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Rice yellow mottle virus

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Rice yellow mottle virus

Summary

  • Last modified
  • 14 July 2018
  • Datasheet Type(s)
  • Invasive Species
  • Pest
  • Natural Enemy
  • Preferred Scientific Name
  • Rice yellow mottle virus
  • Taxonomic Tree
  • Domain: Virus
  •   Family: Unassigned virus family
  •     Genus: Sobemovirus
  •       Species: Rice yellow mottle virus

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Pictures

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PictureTitleCaptionCopyright
Field symptoms: a patch of dry leaves of a rice variety severely attacked by RYMV.
TitleSymptoms
CaptionField symptoms: a patch of dry leaves of a rice variety severely attacked by RYMV.
CopyrightMyimaorga E. Abo
Field symptoms: a patch of dry leaves of a rice variety severely attacked by RYMV.
SymptomsField symptoms: a patch of dry leaves of a rice variety severely attacked by RYMV.Myimaorga E. Abo
Typical symptoms of RYMV: yellow mottle or orange colouration, depending on the genotype.
TitleSymptoms
CaptionTypical symptoms of RYMV: yellow mottle or orange colouration, depending on the genotype.
CopyrightSy & Yacouba, 1996
Typical symptoms of RYMV: yellow mottle or orange colouration, depending on the genotype.
SymptomsTypical symptoms of RYMV: yellow mottle or orange colouration, depending on the genotype.Sy & Yacouba, 1996
Yellow symptoms in the screenhouse. BaDeggi, Nigeria.
TitleSymptoms
CaptionYellow symptoms in the screenhouse. BaDeggi, Nigeria.
CopyrightMyimaorga E. Abo
Yellow symptoms in the screenhouse. BaDeggi, Nigeria.
SymptomsYellow symptoms in the screenhouse. BaDeggi, Nigeria.Myimaorga E. Abo
Note yellowing, orange colouration and stunting of plants. Non-infected plants are uniformly green.
TitleSymptoms
CaptionNote yellowing, orange colouration and stunting of plants. Non-infected plants are uniformly green.
CopyrightMyimaorga E. Abo
Note yellowing, orange colouration and stunting of plants. Non-infected plants are uniformly green.
SymptomsNote yellowing, orange colouration and stunting of plants. Non-infected plants are uniformly green.Myimaorga E. Abo
Maturing plants exhibiting completely sterile and empty spikelets, a characteristic of RYMV infection in highly susceptible varieties.
TitleSymptoms
CaptionMaturing plants exhibiting completely sterile and empty spikelets, a characteristic of RYMV infection in highly susceptible varieties.
CopyrightMyimaorga E. Abo
Maturing plants exhibiting completely sterile and empty spikelets, a characteristic of RYMV infection in highly susceptible varieties.
SymptomsMaturing plants exhibiting completely sterile and empty spikelets, a characteristic of RYMV infection in highly susceptible varieties.Myimaorga E. Abo

Identity

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Preferred Scientific Name

  • Rice yellow mottle virus

Other Scientific Names

  • rice yellow mottle sobemovirus

International Common Names

  • French: bigarure jaune du riz; marbrure jaune du riz; mosaique jaune du riz; panachure jaune du riz

Local Common Names

  • Sierra Leone: pale yellow mottle disease

English acronym

  • RYMV

EPPO code

  • RYMV00 (Rice yellow mottle sobemovirus)

Taxonomic Tree

Top of page
  • Domain: Virus
  •     Family: Unassigned virus family
  •         Genus: Sobemovirus
  •             Species: Rice yellow mottle virus

Description

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RYMV particles were first described by Bakker (1974) and subsequently by Fauquet and Thouvenel (1977) and IITA (1980). Particles are distinctive, being spherical with a diameter of 28±3 nm and containing about 77% protein. Infective virus particles contain ca 23.6±0.5% nucleic acid (Bakker, 1974).

RYMV is a species of the Sobemovirus genus. Sobemoviruses have isometric particles (ca 30 nm in diameter), which are markedly stable in vitro and sediment uniformly at about 115S (Sehgal, 1995). Viruses in this genus are characterized by beetle transmissibility, with a narrow host range. Sobemoviruses exhibit an obligatory dependence on divalent cations (calcium and magnesium) for their conformational stability. The viral capsid is constructed from 180 structural units (each ca 30 kDa), according to a T=3 design (Sehgal, 1995).

The genome is a single-stranded linear, positive-sense RNA molecule, MW 1.4 x 10<(sup)6> (ca 4200 nucleotides). The 5' terminus of the sobemovirus RNA has a genome-linked protein (VPg) and the 3' end is not polyadenylated (Sehgal, 1981; Hull, 1988). Some RYMV isolates possess satellite RNA (Sehgal et al., 1993).

The base composition of RYMV RNA shows a high guanine content (29%), followed by cytosine (26.3%), uracil (25%) and adenine (25%).

The 4550 nucleotide (nt) sequence of RYMV RNA and its predicted genomic organisation was determined by Yassi et al. (1994). The RYMV genomic RNA contains four open reading frames (ORFs). The first ORF (nt 80 to 553) encodes a protein containing 157 amino acids with a predicted M<(sub)r> of 17.8. The function of this protein has not been identified.

ORF2 (nt 608 to 3607) encodes a polyprotein of 999 amino acids, with a predicted M<(sub)r> of 110.7. The first 134 amino acids of ORF2 are predicted to be the genome-linked protein, VPg, followed by the viral protease, the helicase and the RNA-dependent RNA polymerase. ORF3 is within the boundaries of ORF2 and is predicted to encode a polypeptide with 126 amino acids and an M<(sub)r> of 13.7. The function of this protein has not been identified. ORF4 (nt 3447 to 4166), which overlaps the 3' terminus of OFR2, encodes a 26 KDa protein. This polypeptide has been identified as the RYMV coat protein.

Distribution

Top of page Rice yellow mottle disease was first reported in rice at Otonglo near Kisumu area along the shore of Kavirondo gulf of Lake Victoria in western Kenya in 1966 (Bakker, 1970). W. Bakker coined the phrase 'Rice yellow mottle virus' and also described the virus in detail (Bakker, 1974, 1975).

In 1976, Raymundo and Buddenhagen reported a 'pale yellow mottle disease' in Sierra Leone. It was later found that this virus is serologically indistinguishable from an RYMV isolate from Kenya (Rossel and Thottappilly, 1985).

Since RYMV was first reported in Kenya, it has been reported in many East and West African countries (Abo et al., 1998). The potentially devastating RYMV is a distinctly African virus specific to lowland rice and is indigenous to sub-Saharan Africa.

Distribution Table

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The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.

Continent/Country/RegionDistributionLast ReportedOriginFirst ReportedInvasiveReferenceNotes

Asia

TurkeyAbsent, reported but not confirmedKöklü and YIlmaz, 2004; CABI/EPPO, 2010

Africa

Burkina FasoPresentJohn et al., 1984; CABI/EPPO, 2010; EPPO, 2014
BurundiPresentNdikumana et al., 2012; EPPO, 2014
CameroonPresentTraore et al., 2001; CABI/EPPO, 2010; EPPO, 2014
Central African RepublicPresentEPPO, 2014; Longué et al., 2014
ChadPresentTraore et al., 2001; CABI/EPPO, 2010; EPPO, 2014
Congo Democratic RepublicPresentHubert et al., 2013; EPPO, 2014
Côte d'IvoirePresentFauquet & Thouvanel, 1977; CABI/EPPO, 2010; EPPO, 2014
EthiopiaPresentRakotomalala et al., 2014
GambiaPresentAwoderu, 1991a; Anon., 1978; CABI/EPPO, 2010; EPPO, 2014
GhanaPresentAwoderu, 1991a; Anno-Nyako et al., 1995; CABI/EPPO, 2010; EPPO, 2014
GuineaPresentFomba, 1988; CABI/EPPO, 2010; EPPO, 2014
Guinea-BissauPresentFomba, 1988; CABI/EPPO, 2010; EPPO, 2014
KenyaPresentBakker, 1970; Rossel and Thottappilly, 1985; CABI/EPPO, 2010; EPPO, 2014
LiberiaPresentRossel et al., 1982b; CABI/EPPO, 2010; EPPO, 2014
MadagascarPresentReckhaus and Randrianangaly, 1990; CABI/EPPO, 2010; EPPO, 2014
MalawiPresentCABI/EPPO, 2010; Ndikumana et al., 2015
MaliRestricted distributionJohn et al., 1984; CABI/EPPO, 2010; EPPO, 2014
MauritaniaPresentAwoderu, 1991a; CABI/EPPO, 2010; EPPO, 2014
NigerWidespreadJohn et al., 1984; CABI/EPPO, 2010; EPPO, 2014
NigeriaPresentRossel et al., 1982a; Rossel et al., 1982b; CABI/EPPO, 2010; EPPO, 2014
RwandaPresentCABI/EPPO, 2010; EPPO, 2014
SenegalPresentMbodj et al., 1984; CABI/EPPO, 2010; EPPO, 2014
Sierra LeonePresentRaymundo and Buddenhagen, 1976; Raymundo et al., 1979; CABI/EPPO, 2010; EPPO, 2014
TanzaniaPresentRossel et al., 1982b; CABI/EPPO, 2010; EPPO, 2014
-ZanzibarPresentRossel et al., 1982b; Ali and Abubakar, 1995
TogoPresentTraoré et al., 2008
UgandaPresentPinel-Galzi et al., 2006; CABI/EPPO, 2010; EPPO, 2014
ZimbabweRestricted distributionCABI/EPPO, 2010; EPPO, 2014

Europe

Russian FederationAbsent, invalid recordEPPO, 2014
-Southern RussiaAbsent, invalid recordEPPO, 2014
UkraineAbsent, invalid recordEPPO, 2014

Risk of Introduction

Top of page Although RYMV is not transmitted through rice seeds, leaf debris and spikelet contaminants have been implicated in the transmission of the virus. Purity of the seed to be exported or transported to new areas should therefore be emphasised. Insects also transmit the virus.

Hosts/Species Affected

Top of page The host range of Rice yellow mottle virus (RYMV) is narrow, being restricted to species in the Poaceae, mainly in the tribes Oryzeae and Eragrostideae (Bakker, 1974, 1975; Konate et al., 1997a; Abo and Sy, 1998; Abo et al., 1998, 2000a). The occurrence of RYMV was also reported in the wild species Oryza longistaminata, a rhizomatous rice species commonly seen growing in marshy areas (John et al., 1984; Mbodj et al., 1984). Symptoms of RYMV were also observed in O. longistaminata in a fresh water swamp adjacent to a mangrove swamp in Bissau and Carboxanque in the Republic of Guinea-Bissau (Fomba, 1988). However, the disease was not found in O. barthii, an African wild rice species, a progenitor of O. glaberrima collected along the Niger river, Nigeria (John et al., 1984). Awoderu (1991a) established that O. longistaminata, O. glaberrima and O. barthii as well as Eleusine indica, Eragrostis tenuifolia, Echinochloa crus-galli and Panicum maximum are alternative hosts of the virus. Okioma et al. (1983) confirmed E. indica, Dinebra retroflexa and E. tenuifolia as alternative hosts of RYMV. Konate et al. (1997a) reported Oryza species, Echinochloa colona, Ischaemum rugosum and Panicum repens as reservoir hosts of RYMV in the field. Abo (1998) found only Oryza species, Echinocloa crus-pavonis, E. pyrimidalis and Eragrostis ciliaris as the hosts of RYMV in screening tests. Only the Oryza species and E. crus-pavonis were found to be infected in nature (Abo, 1998; Abo et al., 2002). Leersia hexandra, a common lowland weed was found not to be susceptible to RYMV but was a natural host of Trichispa sericea, a vector of RYMV (Abo, 1998; Abo et al., 1999).

Awoderu (1991a) determined that the following plant species are susceptible to RYMV in artificial inoculations: O. longistaminata, O. barthii, O. glaberrima, Eleusine coracana, E. indica, P. maximum, P. repens, Digitaria sanguinalis, E. crus-galli, E. tenuiflora and Cynodon dactylon. Bakker (1974, 1975) reported Oryza sativa, O. punctata, Dinebra retroflexa and Phleum arenarium as diagnostic hosts.

In addition to the hosts listed, RYMV infects Oryza longistaminata, Dinebra retroflexa, Eragrostis ciliaris, E. tenuifolia, Phleum arenarium, Panicum repens and Ischaemum rugosum.

Host Plants and Other Plants Affected

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Growth Stages

Top of page Flowering stage, Seedling stage, Vegetative growing stage

Symptoms

Top of page The disease is characterized by yellow or orange leaf discoloration, stunting, sterility and empty spikelets (see Pictures). Non-synchronous flowering and death of plants occur and grain discoloration is noticed in the field.

Infection by RYMV is mostly systemic, affecting the entire plant and inducing the characteristic symptoms of leaf yellowing of varying intensity, mottling, necrosis and stunted growth. Tillering is reduced, with a weakening of the plant, partial emergence of panicles and spikelet sterility. Plants of highly susceptible cultivars may die following severe infection (Bakker, 1970; Raymundo and Buddenhagen, 1976; John et al., 1984; Masajo et al., 1988). Symptoms of the virus consist of a linear chlorotic mottle on the newly emerged leaves which later spreads into broken or continuous pale-green to yellowish streaks up to 10 cm long (Bakker, 1974). Symptom expression may be strongly influenced by light intensity, day length, humidity, temperature and growth stage of the plant, among other factors (Bakker, 1974).

Young seedlings at the 3-4-leaf growth stage are most susceptible to infection (Bakker, 1970; Fauquet and Thouvenel, 1977). The first newly formed leaves are spirally twisted and have difficulty in emerging. Infected plants exhibit delayed flowering with poorly exserted panicles bearing sterile and discoloured spikelets (Bakker, 1974). Generally, older plants exhibit less conspicuous foliar symptoms and less stunting than younger seedlings. According to Fomba et al. (1989), the secondary effects of infection by RYMV include increased severity of infection by fungi such as brown spot (Cochliobolus miyabeanus), leaf scald (Monographella albescens), sheath rot (Sarocladium oryzae) and sheath blotch (Pyrenochaeta oryzae [Phoma leveillei]).

List of Symptoms/Signs

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SignLife StagesType
Inflorescence / discoloration panicle
Leaves / abnormal colours
Leaves / abnormal forms
Leaves / abnormal patterns
Seeds / discolorations
Stems / stunting or rosetting
Whole plant / dwarfing

Biology and Ecology

Top of page Five viruses are reported from rice in Africa: RYMV (Bakker, 1975), Rice stripe necrosis virus (Fauquet and Thouvenel, 1987), Maize streak virus (Rossel and Thottappilly, 1985; Mesfin et al., 1992), African cereal streak virus (Harder and Bakker, 1973) and Rice crinkle disease (Buddenhagen, 1983). Although all five viruses cause diseases, only RYMV is economically important (Rossel and Thottappilly, 1985) and constitutes a potential threat to the development and expansion of rice production in Africa (Rossel, 1986).

Adult beetles belonging to the family Chrysomelidae, such as Sesselia pusilla, Chaetocnema spp., Dactylispa spp. and Trichispa sericea are thought to be the natural vectors of RYMV (Bakker, 1971, 1974, 1975; Raymundo and Buddenhagen, 1976; Abo, 1998; Abo et al., 1998, 2000a, b; Nwilene, 1999; Banwo et al., 2000, 2001a, b, c). Insects with biting and chewing mouthparts such as the long horned grasshoppers (Conocephalus spp.) have been found to transmit RYMV (Bakker, 1974, 1975; Abo, 1998; Abo et al., 2001b; Banwo et al., 2001b). A phytophagous coccinelid was found to transmit RYMV in screening tests (Abo, 1998; Abo et al., 2000b). A low transmission of RYMV was obtained with mites (Aceria bankerii and Stereotarsomus spinkii) in Kenya. The beetles were able to acquire the virus from diseased rice plants, retain the virus after acquisition, and subsequently infect healthy test plants for several consecutive days after aquisition (Bakker, 1974; Abo, 1998; Abo et al., 2000b). No seed transmission was observed from infected rice plants (Bakker, 1974, 1975; Fauquet and Thouvennel, 1977; IITA, 1985; John et al., 1986a; Abo, 1998; Konate et al., 2001) and no transmission was obtained by growing rice in soil collected from around diseased plants in the field. The virus was readily transmitted mechanically (Bakker, 1970). Transmission of the virus possibly occurs by leaf contact caused by wind (Bakker, 1970). It has been established that RYMV is transmitted when rice roots from infected plants become intertwined with healthy plants, leaf contact from closely spaced plants, rice stubble from incompletely decomposed plant debris and empty rice spikelets from infected rice plants (Abo, 1998). RYMV could be transmitted through guttation fluid and in the irrigation water of heavily infected rice fields (Bakker, 1974, 1975). Farm implements such as the sickle used in harvesting rice have been found to transmit and increase the incidence of the virus in the field (Tsuboi et al., 1995). Transplanted rice rather than direct seeded rice is more vulnerable to attack by RYMV. Trimming and ratooning of rice increases the incidence of RYMV in the field (Abo, 1998). The infection of RYMV in the rice fields normally starts from the edges of the fields and nursery beds. Soil inhabiting nematodes have been found not to transmit RYMV in screening test (Abo, 1998).

Biological Properties of RYMV

The concentration of RYMV reached a maximum in the plant 21 days after inoculation (Fauquet and Thouvenel, 1977). The inoculum progressively loses its capacity for infection between 55 and 70°C. The virus can remain viable in crude extract for at least 34 days at 27°C. At 4°C, it remained infectious in dry leaves for more than 56 days and for several months in frozen leaves. The virus is highly stable in crude sap and can be easily preserved in dry leaves (Fauquet and Thouvenel, 1977).

Strains of RYMV

Serological differences between RYMV isolates have been reported (Mansuour and Bailiss, 1994; Konate et al., 1997b; N'Guessan et al., 2000, 2001; Pinel et al., 2000). Five major strains have been differentiated in Africa, three from West Africa (S1, S2, S3) and two from East Africa (S4, S5) (Pinel et al., 2000). Two serotypes of RYMV exist in Cote d'Ivoire (N'Guessan et al., 2000). RYMV interactions show that in mixed infections S2 dominated over S1 both in viral capsid and RNA contents under temperature regimes (N'Guessan et al., 2000). There was no evidence of interactions in virus accumulation between the West African strains S1 or S2 with the more distantly related East African strain S4. Nucleotide and amino acid divergence between five RYMV strains was up to 11% (Pinel et al., 2000).

An antiserum produced against RYMV from Côte d'Ivoire reacted specifically with crude sap up to a dilution of 1/1024. It also reacted with an RYMV isolate from Kenya up to a dilution of 1/512 (Bakker, 1974). RYMV antiserum against the Kenyan isolate (titre 1/1024) reacted against the virus from the Côte d'Ivoire up to a dilution of 1/512. The presence of spur in gel diffusion tests between the Kenyan and Côte d'Ivoire isolates, as reported by Fauquet and Thouvenel (1977), indicated that they differ and suggest that different strains of the same virus occur in these countries.

Host reactions also indicate that strains of RYMV exist. For example, rice variety IRAT 13, which is resistant to RYMV in Sierra Leone (Raymundo and Konteh, 1980; Fomba, 1988) and in Kenya (Okioma and Sarkarung, 1983) exhibited reactions ranging from moderately susceptible to susceptible when naturally infected in Côte d'Ivoire (Awoderu et al., 1987).

Recent studies conducted by Fomba et al. (1995b) comparing three RYMV isolates showed that the Rokupr isolate from Sierra Leone was more virulent than the isolate from Guinea or Blama, Sierra Leone. Fargette et al. (1995) compared several isolates of RYMV; the results indicated that there are marked differences in virulence among isolates of RYMV.

Impact

Top of page The severe nature of rice yellow mottle disease poses a serious threat to the cultivation of susceptible rice in the wetlands of Africa. The disease mainly affects irrigated rice production of lowland rice in sub-Saharan countries (IITA, 1978; Fomba et al., 1989). However, it may occur sporadically in highly susceptible exotic varieties under rainfed, upland production systems (Raymundo et al., 1979; Awoderu et al., 1987).

In inland swamp regions, disease incidence may reach up to 80%, most probably due to the regeneration of new stems from existing stubble (Raymundo and Konteh, 1980). In irrigated paddy, 5-25% infection was found on many farms where a long duration variety was cultivated in the eastern province of Sierra Leone. Deep flooded or floating rice showed 10% incidence of the disease.

The disease was also observed in farmers' fields in areas around Bida in the Niger flood plains and at Badeggi in Niger State, Nigeria (Rossel et al., 1982a) and in rice project areas of the former Anambra and Imo States, Nigeria (Rossel et al., 1982b).

Traditional African rice varieties, adapted to upland conditions are significantly more tolerant and appear to possess a higher level of resistance to RYMV, regardless of origin, than exotic lowland cultivars (Raymundo et al., 1977; Raymundo and Konteh, 1980; Okioma and Sarkarung, 1983). Most of the indica cultivars showed 20-25% height reductions when infection occurred at the panicle initiation state (Raymundo and Konteh, 1980). Attere (1981) reported that RYMV affects plant height, but not to the extent of stunting growth and reducing tillering capacity.

Severe outbreaks of RYMV have occurred in Kenya (Bakker 1970, 1975), Niger (Reckhaus and Adamou, 1986), Sierra Leone (Raymundo et al., 1979; Raymundo and Konteh, 1980) and Liberia (Raymundo and Buddenhagen, 1976). More recently, RYMV has been reported from Madagascar, where it is considered a serious disease (Reckhaus and Randrianangaly, 1990). Taylor (1989) reported yield losses of 84-97% due to RYMV infection in three susceptible cultivars in Sierra Leone: PN 623-3, TOX 516-12-SLR and ROK 3.

Fomba (1986) reported yield reductions due to RYMV ranging from 19.6% in cv. Angkatta to 95.8% in cv. ROK 5. Reckhaus and Adamou (1986) reported yield reductions ranging from 58 to 68% in Niger following RYMV infection.

Although the effect of RYMV on rice production in Gagnoa, Côte d'Ivoire, was insignificant in 1976, the spread of the disease in this region is currently very high and many ricefields have been abandoned (Yoboue, 1989). In Kenya, extensive yield losses of up to 60% have been recorded in some places, especially where infection occurred up to 30 days after transplanting (Okhoba, 1989).

There is no evidence that RYMV incidence increases in successive seasons under continuous cropping, rainfall or wind speed (Heinrichs et al., 1997).

The incidence of RYMV is very high in the Sahel followed by Sudan Savanna, Guinea Savanna and tropical rainforest in that order (Awoderu, 1991a).

Diagnosis

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The visual assessment based on Standard Evaluation System (SES) for rice on a scale of 1-9 (IRRI, 1988, 1996) and practical score chart for RYMV involving a scale of 0-9 have been developed for detection of RYMV (IITA, 1986; John and Thottappilly, 1987).

Serological diagnostic methods commonly used in the detection of RYMV include precipitation methods (Ouchterlony, 1962) and the Nitrocellulose test (Ayassi et al., 1995). Traoré et al. (2008) developed a broad-spectrum polyclonal antibody which improved serological detection of RYMV. Many variants of Enzyme Linked Immunosorbent Assay (ELISA) (Clark and Adams, 1977; Koenig, 1982; Van Regenmortel, 1982; Clark and Bar-Joseph, 1984; Hébrard, 2008; Ochola and Tusiime, 2011) utilising a panel of polyclonal and monoclonal antibodies (Konate et al., 1997) have been used to detect RYMV.

Electron microscopy is also used in detecting RYMV (Siuzdak et al., 1996; Opalka et al., 2000).

The translation and expression of RYMV could be carried out by using Western and Northern immunoblotting and Polymerase chain reaction (PCR) (Kouassi et al., 1997; Opalka et al., 1998). RT-PCR amplification of the CP gene has also been used to identify RYMV (Hébrard, 2008).

Similarities to Other Species/Conditions

Top of page Most sobemoviruses lack immunological reactions with each other (Bakker, 1975; Thottappilly et al., 1992). Sobemoviruses are characterised by being mechanically transmissable, with the exception of Blueberry shoestring virus. They have narrow host ranges and isometric particles that sediment between 110 and 120S (Hull, 1988). A region of the RYMV small circular RNA (Sc-RNA) exhibits 89% identity to the satellite RNA associated with Australian isolates of Lucerne transient streak virus (Collins et al., 1998). On the basis of hidden Markov model alignment, Potato leaf roll virus coat protein displayed a 33% sequence identity with RYMV (Terradot et al., 2001). The capsid proteins of Southern bean mosaic virus cowpea strian exhibited 20.1% similarity with that of RYMV (Opalka et al., 2000). There is a sequence similarity of Cocksfoot mottle virus coat protein, putative RNA-dependent RNA polymerase (RDRP), and protease with the analogous protein of RYMV (Ryabov et al., 1996). RYMV and Barley yellow dwarf virus are the only two known viruses that have host ranges restricted to monocotyledons and support replication and encapsidation of low molecular weight RNAs (Seghal et al., 1993).

Prevention and Control

Top of page Host-Plant Resistance

The use of natural host-plant resistance is an appropriate way to control RYMV (Fomba, 1988; Thottappilly and Rossel, 1993). Exotic varieties of rice are generally highly susceptible to RYMV whereas traditionally grown, African upland varieties are usually moderately resistant or tolerant to the virus (IITA, 1979; Raymundo et al., 1979; Raymundo and Konteh, 1980; Rossel and Thottappilly, 1985; Fomba, 1988; Taylor, 1989; Awoderu, 1991b, Thottappilly and Rossel, 1993). Among indigenous African Oryza species like O. glaberrima and O. barthii, several accessions were identified as highly resistant (Attere and Fatokun, 1983; Thottappilly and Rossel, 1993). Breeding for resistance to RYMV was initiated in many countries (Masajo et al., 1988; Paul, 1992; Masajo and Rasoafalimanana, 1995; Paul et al., 1995a, b; Vales et al., 1995; John et al., 1985, 1986b; Ahmadi and Cisse, 1995a, b; Singh, 1995; Fomba et al., 1995a; Alluri et al., 1995; Bouet et al., 1995).

RYMV resistance has been found to be under polygenic determinism and 15 quantitative trait locus (QTL) have been detected on seven chromosomal fragments (Loriuex et al., 1996; Ghesquiere et al., 1997; Albar et al., 1998).

Paul et al. (1995a, b) studied the inheritance of RYMV in O. glaberrima. The genetic analysis of the crosses revealed that two recessive genes conferred RYMV resistance. Also studies on the inheritance of resistance/tolerance to RYMV disease were conducted by Mansary (1995a, b) and Kumwenda et al. (1995). The former reported that resistance in O. sativa is controlled by partial dominant genes and that variety CT 19 possesses excess recessive genes conditioning resistance to RYMV. The latter indicated that 1-2 recessive pair of alleles confers resistance to ITA 235 and 3 dominant pairs of alleles confer resistance to RYMV for LAC 23. Two dominant genes condition resistance in LAC 23 (IITA, 1980); TOS 3554 has excessive dominant genes conditioning resistance to RYMV and other plant characters. However, Ahmadi and Singh (1995) indicated that resistance to RYMV in O. sativa is weak, bi-directional and conditioned by partial dominant genes. Ndjiondjop et al. (1999) produced results consistent with the presence of a single recessive resistance gene common to Tog 5681 and Gigante varieties of rice. Resistance of japonica cultivars has been introduced into indica cultivars in some instances (Singh, 1995). Also, transformed lines encoding the RNA dependent polymerase of RYMV are resistant to RYMV due to post-transcription gene silencing (Pinto et al., 1999). Resistance is also affected by a morphology dependent mechanism and significant epistatis (Pressoir et al., 1998).

Studies have been carried out to characterise the genetic basis of RYMV resistance and to map resistant genes in rice (Albar et al., 1998; Pressoir et al., 1998; Ioannidou et al., 2000; Pinel et al., 2000). One RAPD marker is already linked to RYMV resistance (Albar et al., 1995). Efforts are concentrating on developing non conventional virus resistance using genetic engineering by incorporating viral genes into rice (Baulcombe, 1995; Kouassi et al., 1995; Pinto, 1995).

Integrated Pest Management

Reckhaus and Andriamasintseheno (1995, 1997) advocate an IPM strategy for control of RYMV in Madagascar. This strategy includes the destruction of crop residues, delaying planting time where irrigation facilities are available, the use of tolerant varieties, and chemical control of vectors. Important factors in the spread and management of RYMV in Africa have been advanced by Abo and Alegbejo (1997), Abo et al. (1998), Abo and Sy (1998), Imolehin et al. (1998) and Nwilene (1999). For example, farmers are advised not to use chemicals indiscriminately against the insect vectors unless economic thresholds (ET) and economic injury levels (EIL) are reached (Reckhaus and Adamou, 1986; Thresh, 1989, 1991). The farmers are always advised to use tolerant/resistant varieties where the disease occurs. Synchronous planting coupled with withholding irrigation water between planting to provide a rice-free period has been advocated in Niger (Reckhaus and Adamou, 1986; Thresh, 1989). According to Coulibaly (1995) and Hamadoun and Traore (1996) management practices have been advocated for rice farmers in Mali. These pratices include discontinuation and substitution of susceptible varieties with resistant/tolerant ones, burning of rice straws after harvest, change of nursery sites, early planting, rouging and destruction of infected plants in the field. Other practices include reduction of fertilizer (e.g. urea) application on infected plots, flooding of tilled plots, careful use of insecticides against the beetle vectors and timely weeding.

References

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