunosorbent assay (ELISA), immunocapture
polymerase chain reaction (IC-PCR) and/or IC-reverse transcription-PCR (IC-RTPCR)
revealed the occurrence of CMV, YMV, YMMV, yam-infecting badnaviruses
(DaBV and/or DsBV), but not Dioscorea mottle virus (DMoV, Como-like virus).
Incidence of the yam-infecting badnaviruses was highest, followed by YMV and
YMMV. Incidence of CMV was lowest, but this finding is the first official report of
its occurrence in these countries. Polyclonal antibodies against a yam isolate of CMV
and an isolate of yam-infecting badnavirus from a D. alata in Nigeria were produced
in rabbits. CMV antisera had an end-point titre of 1:25,600 and 1:64,000 by PAS- and
ACP-ELISA, respectively; whereas badnavirus antisera had a titre of 1:1280 using both the methods. The antibodies of each virus detected homologous antigen in
infected yam leaves from Ghana, Togo, Benin and Nigeria. Diversity among 19
badnavirus isolates representing diverse production zones in Ghana, Togo, Benin, and
Nigeria was assessed by analyzing nucleotide sequence of partial RT/RNaseH-coding
region. Homology between the deduced amino acid sequences of the 19 isolates
ranged from 73 to 100%. Phylogenetic analysis revealed that the badnavirus isolates
analyzed in this study correspond to two distinct species, Dioscorea alata bacilliform
virus (DaBV) and Dioscorea sansibarensis bacilliform virus (DsBV). One isolate
from Benin had 77% and 75% amino acid identities with DaBV and DsBV,
respectively and may represent a third species. This result suggests the occurrence of
three different bacilliform viruses in yam in West Africa. Information on the
aetiology, geographical distribution and variability of viruses naturally occurring in
yam in Ghana, Benin and Togo and the diagnostic tools developed in this study
would enhance germplasm monitoring and disease control efforts.
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