An overview of viruses infecting Dioscorea yams in sub-Saharan Africa by els87028

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									Plant virology in sub-Saharan Africa




  An overview of viruses infecting Dioscorea yams
               in sub-Saharan Africa

                    L. Kenyona, S.A. Shoyinkab, J.d’A. Hughesc, and B.O. Oduc
                a
                Natural Resources Institute, Central Avenue, Chatham, Kent, ME4 4TB, UK
            b
             Institute of Agricultural Research and Training, Obafemi Awolowo University
                                   Moor Plantation, Ibadan, Nigeria
    c
     International Institute of Tropical Agriculture, c/o L.W. Lambourn and Co., 26 Dingwall Road,
                                        Croydon CR9 3EE, UK


                                            Abstract
Viruses of the genera Potyvirus, Potexvirus, Badnavirus, Cucumovirus, and Carlavirus
infect yams of different species worldwide. These viruses cause a range of symptoms
including mosaics, mottle, vein clearing, chlorosis, stunting, and distortion. These lead
to a chronic and sometimes severe disease situation in all yam growing areas.
   In sub-Saharan Africa, where more than 90% of the world’s yams are produced, Yam
mosaic virus (YMV), genus Potyvirus is an ubiquitous pathogen. It is the only well-
characterized virus infecting yams. Other viruses infecting yams in sub-Saharan Africa
are Dioscorea alata virus (DAV), genus Potyvirus; Dioscorea alata bacilliform virus
(DaBV), genus Badnavirus; Cucumber mosaic virus (CMV), genus Cucumovirus; and
Dioscorea dumetorum virus (DdV), genus Potyvirus. Dioscorea mottle virus (DMoV),
a possible member of the genus Comovirus, was recently isolated from Dioscorea
alata in Nigeria.
   Transmission studies show that YMV, DAV, DaBV, and CMV are mechanically
transmissible between yam plants. All these viruses are transmitted by aphids, except
DaBV, which is transmitted by mealybugs (Planococcus citri). Although tuber yield
losses have been attributed to virus infection, the influence of these viruses on yield
and growth of the yam plant has not been well studied. Tremendous improvement has
been achieved in the detection of yam viruses with the development of highly sensitive
and more specific diagnostic techniques such as enzyme-linked immunosorbent assays
(ELISA), immunosorbent electron microscopy (ISEM), and polymerase chain reaction
(PCR). The reliability of these diagnostic tools has enabled the production of virus-tested
in-vitro germplasm which can be safely distributed internationally.



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                                        An overview of viruses infecting Dioscorea yams


                                       Résumé
Les virus des genres Potyvirus, Potexvirus, Badnavirus, Cucumovirus et Carlavirus
s’attaquent à différentes espèces d’igname de par le monde. Ils sont responsables de
divers symptômes dont la mosaïque, la marbrure, l’éclaircissement des nervures, la
chlorose, le rabougrissement et la déformation. Ceux-ci sont à l’origine d’affections
chroniques, voire graves dans toutes les zones de culture d’igname.
En Afrique subsaharienne où plus de 90% des ignames du monde sont produites, le
virus de la mosaïque de l’igname, YMV du genre potyvirus est un pathogène omni-
présent. C’est le seul virus de l’igname qui ait été bien caractérisé. Les autres virus
qui infectent l’igname en Afrique subsaharienne sont : le virus de Dioscorea alata
(DAV), du genre Potyvirus; le virus bacilliforme de Dioscorea alata (DaBV), du genre
Badnavirus; le virus de la mosaïque du concombre (CMV), du genre Cucumovirus et
le virus de Dioscorea dumetorum (DdV), du genre Potyvirus. Le virus de la marbrure
de Dioscorea (DMoV), membre possible du genre Comovirus, fut récemment isolé de
Dioscorea alata au Nigeria.
    Des études conduites sur la transmission de ces virus montrent que YMV, DAV, DaBV,
et CMV se transmettent de façon mécanique d’un pied d’igname à un autre. Tous les
virus sont transmis par des pucerons, sauf le DaBV qui est transmis par les cochenilles
(Planococcus citri). Bien que les pertes de rendement en tubercules soient attribuées
aux viroses, l’impact de ces virus sur le rendement et le développement de l’igname n’a
pas encore fait l’objet d’une étude approfondie. Des améliorations considérables ont été
obtenues dans la détection des virus de l’igname avec la mise au point de techniques
de diagnostic très sensibles et plus spécifiques telles que l’essai par l’immunosorbant
lié à une enzyme (ELISA), la microscopie électronique immunosorbante (ISEM), et la
réaction en chaîne polymérase (PCR). Très fiables, ces outils de diagnostic ont permis
la production de matériel végétal in vitro, testé contre les virus et pouvant être distribué
sans risque à l’échelle internationale.

                                    Introduction
Yams are the starchy tubers produced by various Dioscorea species (Dioscoreaceae).
They are a traditional crop and an important staple food over much of sub-Saharan
Africa. Of the 600 or so species of Dioscorea recognized, only about 10 species are
grown for food in different tropical and subtropical parts of the world. About 30 other
species are grown on a small-scale for extraction of the pharmaceutical compounds
dioscorin and diosgenin. Species of Dioscorea are native to a wide range of tropical
and subtropical ecologies from semiarid scrub land to tropical rain forests. Most are
climbing vines found in tropical forests and forest margins. Approximately 95% of all


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food yam production occurs in the equatorial “yam-belt” of sub-Saharan Africa, with
about three quarters of this being confined to the south and central states of Nigeria.
   The preferred yam species in West Africa is the white yam (D. rotundata) since this
generally produces tubers with a high dry-matter content that is good for pounding.
This is thought to be a tetraploid (2n = 40) which resulted from hybridization between
weedy species (Dumont and Vernier 2000). Some authorities regard the yellow yam
as a separate species (D. cayenensis), while others group it with the white yam in
the D. rotundata–cayenensis complex. There is some evidence that domestication
of this species (complex) is an ongoing process since in some countries (e.g., Benin
Republic), there appears to be a replenishment of the germplasm of cultivated types
by the collection of wild types from forests (Berthaud et al. 2001).
   Water yam (D. alata, native to southeast Asia) is gaining popularity in sub-Saharan
Africa because it is easier to propagate than the native white yam. However, it is less
acceptable to many consumers because of the higher water content of the tubers, which
means it cannot be pounded to produce fufu of the right consistency. Other species grown
(or sometimes harvested from the wild) for other culinary uses in West Africa include
the aerial yam (D. bulbifera) and the bitter yam (D. dumetorum).
   Most of the edible yam species are relatively infertile and if or when they do set true
seed, most seed is not viable. Thus, most propagation and multiplication of yam is by
vegetative means through the planting of small tubers (seed yams) or pieces of tuber
(setts). This vegetative propagation allows the perpetuation and accumulation of some
diseases, including those caused by viruses. The planting of smaller tubers saved from
the previous harvest may in effect be selection of the most infected lines.
   There is a desire for increased international movement and exchange of yam
germplasm to facilitate selection and breeding for improved genotypes adapted to
local conditions and uses. However, the presence of any virus in the vegetative planting
material poses a great risk as new viruses or strains may be distributed to new sites
where their effects on tuber yield or quality may be catastrophic. There is thus the need
to develop sensitive and robust methods for detecting and diagnosing yam-infecting
viruses that can be used to prevent the international movement of these viruses.
   Virus species from at least six different genera identified infecting yam in different
parts of the world have been characterized to a greater or lesser extent (Table 1). The
virus species most commonly encountered in yam in sub-Saharan Africa are Yam mosaic
potyvirus, Dioscorea alata potyvirus, and the badnaviruses.




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      Table 1. Identified virus species infecting Dioscorea yams.

      Virus                                                   Reported geographical distribution   Susceptible Dioscorea spp.             Antiserum

      Chinese yam necrotic mosaic virus (ChYNMV),
      genus Macluravirus (?)                                  Asia (JP)                            D. batatas
      Synonym: Naga-Imo virus                                                                      D. batatas, f. typica

      Cucumber mosaic virus (CMV), genus Cucumovirus          Africa (CI, NG)                      D. alata
                                                                                                   D. trifida
      Dioscorea alata badnavirus (DaBV), genus Badnavirus     Africa (BJ, GH, NG, TG), Asia (JP)   D. alata, D. dumetorum
                                                              Caribbean (BB, GD)                   D. nummularia, D. opposita
                                                              Pacific (PG, VU)                      D. pentaphylla, D. trifida
                                                              South America (BR, GY)               D. rotundata–cayenensis
      Dioscorea alata virus (DAV), genus Potyvirus            Africa (BJ, CM, GH, GN, NG)          D. alata
      Synonyms: Yam virus I (YVI)                             Asia (ID, LK) Caribbean (BB, PR)     D. esculenta
      Yam mild mosaic virus (YMMV)                            Pacific (PG, SB, VU)




                                                                                                                                                      An overview of viruses infecting Dioscorea yams
      Dioscorea bulbifera badnavirus (DbBV), genus            Africa (NG, GH)
435




      Badnavirus                                              Caribbean (PR)                       D. bulbifera
      Dioscorea dumetorum virus (DDV), genus (?)
      Potyvirus                                               Africa (NG)                          D. alata
                                                              Asia (BD, LK)                        D. dumetorum
      Dioscorea esculenta virus (DEV), genus (?)Potyvirus     Asia (LK)                            D. esculenta
      Dioscorea latent virus (DLV), genus Potexvirus          Caribbean (PR)                       D. bulbifera, D. composita
                                                              Pacific (VU)                          D. floribunda, D. nummularia
      Dioscorea mottle virus (DMoV), genus (?)Comovirus       Africa (NG)                          D. alata
      Syn: Dioscorea mild chlorosis virus
      Dioscorea trifida virus (DTV), genus (?)Potyvirus        Caribbean (GP)                       D. alata, D. rotundata–cayenensis
                                                                                                   D. trifida
      Japanese yam mosaic virus (JYMV), genus Potyvirus       Asia (JP)                            D. japonica
      Yam internal brown spot virus (IBSV), genus (?)         Caribbean (BB)                       D. alata
      Badnavirus                                                                                   D. rotundata–cayenensis?
      Yam mosaic virus (YMV), genus Potyvirus                 Africa (BJ, CM, CI, GH, NG, TG)      D. alata, D. dumetorum
      Synonyms: Dioscorea rotundata potyvirus (DRPV)          Asia (ID, MY, PH)                    D. esculenta
      Dioscorea green-banding mosaic potyvirus (DGMV)         Caribbean (BB, GP, PR)               D. praehensilis
                                                              Pacific (PG, SB) South America (GY)   D. preussii, D. rotundata–cayenensis
Plant virology in sub-Saharan Africa


                 Genus Potyvirus, family Potyviridae

Yam mosaic virus (YMV)
Yam mosaic virus (YMV) is one of the most important viruses infecting yams in sub-
Saharan Africa. It was first isolated and characterized in the Côte d’Ivoire by Thouvenel
and Fauquet (1979), and has since been detected throughout the yam growing regions
of Africa (Goudou-Urbino et al. 1996), the Caribbean, and the Pacific. Potyviruses are
flexuous rods 650–900 nm in length and with a single stranded RNA genome of 9–12
kbp. Typical symptoms of YMV are mosaic, green-vein banding, green spotting or
flecking, blistering and leaf mottling, vein yellowing, and leaf deformation (curling).
YMV can be transmitted in a nonpersistent manner by a wide range of aphid species.
It can also be mechanically transmitted in sap, though the main means of spread is
probably in infected planting material. Thus, the main method of control is to use
clean planting material collected from uninfected plants. Some resistance to YMV has
been identified in some breeding lines of D. rotundata and attempts are being made to
incorporate this into more agronomically useful varieties. Monoclonal and polyclonal
antisera are available for use in ELISA, and RT-PCR using specific YMV primers has
been shown to be reliable for testing for the virus. Nicotiana benthamiana is a good
test plant for YMV.

Dioscorea alata virus (DAV)
Dioscorea alata virus (DAV) (= yam virus I, YVI) probably occurs in all areas where
water yam is grown. Symptoms in water yam range from very mild chlorosis through
leaf mosaic to severe stunting, though the most common symptom is a mild mosaic. This
has led some authors to name the virus Yam mild mosaic virus (YMMV). Sequencing
part of the genome of potyviruses from yam suggests that there are two related clades
of DAV, one from Africa and the other from southeast Asia, but that isolates from both
these clades have been found infecting other Dioscorea species as well as D. alata. So
far, it has proved impossible to transmit DAV mechanically by sap inoculation, though
several aphid species are efficient vectors (Odu et al. 2001). Polyclonal antisera and
DAV-specific PCR primers are available for detection of DAV.

Other Potyviridae
Sequence analysis of part of the genome of the potyviruses isolated from yam indicates
that as well as YMV and DAV, there are several other distinct potyviruses that infect
yam. These include Dioscorea esculenta virus (DEV), Dioscorea dumetorum virus
(DDV), Dioscorea trifida virus (DTV), and Japanese yam mosaic virus (JYMV), each


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                                     An overview of viruses infecting Dioscorea yams


named after the host species where they are most commonly detected. Chinese yam
necrotic mosaic virus (ChYNMV) from Japan has recently been shown to be in the
genus Macluravirus of the Potyviridae. Details of the geographical distributions, host
range, symptoms, and transmission characteristics of each of these remain sparse.

Genus Badnavirus, family Caulimoviridae
The badnaviruses are a genetically and serologically diverse genera of virus having
bacilliform particles of 25–30 × 120 nm and a circular double stranded DNA genome
of 7.3–8.3 kbp. Several bacilliform viruses have been associated with yam disease. In
the Caribbean, a bacilliform virus was associated with internal brown spot disease.
Dioscorea alata badnavirus (DaBV) and the serologically related Dioscorea bulbifera
badnavirus (DbBV) have been detected in a range of yam species from West Africa using
specific antisera. It was also demonstrated that they can be mechanically transmitted
from partially purified samples and also in a semipersistent manner by some mealybug
species including the citrus mealybug Planococcus citri. Sequencing and phylogenetic
analysis of part of the genome of badnaviruses isolated from yam indicates that a
wide range of genetically diverse strains or quasi-species of Badnavirus infects yam
in different parts of the world. Symptoms of badnavirus infection in yam depend on
the virus strain, host genotype, and environmental conditions and can range from no
symptoms to severe chlorosis and leaf distortion and curling.

Cucumber mosaic virus genus Cucumovirus
Cucumber mosaic virus (CMV), genus Cucumovirus, is a species with several serologi-
cally and genetically distinct strains or subspecies. The virus particles are isometric
and about 30 nm in diameter. The species as a whole has a very wide host range, but
there tends to be some specialization within strains or subspecies. CMV infections
of yam tend to be sporadic, suggesting they arise from a chance encounter between
a viruliferous vector and the yam plant. However occasionally, CMV incidence in
yam can be high locally. Strains of CMV have been reported infecting D. alata, D.
trifida, and D. rotundata in West Africa, the Caribbean, South America, and the South
Pacific (Migliori and Cadilhac 1976). Typically, CMV infection of yam causes severe
leaf chlorosis and mosaic symptoms; it may also cause leaf distortions and stunting.
CMV is efficiently transmitted in nature in a nonpersistent manner by a wide range
of aphid species. Experimentally, it is mechanically transmitted to a wide range of
indicator plants. Mechanical transmission of CMV produces chlorotic local lesions on
Chenopodium amaranticolor and C. quinoa, systemic mosaic symptoms on Cucumis
sativus and necrotic local lesions on Vigna unguiculata. Numerous different more or
less strain-specific antisera are available for use in ELISA.


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Dioscorea latent virus genus Potexvirus
Dioscorea latent virus (DLV) is the only member of the Potexvirus genus known to infect
yams. It is known to spread in Puerto Rico and causes no symptoms in D. composita
or D. floribunda (Lawson et al. 1973). It can be transmitted mechanically to various
herbaceous indicator species such as N. benthamiana and N. megalosiphon, again caus-
ing symptomless infection. DLV is often found in association with DAV, and detection
is usually by ELISA using polyclonal antiserum.

Dioscorea mottle virus genus Comovirus
Dioscorea mottle virus (DMoV) belongs to the genus Comovirus having particles that
are isometric, 20–30 nm in diameter, and a bipartite genome of single stranded RNA.
DMoV has been found infecting D. alata in Nigeria, and is likely to be distributed
across West Africa. Symptoms in D. alata include mild chlorosis (mild chlorosis
strain), mottling (mottle strain), and necrosis (necrosis strain). DMoV is mechanically
transmissible from D. alata, and the natural vector is thought to be a beetle. DMoV can
be mechanically transmitted to the indicator plants Vigna unguiculata, Glycine max,
Chenopodium murale, C. amaranticolor, and C. quinoa. Antisera are being developed
for use in diagnosis.

                                     Conclusion
With the great range of virus species and strains that commonly affect yams, and the
finding that nutrient or trace element deficiencies can mimic or enhance the symptoms
caused by virus infections, it has up to now been extremely difficult to assess how much
damage in terms of the yield or quality reduction infection by a specific virus can cause.
Multiple infections with different species or strains of virus are also relatively common.
Generally, the effect of the mixture is greater than that of the strains/species alone. Thus,
it is important not to move viruses between different regions so as to avoid the risk of
creating even more devastating mixtures. Since most of the spread of these viruses is
from one generation and season to the next through the vegetative propagation of the
yams, one of the main methods to limit the spread is to ensure that only virus-tested
planting materials are used. These can be plantlets derived directly from tissue culture or
from mini-tubers produced in vector-proof conditions. If virus-tested planting materials
are not available, farmers should ensure that their planting setts are derived from parent
material that at least looked healthy during the growing season.

                                     References
Berthaud, J., M. Bousalem, O. Daïnau, J. Dubern, B. Malaurie, and S. Tostain. 2001. Can yam
domestication and participatory breeding be new ways to improve this crop and conserve its


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                                          An overview of viruses infecting Dioscorea yams


genetic resources? Pages 404–409 in Root crops in the 21st Century. Proceedings of the 7th
Triennial Symposium of the International Society for Tropical Root Crops—Africa Branch,
11–17 October 1998, Cotonou, Benin.

Dumont, R. and P. Vernier. 2000. Domestication of yams (Dioscorea cayenensis-rotundata) within
the Bariba ethnic group in Benin. Outlook on Agriculture 29(2): 137–142.

Goudou-Urbino, C., G. Konate, J.B. Quiot, and J. Dubern. 1996. Aetiology and ecology of a yam
mosaic disease in Burkina Faso. Tropical Science 36: 34–40.

Lawson, R.H., S.S. Hearon, F.F. Smith, and R.P. Kahn. 1973. Electron microscopy and separation
of viruses in Dioscorea floribunda. Phytopathology 63: 1435.

Migliori, A. and B. Cadilhac. 1976. Contribution to the study of a virus disease of yam Dioscorea
trifida in Guadeloupe. Annales de Phytopathologie 8: 73–78.

Odu, B.O., S.A. Shoyinka, J.d’A. Hughes, R. Asiedu, and A.O. Oladiran. 2001. Yam viruses of
Nigeria. Pages 631–633 in Root crops in the 21st Century. Proceedings of the 7th Triennial
Symposium of the International Society for Tropical Root Crops—Africa Branch, 11–17 October
1998, Cotonou, Benin.

Thouvenel, J.C. and C. Fauquet. 1979. Yam mosaic: a new potyvirus infecting Dioscorea cayenen-
sis in Ivory Coast. Annals of Applied Biology 93: 279–283.




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