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<http://www.promedmail.org>
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International Society for Infectious Diseases <http://www.isid.org>

Date: January 31, 2008
Source: Viet Nam News Agency [edited]
<http://vietnamnews.vnagency.com.vn/showarticle.php?num=01AGR310108>

Mekong Delta crops hit by rice diseases

Nearly 10 percent of winter-spring crops in the Cuu Long (Mekong) Delta suffered from rice diseases, said Nguyen Quang Minh, director of the Agriculture and Rural Development Ministry's Plant Protection Department. He told Viet Nam News yesterday [30 Jan 2008] that more than 91 000 ha (hectares) (224 866 acres) of crops were attacked by brown planthoppers, small insects causing yellow and stunt rice diseases, an increase of almost 21 000 ha (51 892 acres) over last January [2007].

The worst hit provinces were Hau Giang, Vinh Long, Bac Lieu, Soc Trang, and Long An with an insect density of 750-3000 insects per sq m. The density was as bad as 6000-8000 insects per sq m in some areas, said Minh. The department forecast that brown planthoppers would quickly spread out during Tet [festival], from 5-11 Feb 2008, when the crops would be at the most important period of development.

Incidents of rice blast disease also increased this year [2008], affecting around 50 000 ha (123 553 acres) of crops, said Minh. Rice blast disease is a disease caused by fungus, which results in yellow spots on leaves, reduces the process of photosynthesis, and affects yields.

The department has asked provincial-level agriculture and rural development departments and plant protection centres in southern provinces to tighten supervision over rice diseases and instruct farmers how to protect their crops. The Cuu Long Delta has a total of 1.6 million ha (3.954 million acres) of crops.

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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 and is also the vector of _Rice grassy stunt virus_ and _Rice ragged stunt virus_. A number of other rice viruses are spread by different species of plant- or leafhoppers, including _Rice yellow stunt virus_. These and other rice viruses are present in the area and it is not clear which particular virus species are involved in the present epidemic. In tropical regions, levels of virus infection and vector density may be very high. Crop losses of up to 100 percent due to some virus diseases have been reported. Generally for these viruses, disease management includes vector control, cultural practices to minimise inoculum, and the use of rice varieties resistant to the vector, the virus, or both. Cultivars resistant to the vectors have usually low virus disease incidence.

Planthoppers also cause wounds on the leaf surfaces (by feeding or oviposition) exposing the plant to fungal and bacterial infections.
Rice blast is caused by the fungus _Magnaporthe grisea_ (synonyms _Pyricularia oryzae_, _P. grisea_). It is one of the most destructive diseases of rice worldwide with reported losses of more than 50 percent in infected fields. Symptoms include lesions on all parts of the shoot, stem rot, and panicle blight. When nodes are infected, all plant parts above the infection die and yield losses are severe. When infection occurs at the seedling or tillering stages, plants are often completely killed. More than 50 species of grasses and sedges can be affected by different strains of the fungus, but most strains isolated from rice can only infect a limited number of cultivars.
Symptom severity and spread of the blast fungus are influenced by climatic conditions. The disease is spread by infected plant debris, mechanical means, water, and wind. Disease management may include fungicides and cultural practices, but relies mainly on resistant varieties. However the fungus is highly variable and new strains are emerging breaking down host resistance.

Serious problems due to diseases and insect pests of rice have been reported from this area recently. Collaborative projects with international scientific organisations have been established to combat these problems and increase food security (see ProMED-mail 20080118.0221).

Maps
Viet Nam:
<http://www.lib.utexas.edu/maps/middle_east_and_asia/vietnam_pol01.jpg> and <http://healthmap.org/promed?v=14.9,108.5,5>
Viet Nam provinces:
<http://www.angelfire.com/co/hongnam/vnmap.html>
Distribution of several rice viruses in southern Viet Nam:
<http://www.kper.or.kr/img/board/75ho_info_05.jpg>

Pictures
Brown planthoppers:
<http://www.agric.nsw.gov.au/Hort/ascu/fulgor/nlugens.htm> and <http://www.dpvweb.net/dpv/showfig.php?dpvno=320&figno=03>
_Rice grassy stunt virus_ symptoms:
<http://seedcenter17.doae.go.th/farmer/pest/Image_Disease/rice_grassy%20stunt-05-014_B.jpg>
and
<http://www.dpvweb.net/dpv/showfig.php?dpvno=320&figno=04>
_Rice ragged stunt virus_ symptoms:
<http://www.ricethailand.go.th/rkb/data_005/Image_Disease/rice_ragged%20stunt-05-012_B.jpg>
Rice blast:
<http://www.imtech.res.in/raghava/rbpred/lesion.jpg>

Links
Brown planthopper fact sheet:
<http://www.knowledgebank.irri.org/RiceDoctor/Fact_Sheets/Pests/Planthopper.htm>
_N. lugens_ taxonomy and distribution map:
<http://www.environment.gov.au/cgi-bin/abrs/fauna/details.pl?pstrVol=FULGOROIDEA;pstrTaxa=1909;pstrChecklistMode=2>
_Rice grassy stunt virus_ fact sheet:
<http://www.knowledgebank.irri.org/RiceDoctor/Fact_Sheets/Diseases/Rice_Grassy_Stunt_Virus_(RSGV).htm>
Review of rice viruses, biology, and epidemiology:
<http://arjournals.annualreviews.org/doi/abs/10.1146/annurev.phyto.34.1.249?journalCode=phyto>
Taxonomy and descriptions of rice viruses via:
<http://www.ncbi.nlm.nih.gov/ICTVdb/Ictv/index.htm>
Information on rice blast:
<http://www.imtech.res.in/raghava/rbpred/home.html>,
<http://ascus.plbr.cornell.edu/blastdb/about.html>,  and <http://www.knowledgebank.irri.org/ricedoctor/default.htm#Fact_Sheets/Diseases/Rice_Blast.htm>
_M. grisea_ taxonomy:
<http://www.indexfungorum.org/Names/SynSpecies.asp?RecordID=317113>
Illustrated fact sheets for all major diseases and pests of rice via:
<http://www.knowledgebank.irri.org/RiceDoctor/default.htm>.  - Mod.DHA]