Rice tungro bacilliform virus (rice tungro)

In ricefields that are affected with tungro, plants are infected with either both RTBV and RTSV, or with each virus alone. Doubly infected plants show typical so-called 'tungro symptoms', including plant stunting, yellow or orange discoloration of leaves, and reduced tiller number. The discoloration starts from the leaf tip and extends to the lower part. Discoloured leaves may have irregular, small, dark-brown blotches. Younger plants may show interveinal chlorosis. Plants infected with RTBV alone show milder symptoms; plants infected with RTSV alone show very mild stunting but no yellowing of the leaves. RTBV-infected plants may not form panicles; if formed, panicles may be unfilled or with discoloured grains. Panicle exsertion is often incomplete or delayed. Generally, symptoms are severe when plants have been infected at the seedling stage. The symptoms appear a week after inoculation and may become milder again as the plant grows. The symptoms are enhanced when plants are also infected with RTSV.RTBV is limited to the phloem sieve tubes and phloem and xylem parenchymatous cells (Sta Cruz et al., 1993). The virus particles are scattered or aggregated in the cytoplasm of infected cells.

Introduction

Control measures are required in irrigated areas where rice planting is staggered in time.

IPM Programmes

An integrated management scheme has been practised in South Sulawesi, Indonesia (Sama et al., l991). The scheme has four components: planting two crops in the months when the vector population is low; scheduled planting to ensure nearly synchronous growth in each area; rotation or deployment of cultivars with different degrees of resistance to the vector Nephotettix virescens; and applying insecticide to reduce vector density in fields that are affected with tungro. Tungro incidence has been low during the scheme: this may be explained by the slow rate of vector population increase and the few or no sources of tungro in the area.

RTBV is widely distributed in rice-growing areas in South and South-East Asia, and in southern China, where rice is the staple food. Tungro is the major threat to stable rice production in irrigated areas in South and South-East Asia (Hibino, 1989; Thresh, 1991; Bos, 1992). It has become increasingly important since the mid-1960s; major outbreaks occurred in 1969 and thereafter repeatedly in many countries (Hibino, 1989).

Tungro became a serious problem after the release of high-yielding rice cultivars which are short statured and photo-insensitive, have shorter growth duration, and allow rice growers two crops of rice in a year. Staggered planting under the double-cropping rice systems is the major reason why tungro disease predominated after the 1960s. The genetic uniformity of rice planted after the introduction of such cultivars has also had an effect (Thresh, 1991; Bos, 1992; Hibino, 1995).

Rice production in these regions relies largely on irrigated areas where tungro is endemic; its incidence is low or nil in rainfed or upland rice fields. When it reaches epidemic proportions, tungro is disastrous and can damage rice fields in a huge acreage even in rainfed or upland areas (Hibino, 1989; Thresh, 1991; Bos, 1992). Tungro spreads fast even in fields where the vector population is not too high; rice is often totally damaged in one area.

RTBV does not occur as an independent disease, but is associated with its helper virus Rice tungro spherical virus (RTSV). Together they cause rice tungro disease. RTBV causes the tungro symptoms, while RTSV does not cause any distinct symptoms except mild plant stunting. However, it does enhance the symptoms caused by RTBV (Hibino et al., 1978).

Under experimental conditions, yield loss due to tungro infection was 20-100% depending on the cultivars tested (Lin and Palomar, 1966; Hino et al., 1972; Chowdhury and Mukhopadhyay, 1975; Srinivasan, 1979; Rao and Anjaneyulu, 1980; John and Ghosh, 1981; Rao et al., 1983; Yadaf and Mishra, 1985). In an experiment using rice plants infected with RTBV and RTSV separately or together, yield reduction was 18-97% in RTBV and RTSV infected plants and 16-91% in RTBV alone infected plants (Hasanuddin and Hibino, 1989). Thus, losses due to tungro are largely due to RTBV infection.

Rice tungro disease is one of the most destructive diseases of rice in the Asian tropics. The incidence of tungro is generally high in irrigated areas, where rice is planted throughout the year. It is generally endemic but occasionally becomes epidemic and causes devastating losses over huge areas. Tungro has caused average annual losses of more than US$ 1500 million (Herdt, 1988).

Tungro-like diseases have been known to occur in Indonesia since 1859, in Malaysia since 1938, and in the Philippines since 1940 (Ou, 1985) before the viral nature of tungro disease was reported (Rivera and Ou, 1965). At least some of these diseases are now considered to be tungro disease.

In India, tungro reached an epidemic level in West Bengal, Bihar and Uttar Pradesh in 1969, and in Kerala in 1973-1974 (Anjanyulu et al., 1994). In 1981, a high incidence of tungro was recorded in Bihar and West Bengal (Singh et al., 1982), and in Andhra Pradesh, Orissa and West Bengal 90,000 ha of land were affected (Anjyanyulu, 1994). In 1984-1985, a tungro outbreak occurred in Tamil Nadu and Andhra Pradesh. The area damaged was 30,000 ha with an estimated yield loss of 20,000 t in Tamil Nadu (V. Mariappan, personal communication, Tamil Nadu Agricultural University, Coimbatore, India); 49,000 ha were affected in Andhra Pradesh (G. Venkata Rao, personal communication, Andhra Pradesh Agricultural University, Hyderabad, India). In 1998, an outbreak of tungro-like yellow stunt syndrome occurred in the Punjab and damaged 40,000 ha of the 490,000 ha planted to rice in the area (Azzam et al., 1999). Yield losses on the affected land were estimated at 30-100%.

In Indonesia, a disease called 'mentek', which causes tungro-like symptoms, damaged an average 30,000 ha of rice in Central Java during 1840-1859 (van der Vecht, 1953) and affected 30,000-50,000 ha of rice in 1934-1936 (van der Vecht, 1953). Major outbreaks of tungro have been recorded from time to time in Sulawesi and Sumatra during the period 1969-1985, when the total damaged area was estimated as 140,000 ha and the total loss reached 220,000 t of rice, equivalent to $70 million (Tantera, 1986). In 1980-1985, tungro became epidemic in Bali, Java and Nusa Tungara damaging 40,000 ha of land and 66,000 t of rice, equivalent to $20 million (Tantera, 1996). In another estimate, the total area affected with tungro disease during the period 1969-1984 in Indonesia reached 200,000 ha (Manwan et al., 1985). In 1989-1997 in Bali and in 1994-1995 in Java, a tungro outbreak occurred again. In 1995 alone, tungro damaged 12,000 ha of land and caused a yield loss of US$1.9 million in Surakarta regency, Central Java (Daradjat et al., 1999).

In Malaysia, tungro disease has been known since 1934 and is called 'penyakit merah'. A high incidence of tungro was recorded for the first time in Perak in 1964 (Ou et al., 1965) and again in 1969. In 1981-1984, tungro outbreaks occurred at Kedah, Penang, Perak and Perlis. Areas affected with tungro reached 12,700 ha in 1981, 17,500 ha in 1982, 12,900 ha in 1983, 3,100 ha in 1984, and 1,000 ha in 1985 (Imbe and Habibuddin, 1989). Losses due to tungro were estimated to be 5,600 million t in 1981, 15,300 million t in 1982, and 15,500 million t in 1983 (Supaad, 1989). The total losses during the period 1981-1983 amounted to US$ 8.8 million.

In the Philippines, tungro was recorded as early as the 1940s in major rice growing regions and reduced grain yield by 30% amounting to1.4 million t annually (Serrano, 1957). Major outbreaks have occurred from time to time during 1962-1998. In 1970, outbreaks damaged rice on 100,000 ha in central Luzon and at Cotabato (Ou et al., 1974). Yield losses due to tungro in 1971 were estimated as 456,000 tonnes of rough rice (Ling et al., 1983).The crop loss estimated by the Provincial office of the Department of Agriculture for the outbreak in Davao del Norte in 1993 alone reached US$406,500 (Truong et al., 1999).

In Thailand, a tungro epidemic started in the southern part of the Central Plain in 1964. In 1966, 350,000 ha of rice fields were severely diseased and 310,000 ha were moderately affected (Lamey et al., 1967). The area damaged reached about 10% of the total land planted for rice. In 1968, tungro damaged 50,000 ha of land, and the yield loss reached to 50% (King, 1968). Based on data from the national Extension Department, the area of tungro affected land was 134,000 ha in 1968, 254,000 ha in 1969, 210,000 ha in 1970, 163,000 ha in 1971, 95,000 ha in 1972, 75,000 ha in 1973, 152,000 ha in 1974, and 146,000 ha in 1975 (Wathanakul and Weerapat, 1977).

In Vietnam, tungro was first observed in 1982. It reached epidemic levels in 1990 on 20,000 ha in central Vietnam (Ngo et al., 1994).