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Plantwise Technical Factsheet

(Rice grassy stunt virus)

Host plants / species affected
Cynodon dactylon (Bermuda grass)
Cyperus rotundus (purple nutsedge)
Echinochloa colona (junglerice)
Leersia hexandra (southern cut grass)
Monochoria vaginalis (pickerel weed)
Oryza glaberrima (African rice)
Oryza rufipogon (wild rice)
Oryza sativa (rice)
List of symptoms/signs
Inflorescence  -  discoloration panicle
Leaves  -  abnormal colours
Leaves  -  abnormal forms
Leaves  -  abnormal patterns
Leaves  -  wilting
Roots  -  reduced root system
Seeds  -  discolorations
Stems  -  stunting or rosetting
Stems  -  witches broom
Whole plant  -  dwarfing
Whole plant  -  plant dead; dieback
Symptoms
Notes on Strains

There are strains and variants reported for RGSV. These strains have been differentiated based on symptomatology, insect transmission and cultivar reaction.

Rice Grassy Stunt Virus - Strain 1

Plants infected by rice grassy stunt virus strain 1 (RGSV1) show severe stunting, excessive tillering and an erect growth habit. The leaves are short, narrow, pale-green or pale-yellow and often have numerous dark-brown or rust-coloured spots of various shapes and sizes, which may form blotches. Young leaves may also be mottled or striped. However, the leaves of infected plants may remain green when supplied with adequate amounts of nitrogenous fertilizer (Rivera et al., 1966; Ling, 1972; Tantera et al., 1973).

Symptoms appear between 6 and 19 days after inoculation (Anjaneyulu, 1974; Kulshrestha et al., 1974; Das et al., 1974; Rivera et al., 1966). Plants inoculated at later stages of growth may not develop symptoms before harvest, but typical symptoms appear on the regenerated growth after ratooning (Ling, 1976).

Rice plants infected by RGSV strain 2 (RGSV2) at an early stage show stunting, yellowing of the lower leaves, and narrowing of the leaf blade. Initial symptoms usually develop 7-14 days after inoculation of 10-day-old TN1 seedlings. Irregular rust-coloured blotches on the lower leaves, and mottling on the young leaves, may also occur. Severely infected seedlings show pale, narrow leaves and profuse tillers, and die 3-4 weeks after inoculation. However, mildly infected plants show profuse tillering similar to the ordinary strain (RGSV1), 3-6 weeks after inoculation. The leaves remain yellow or pale-yellow, even when adequately fertilized.

Plants infected at the seedling stage rarely reach maturity. However, TN1 plants infected at 30 days or older show symptoms similar to those on plants infected with rice tungro viruses. Plants at a later stage of growth do not develop the symptom of profuse tillering (Cabauatan and Hibino, 1983; Cabauatan et al., 1985).

Taiwan isolates

Three isolates of RGSV have been identified in Taiwan, namely wilted stunt virus isolate (GSW), grassy stunt B (GSB), and grassy stunt Y (GSY) (Chen and Chiu, 1982). It is not yet known whether these different isolates are strains of RGSV. The severity of symptoms caused by these isolates varied with host variety and season.

GSW

This isolate of RGSV is distinctive among the three isolates as it causes severe stunting, reduced tillering, yellowing, twisting and trapping of leaves, and premature death. Initial symptoms usually appear about 10 days after inoculation. Symptoms of GSW are similar to those of plants infected with RGSV2 (Cabauatan et al., 1985) especially during the summer if plants are infected at an early stage of growth. However, the tungro-like symptoms in plants at later stages of growth are not described in GSW-infected plants. Premature death of infected plants is a common reaction to both RGSV2 and GSW. Symptoms appear 3-14 days after inoculation of rice seedlings.

GSB

GSB is a mild isolate of RGSV, causing excessive tillering, reduced stunting, and pale-green leaves with conspicuous mottling.

GSY

Plants infected by this isolate of RGSV are markedly stunted with narrow, striped and erect leaves. The number of tillers is normal during winter but increases in summer.

Philippines RGSV variants

Different variants of RGSV in the Philippines were observed in 1990 (Miranda et al., 1991), namely very mild (M3), moderately mild (M2), moderately severe (SC) and severe (S2). The relationship with existing strains such as RGSV1, GSV2 and the Taiwan isolates has not been determined. These isolates are characterized based on symptomatology, insect transmission and varietal reaction. Symptoms exhibited on infected TN1 plants are described below:

* M3 - This isolate is the mildest and induces symptomless infection. Symptoms may show 19-38 days after inoculation.

* M2 - Plants infected with the M2 isolate exhibit slight stunting and fewer tillers with an erect growth habit. Affected leaves become narrow and slightly yellow. Symptoms appear 18-32 days after inoculation.

* SC - TN1 plants infected with the SC isolate show profuse tillering, characteristic of this isolate. Plants are shorter than those infected by M2. Leaves are narrow with pale-green to yellow colouring and rust-coloured necrotic leaf spots. Symptoms appear 7-20 days after inoculation.

* S2 - The S2 isolate causes severe stunting, less profuse tillers, and a spreading growth habit. The leaves are narrow and yellow with rust-coloured necrotic spots. Symptoms initially appear 8-19 days after inoculation.

Notes on Strains

There are strains and variants reported for RGSV. These strains have been differentiated based on symptomatology, insect transmission and cultivar reaction.
Prevention and control

RGSV is relatively difficult to control as the insect vector must be also be considered.

Host Plant Resistance

RGSV has been successfully controlled in Indonesia, the Philippines, India and Vietnam by planting resistant varieties of rice (Ling et al., 1970; Kush and Ling, 1974; Nuque et al., 1982). These varieties carry sources of resistance derived from Oryza nivara, a wild rice species. However, O. nivara became susceptible when RGSV2 arose in the 1980s (Cabauatan et al., 1985; Hibino et al., 1985a). There are currently no rice varieties resistant to both RGSV1 and RGSV2 (IRRI, 1993).

Chemical Control

Chemical methods are usually used to control the insect vector of RGSV. A mixture containing buprofezin and fenobucarb, formulated as a driftless dust, is also effective in reducing the planthopper population and incidence of grassy stunt (Hirao et al., 1983). The application of insecticides is not always successful in controlling vector populations, especially in regions where rice is grown throughout the year (Hibino, 1986b). The insect has a tendency to adapt to the extensive use of insecticides.

Biological Control

Neem seed oil (from Azadirachta indica) was highly effective in reducing the survival of Nilaparvata lugens, and in suppressing the transmission of RGSV and rice ragged stunt oryzavirus (RRSV) to rice under laboratory conditions (Saxena and Kahn, 1984; 1985).

Cultural Control

Cultural control methods contribute to a reduction in the planthopper population and the elimination of the virus inoculum source (Oka, 1987; Mochida, 1979). Methods used include: simultaneous cropping rotated with secondary crops; immediate destruction of rice stubble and ratoons of harvested rice crops; raising the water level in the field or complete drainage over a few days; and plant spacing which allows sunlight to reach the basal portion of the plant.

Integrated Pest Management

The integration of host plant resistance, chemical, biological and cultural control measures is the most effective approach to the control of both RGSV and the planthopper vector (Mochida, 1986).

Impact
RGSV is one of the major diseases of rice (Ling, 1976). The importance of RGSV has increased since 1970 due to the high population of the brown planthopper, Nilaparvata lugens, in central tropical Asian countries, mainly in South, East, and South-East Asia (Dyck and Thomas, 1979; Hirao et al., 1985). N. lugens is also a vector of Rice ragged stunt virus (RRSV) (Ling et al., 1978; Hibino et al., 1979) and causes feeding damage known as hopperburn (Mochida, 1979; Mochida and Suryana, 1979).

Actual yield losses are difficult to estimate because the damage caused by viruses cannot be distinguished from that caused by N. lugens (Palmer et al., 1978). In 1971, RGSV and N. lugens affected 8000 ha of rice in central Java, Indonesia, causing an estimated yield loss of 77.8% (Tantera et al., 1973). By 1974-1977, RGSV was considered the most damaging and widespread rice virus in Indonesia especially in Java, Bali, Sumatra and Sulawesi, wherein a total of 1.2 million ha was affected by the vector and virus. Combined yield losses exceeded 3 million tons of paddy rice amounting to more than US $510 million (Mochida and Suryana, 1975; Palmer et al., 1978; Palmer and Rao, 1981).

A major outbreak of RGSV was reported in 1973 in Laguna (IRRI, 1973) and in Mindanao (Ou, 1985), the Philippines; estimated yield losses due to virus and planthopper was about $26 million (Dyck and Thomas, 1979). In Kerala, India, 15,000 ha were affected by RGSV (Gopalakrishnan et al., 1973; Anjaneyulu, 1974; Kulshrestha et al., 1974; Santhakumari et al., 1982) and yield losses due to the virus and planthoppers were estimated at $20 million (Dyck and Thomas, 1979).
Related treatment support
Plantwise Factsheets for Farmers
Latt Latt Khaing; CABI, 2014, English language
Thailand, Bureau of Rice Research and Development; CABI, Thai language
Cambodia, General Directorate of Agriculture; CABI, 2014, Cambodian language
Cambodia, General Directorate of Agriculture; CABI, 2014, English language
Malarvannan, S.; CABI, 2012, Tamil language
 
Pest Management Decision Guides
Cambodia, General Directorate of Agriculture; CABI, 2014, English language
Cambodia, General Directorate of Agriculture; CABI, 2014, Cambodian language
 
External factsheets
IRRI Rice Knowledge Bank Factsheets, International Rice Research Institute (IRRI), English language
Bayer CropScience Crop Compendium, Bayer CropScience, English language
PlantVillage disease guide, PlantVillage, English language
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