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Date: February, 2008
Source: The American Phytopathological Society, Plant Disease 2008; 92(2): 316 [edited]
<http://apsjournals.apsnet.org/doi/abs/10.1094/PDIS-92-2-0316B>

[Reference: Y Sere, et al: First report of _Rice yellow mottle virus_ in rice in The Gambia. Plant Dis 2008; 92(2): 316; DOI: 10.1094/PDIS-92-2-0316B]

_Rice yellow mottle virus_ (RYMV) of the genus _Sobemovirus_ is a major biotic constraint to rice (_Oryza sativa_) production in Africa. First reported in Kenya during 1966, RYMV was later found in most countries in Africa where rice is grown. In countries in westernmost Africa (The Gambia, Guinea-Bissau, Mauritania, and Senegal), plants with leaf yellowing and mottling symptoms were observed, but RYMV was never isolated. Rice is the staple food in The Gambia.

In 2006, 4 samples were collected from local rice varieties in the Kuntaur Region in the center of The Gambia. Mechanical inoculation with leaf extracts from all samples caused typical yellow mottle symptoms on susceptible rice varieties. RYMV was detected in the 4 samples collected by ELISA [enzyme linked immunosorbent assay]. The 720-nucleotide coat protein gene was amplified for each isolate by reverse-transcriptase-PCR [polymerase chain reaction]. The RT-PCR products were directly sequenced and then compared with a pool of RYMV coat protein sequences from West African isolates. Phylogenetic reconstruction indicated that the isolates from The Gambia formed a monophyletic group with over 97 percent nucleotide identity, and are closely related to isolates of other countries in West Africa (Burkina Faso, Cote d'Ivoire, Guinea, Mali, and Sierra-Leone) with 91 to 94 percent identity.

Detection of RYMV in The Gambia indicates that RYMV is present in westernmost Africa, which is referred to as the 'rice belt' of Africa, and shows that RYMV is widely distributed from eastern Africa (Tanzania) to the western part of the continent.

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[_Rice yellow mottle virus_ (RYMV, genus _Sobemovirus_, not yet assigned to family or order) was originally confined to one district of Kenya. However, large-scale rice irrigation projects lead to build up and wide dissemination of the virus and its vectors in sub-Saharan Africa. It is now the most serious pathogen of irrigated and lowland rainfed rice in West and Central Africa, and losses of up to 100 percent have been reported.

Besides rice, the virus also affects a number of other grass species. Symptoms include stunting, reduced tillering, mottling and yellowish streaking of the leaves, delayed flowering or incomplete emergence of the panicles and, in extreme cases, death of plants. RYMV is spread by a number of insect vectors, mainly by several species of beetles (for example _Chaetocnema pulla_). It does not appear to be seed transmitted in rice, but it can be spread by mechanical means, which is thought to account for much of its spread in the field. For example, a severe increase of RYMV incidence was noted in areas that switched rice cultivation methods from direct seeding to transplanting of seedlings. The virus survives in crop residues, and volunteer rice and grassy weeds serve as pathogen reservoirs. Disease management includes cultural practices as well as control of vectors and reservoir hosts.

The virus is highly variable, and different strains may have different levels of virulence. RYMV isolates originating in closely related agro-ecological zones show variability in the coat protein. As a consequence, very few rice varieties resistant or tolerant to RYMV are known. Resistance breeding programmes are in progress at the Africa Rice Centre (WARDA), one of the research centres of the Consultative Group on International Agricultural Research (CGIAR).

Maps
The Gambia:
<http://www.lib.utexas.edu/maps/africa/gambia.gif>  and
<http://healthmap.org/promed?v=13.5,-15.5,6>
Location of Kuntaur Region:
<http://www.gambia.co.uk/docs/about_the_gambia/points_of_interest/kuntaur.aspx>

Pictures
RYMV symptoms:
<http://www.innovations-report.de/bilder_neu/70653_Unbenannt-1.jpg>  and
<http://www.ird.fr/fr/imagesTEST/34971.jpg>
_C. pulla_ vector:
<http://www.dpvweb.net/dpv/showfig.php?dpvno=149&figno=05>
RYMV particle:
<http://www.ncbi.nlm.nih.gov/ICTVdb/WIntkey/Images/RENDER/1f2n.jpg>

Links
Information on RYMV:
<http://www.warda.org/publications/AR2000/F3.pdf>
Research on RYMV host resistance and epidemiology:
<http://www.innovations-report.de/html/berichte/agrar_forstwissenschaften/bericht-70653.html>,
<http://www.agbioforum.org/v7n12/v7n12a06-datta.htm>  and via
<http://www.warda.org/africa-rice-congress/presentations.html>
Detection and transmission of RYMV:
<http://library.wur.nl/wda/dissertations/dis3747.pdf>
RYMV strains:
<http://www.springerlink.com/content/2lja0tcu3dc27qwb/>
RYMV taxonomy:
<http://www.ncbi.nlm.nih.gov/ICTVdb/ICTVdB/00.067.0.01.005.htm>
Africa Rice Centre:
<http://www.warda.org/>
CGIAR:
<http://www.cgiar.org>.  - Mod.DHA]