Ditylenchus angustus
(rice stem nematode)

D. angustus survives between crops in an anhydrobiotic state in rice stem tissues and in partially or fully enclosed panicles (Cox and Rahman, 1979; Kinh, 1981). D. angustus is dependent on environmental conditions to survive dehydration (Ibrahim and Perry, 1993) and successful anhydrobiosis relies on slow drying, which occurs, for example, in dry soil (Cuc, 1982) or within plant tissues. Butler (1913) recovered nematodes after 7-15 months from plant material. D. angustus can be present in freshly harvested mature true seed and in sterile spikelets (Butler, 1919; Hashioka, 1963; Cuc and Giang, 1982; Ibrahim and Perry, 1993) and although transmission in seed can occur (Sein, 1977a), the risk of such transmission is minimal, particularly after thorough sun drying (Seshadri and Dasgupta, 1975). The anhydrobiotic population is predominantly fourth-stage juveniles (Ibrahim and Perry, 1993): this stage has the greatest infective and reproductive potential (Plowright and Gill, 1994).

Nematodes are spread in water or by leaf contact at high relative humidities (>75% RH) (Rahman and Evans, 1987); long-distance spread in water is possible (Bridge et al., 1990). D. angustus invades rice plants at the water surface (Plowright and Gill, 1994) and rapidly enters the innermost leaf interstices, where it feeds on meristematic tissues and on young leaves within the leaf sheath. Young plants are more easily invaded than old (Rahman and Evans, 1987). It has a short life cycle, 10-20 days at 30°C, and populations tend to have a stable demographic equilibrium in which fourth-stage juveniles predominate (Plowright and Gill, 1994).

Incidence

Prasad and Varaprasad (2002) have shown that D. angustus is seedborne. The nematode was found infecting irrigated rice in Godavari delta, India, which is located away from the ufra endemic zone (Assam, india). Live nematodes were recovered from the dried seeds, mainly located in the germ portion, 3 months after harvest. D. angustus can be present in freshly harvested, mature, true rice seed and in sterile spikelets (Butler, 1919; Hashioka, 1963; Sein, 1977a; Cuc and Giang, 1982; Ibrahim and Perry, 1993). In Burma, rice plants heavily infested with D. angustus produced panicles containing many unfilled grains. An average of 15.4 nematodes was found in each unfilled 'grain' compared with 5.3 in mature grain. The desiccation survival of third- and fourth-stage juveniles and adults of D. angustus was examined at different relative humidities on glass slides, on agar and agarose substrates, and in infested rice stems and seeds. Individual nematodes show no intrinsic ability to control water loss and survive severe desiccation. Nematodes of all three stages are dependent on high humidities and/or protection by plant tissue for long-term survival (Ibrahim and Perry, 1993). In another study, 2-day-old seeds of rice varieties Nep mua and Lua Tieu infested with D. angustus at 16.5-17% RH had 7 and 63 nematodes per 100 unfilled grains, and 25 and 167 nematodes per 100 filled grains, respectively. When infested seeds were dried in the sun (44-45°C) for 4 days (6 h/day), seed humidity decreased to below 12% and the nematodes were killed (Cuc and Giang, 1982).

Seed Transmission

When naturally infected seeds were used as an inoculum source, large numbers of seeds grown together resulted in diseased plants (Sein, 1977a). However, the risk of such transmission is minimal, particularly after thorough sun drying (Seshadri and Dasgupta, 1975).

Seed Treatments

Soaking rice seed for 24 hours in thiabendazole (0.1%) or ethylthiocyanoacetate (0.13%) was effective and economically feasible for control of D. angustus (Vuong Huu Hai and Rodriguez, 1972). Chakraborti (2000) tested a variety of treatments to rice seeds or seedlings for control of rice stem nematode and found that using a trap crop (cv. Shalibahan) in the seed bed combined with the application of azadirachtin and neem (Azadirachta indica) oil to seedlings for 30 minutes gave the lowest mean percentage of infected tillers (1.8 per 25 hills) and nematode population (12.3 per 250 g of soil) and the highest yield (3.0 t/ha). Other treatment combinations or individual treatments also effectively suppressed the nematodes, but some were only as efficient as the control. Rahman (1996) examined the management of ufra disease, caused by D. angustus, with or without stubble cleaning, ZnSO4, neem cake and neem seed dust in transplanted Aman and irrigated Boro rice. Neem seed dust controlled ufra disease with 72.2-91.0% healthy panicles producing 5.3 to 6.7 t/ha yield. Simple stubble cleaning had less ufra infestation and produced two to three times more yield than the control (diseased) and no stubble cleaning treatments.

D. angustus feeds on tissue developing within the leaf sheath, so the youngest emerging leaf should be examined for symptoms in the field. Symptoms will develop within 1-3 weeks of infection, depending on the size and source of the initial inoculum. Infected plant residues, before flooding, provide primary infection sources in a field, whilst secondary and tertiary infection occurs from water with the onset of flooding.

Symptoms of low infection are difficult to detect: to accurately assess or confirm infection it is therefore necessary to sample tillers in the field. Tillers should be cut above the peduncle because nematodes are not found in the internodes below the growing point. 1-cm sections of the leaf sheath should be split longitudinally and placed in water for 24 h on a Baermann funnel placed in a bijou bottle or similar vessel. The nematodes will migrate into the water and can then be collected and counted. Such extracts may become foul within 24 h. For immediate examination of samples, leaf sheaths can be teased apart in water in a Petri dish to release nematodes and observe directly.

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

Introduction

The Deepwater Rice Management Project (Anon., 1987) listed control measures which may be appropriate against D. angustus:

- thorough burning of crop residues to eliminate all infested stem terminals

- extending the overwintering period by delayed planting

- the use of shorter-duration cultivars

The use of resistant cultivars, when they become available, should prove to be the most effective measure.

Cultural Control

Burning of infested crop residues gives very effective control and has long been advocated (Butler, 1919). Thorough burning is essential, although it is not always possible where soil remains water-logged after harvest or when a large proportion of the straw is removed for other purposes (e.g. for cattle fodder), leaving insufficient for effective burning (McGeachie and Rahman, 1983).

Ploughing in crop residues can reduce ufra (caused by D. angustus) because nematodes decline more rapidly in moist soil than in foliar remains (Butler, 1919). This is not always possible and depends on local circumstances and soil conditions.

Growing a non-host crop, such as jute, in rotation with deepwater rice can reduce the incidence of ufra in fields where the rise of floodwater is not excessively fast (McGeachie and Rahman, 1983). Lowland transplanted rice rotated with a non-host, mustard, was less affected by ufra than continuously cultivated rice (Miah and Rahman, 1985).

Removal of volunteer and ratoon rice plants, wild rice and other host weeds will help prevent the carryover between seasons, and banks between plots may also be beneficial (Sein and Zan, 1977).

D. angustus survives for a limited period: lengthening the overwintering period can reduce primary infection (Cox and Rahman, 1980; McGeachie and Rahman, 1983). This can be achieved with deepwater rice by using short-duration cultivars (which would reduce population densities of the nematode at harvest) or by late sowing and transplanting. Manipulation of rice cropping patterns and cultivation techniques is a promising means of control (McGeachie and Rahman, 1983).

Where it is possible, submerging young seedlings for a week would reduce infection in lowland rice. Maintaining water levels well below the top of the leaf-sheath collar throughout the crop in lowland rice reduces nematode dispersal and secondary and tertiary infection. It is essential to protect seedlings (that are in seed beds and destined to be transplanted) from early infection, especially for the first 14 days after sowing.

Rahman (1996) reported upon ufra disease management in rainfed lowland and irrigated rice in Bangladesh. Rahman (1996) found that neem seed dust was effective in controlling ufra disease. Stubble cleaning was also effective at reducing infection.

Host-Plant Resistance

A large number of deepwater and lowland rice cultivars have been tested against D. angustus.

In Vietnam, four high-yielding, local improved breeding lines (IR9129-393-3-1-2, IR9129-169-3-2-2, IR9224-117-2-3-1, IR2307-247-2-2-3) and three cultivars (BKN6986-8, CNL53, Jalaj) are described as slightly infected (Kinh and Phuong, 1981; Kinh and Nghiem, 1982). A Burmese cultivar (B-69-1) from the Irawaddy Delta was tolerant of ufra disease (Sein, 1977b), and a Thailand cultivar (Khao Tah Ooh) was relatively less susceptible (Hashioka, 1963). Two cultivars in West Bengal, India (IR36 and IET4094) were also less susceptible (Chakrabarti et al., 1985). Complete resistance to D. angustus has been found in wild rice, Oryza subulata, and a deepwater cultivar, RD-16-06 (Miah and Bakr, 1977a). The Rayada group lines are highly resistant to D. angustus in Bangladesh, and others showing moderate resistance are CNL-319, BR306-B-3-2, BR308-B-2-2, Bazail 65 and Dalkatra (Rahman, 1987). Improved cultivars could become available to farmers in the near future (Anon., 1987).

The cultivars Padmapani and Digha are not attacked by D. angustus in areas of India and Bangladesh. It is suggested that they escape the disease because of their short growth duration (Mondal and Miah, 1987; Rathaiah and Das, 1987).

Plowright et al. (1996) recorded the induction of phenolic compounds in rice after infection by D. angustus. Chlorogenic acid occurred in small quantities and was only found in resistant plants. Levels increased in response to infection. Sakuranetin, a phytoalexin, was isolated from two resistant selections of Rayada 16-06. There was correlative evidence that the compound had a functional role in resistance.

Das and Sarmah (1995) claimed 80% resistance or more to D. angustus in RDA 14, RDA 4, RDA 2, RDA B4, RDA 16-06/1/2, RDA B8, RDA 3, Bazail 65, RDA B5 and RDA B3 genotypes. The resistance was expressed in field trials in Assam.

Sarma et al. (1999), working in Assam, evaluated 19 advanced breeding lines from Bangladesh. Three resistant lines were recommended for deeply flooded areas of Assam while the remaining lines may be more suitable for less flooded areas.