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Date: June 12, 2007
Source: International Rice Research Institute (IRRI) Bulletin [edited] <http://bulletin.irri.cgiar.org/bulletin/2007.22/default.asp>

Looking for solutions to the rice virus problem in the Mekong Delta

In the last 2 years, rice production in Viet Nam, particularly in the Mekong Delta, suffered a major setback when outbreaks of virus diseases carried by the brown planthopper (BPH) caused a loss of about 400 000 tons (or 1.1 percent of Viet Nam's total). Viet Nam's rice exports were threatened and the infestations prompted officials and farmers to spray insecticides in excess in attempts to curb the spread of the viral infections, which, in turn, escalated to a high usage of insecticides. There is also an imminent danger of the virus problem spreading north and west from the current epicenter in the Mekong Delta, which might affect rice production in Cambodia, Laos, and central and northern Viet Nam. The virus has been reported in Cambodia and in Nha Trang, about 400 km (248.5 mi) north of the Delta. During this time, extensive outbreaks of BPH were also reported in China, Korea, and Japan, causing yield losses of more than 3 million tons of rice.

To focus on identifying research and implementation issues related to management of the BPH/virus problem, the Australian Centre for International Agricultural Research (ACIAR) is providing funds to Vietnamese and IRRI scientists to undertake an 8-month project that will review literature, historical data, and reports, and collect preliminary data to scope for research and implementation issues, provide some understanding of the vector-virus relationships, vector migrations, and vector variability, and explore farmers' and extension's perceptions of viral diseases and their management. This information is needed for the development of an integrated management of the BPH/virus problem to prevent its spread and to develop communication management strategies to help farmers adapt.

The research team met at the Plant Protection Department (PPD) office in Ho Chi Minh City on 30-31 May [2007] to develop detailed work plans. After the intensive scoping study, a workshop will be organized in early 2008 to report and advise on potential next steps in research and implementation to manage the problem. The materials gathered in the study will also be used to develop a full research proposal on the management of BPH and the viruses.

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[The Asian brown planthopper (_Nilaparvata lugens_) is one of the worst pests of rice in the Australasian and Pacific Island region.
The specific virus diseases causing concern are not mentioned in this report, however, _N. lugens_ is the vector of _Rice grassy stunt virus_ (RGSV; genus _Tenuivirus_, family _Bunyaviridae_) and _Rice ragged stunt virus_ (RRSV; genus _Oryzavirus_, family _Reoviridae_).
A number of other rice viruses are spread by the green planthopper (_Nephotettix virescens_) including _Rice tungro bacilliform virus_ and _Rice tungro spherical virus_, which are both also present in southern Viet Nam.

RGSV can also be transmitted by _N. bakeri_ and _N. muiri_. Rice wilted stunt and rice rosette are at present considered to be strains of RGSV. The virus causes severe stunting, excessive tillering, erect growth habit, and reduced pale leaves on rice. Symptoms may be confused with rice yellow dwarf and rice dwarf diseases. RGSV is usually not a widespread problem but has been reported to cause serious damage in limited areas during sporadic outbreaks. Other virus species in the genus _Tenuivirus_ affecting the same host are _Rice stripe virus_ and _Rice hoja blanca virus_, however their insect vectors are different species of planthoppers.

RRSV causes stunting, enations on leaf veins and sheaths, ragged leaves, and flower suppression. Yield losses of up to 90 percent have been recorded depending of the percentage of plants infected. RRSV can also infect barley and maize. Control measures include vector control and the use of rice varieties resistant to the vector, the virus, or both. Cultivars resistant to the vector have low disease incidence.

Both RGSV and RRSV are transmitted by the vector insects in a persistent manner, multiply in the vector, are retained during moulting, but are not transmitted congenitally to the progeny of the insects. Neither virus is transmitted by seed, pollen, or mechanical means. Both viruses are spreading in large parts of the rice-growing areas of Asia, and in tropical regions levels of infection and vector density are often very high.

Map of Viet Nam and neighbors
<http://www.lib.utexas.edu/maps/middle_east_and_asia/vietnam_pol01.jpg>
Distribution map of several rice viruses in southern Viet Nam <http://www.kper.or.kr/img/board/75ho_info_05.jpg>
Pictures
Brown planthopper:
<http://www.agric.nsw.gov.au/Hort/ascu/fulgor/nlugens.htm> and <http://www.dpvweb.net/dpv/showfig.php?dpvno=320&figno=03>
RGSV symptoms on whole rice plants
<http://www.knowledgebank.irri.org/RiceDoctor_MX/projectImages/image138.jpg>
and
<http://www.dpvweb.net/dpv/showfig.php?dpvno=320&figno=01>
RGSV symptoms on rice leaves:
<http://www.dpvweb.net/dpv/showfig.php?dpvno=320&figno=04>
Electron micrograph of _Tenuivirus_ particles:
<http://image.fs.uidaho.edu/vide/images/a9.jpg>
RRSV symptoms:
<http://www.knowledgebank.irri.org/diseaseDiag/diseaseDiagnosis_files/image064.gif>
Electron micrograph of RRSV particles:
<http://www.iah.bbsrc.ac.uk/dsRNA_virus_proteins/Oryzavirus-RRSV-image.gif>
Links
_N. lugens_ taxonomy and distribution map:
<http://www.environment.gov.au/cgi-bin/abrs/fauna/details.pl?pstrVol=FULGOROIDEA;pstrTaxa=1909;pstrChecklistMode=2>
Brown planthopper biotypes:
<www.springerlink.com/index/V2N89L31T2573778.pdf>
RGSV description:
<http://image.fs.uidaho.edu/vide/descr688.htm>
RGSV disease information:
<http://www.knowledgebank.irri.org/RiceDoctor_MX/Fact_Sheets/Diseases/Rice_Grassy_Stunt_Virus_(RSGV).htm>
RRSV description:
<http://image.fs.uidaho.edu/vide/descr691.htm>
RRSV disease information:
<http://www.knowledgebank.irri.org/ricedoctor_mx/Fact_Sheets/Diseases/Rice_Ragged_Stunt.htm>
News item in CropBiotech update 8 Jun 2007:
<http://www.isaaa.org/kc/cropbiotechupdate/2007/06/08.html#14>. - Mod.DHA]

[see also in the archive:
2006
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Ragged and grassy stunt, brown planthopper, rice - Viet Nam 0061103.3157
1997
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Rice hoja blanca & rice stripe viruses: RFA 19971015.2132]

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