Aphelenchoides besseyi
(rice leaf nematode)

A. besseyi is very widely distributed and now occurs in most rice growing areas (Ou, 1985). Its wide distribution has resulted from dissemination in seed.

See also CABI/EPPO (1998, No. 157).

Rice

After sowing, anabiotic A. besseyi rapidly become active and are attracted to meristematic areas. During early growth, it is found in low numbers within the innermost leaf sheath, feeding ectoparasitically around the apical meristem (Yoshii and Yamamoto, 1950b; Goto and Fukatsu, 1952; Todd and Atkins, 1958). The main stem is frequently more infected than subsequent tillers (Goto and Fukatsu, 1952). A rapid increase in nematode numbers takes place at later tillering (Goto and Fukatsu, 1952) and is associated with the reproductive phase of plant growth (Huang and Huang, 1972).

Nematodes are able to enter spikelets before anthesis, within the boot, and feed ectoparasitically on the ovary, stamens, lodicules and embryo (Dastur, 1936; Huang and Huang, 1972). However, A. besseyi is more abundant on the outer surface of the glumes and enters when these separate at anthesis (Yoshii and Yamamoto, 1950b). As grain filling and maturation proceed, reproduction of the nematode ceases, although the development of J3 to adult continues until the hard dough stage (Huang and Huang, 1972). The population of anabiotic nematodes is predominantly adult females (Huang et al., 1979). These nematodes coil and aggregate in the glume axis. More nematodes occur in filled grain than in sterile spikelets (Yoshii and Yamamoto, 1950b) and infected grain tends to occur more towards the middle of the panicle (Goto and Fukatsu, 1952).

A. besseyi is amphimictic (Huang et al., 1979) and males are usually abundant, however, reproduction can be parthenogenetic. The optimum temperature for oviposition and hatching is 30°C. At 30°C the life cycle is 10±2 days and lengthens significantly at temperatures <20°C (Huang et al., 1972). No development occurs below 13°C (Sudakova, 1968).

A. besseyi aggregate in the glume axis of maturing grain and slowly desiccate as kernel moisture is lost. They become anabiotic and are able to survive for 8 months to 3 years after harvest (Cralley, 1949; Yoshii and Yamamoto, 1950b; Todd, 1952; Todd and Atkins, 1958). Survival is enhanced by aggregation and a slow rate of drying (Huang and Huang, 1974), but the number (Yoshii and Yamamoto, 1950b; Sivakumar, 1987a) and infectivity (Cralley and French, 1952) of nematodes is reduced as seed age increases. It is ironic that good seed storage conditions probably prolong nematode survival.

A. besseyi is not thought to survive long periods in soil between crops (Cralley and French, 1952; Yamada et al., 1953) although anabiotic nematodes may survive on rice husks and plant debris. Sivakumar (1987b) found A. besseyi reproducing on Curvularia and Fusarium in straw after harvest.

The principal dispersal method for A. besseyi is seed. It can be transmitted in flood water in lowland rice (Tamura and Kegasawa, 1958) but the survival of nematodes in water decreases as temperature increases from 20 to 30°C (Tamura and Kegasawa, 1958). High seeding rates in infected seed beds facilitates local dispersal (Kobayashi and Sugiyama, 1977).

Strawberry

A. besseyi is a foliar pest of strawberry and may be found between leaves in buds. The nematode has rapid life cycles (2-3 weeks) and thrive in moist conditions which enable them to move over plant surfaces in water films (Brown et al., 1993).

Incidence

Surveys have shown large numbers of seed lots to be infected with, and high incidences of infection by, A. besseyi throughout the main rice producing areas of the world. In Tanzania, A. besseyi was reported in 12.8% of rice seed lots with infection levels ranging from 2 to 82% within lots (Taylor et al., 1972). About 70% of the samples of rice grains from 32 plantations in central west Brazil were infected. The numbers of nematodes ranged from 10 to >140 nematodes/100 grains (Huang et al., 1977). A. besseyi was recovered from 5.5% of 474 seed samples obtained from rice seed warehouses in Louisiana, USA (McGawley et al., 1984). In Malaysia, 80% of seed lots from four major rice growing areas showed the presence of A. besseyi (Rahmin, 1988). A. besseyi has also been found in the seeds of Stylosanthes hamata (Gokte et al, 1992), Fraxinus americana (Gokte et al., 1989) and Panicum maximum (Merny et al., 1985).

The process of infection of the seeds begins after sowing, when anabiotic A. besseyi rapidly become active and are attracted to meristematic areas of the developing seedling (Nandakumar et al., 1975). During early growth, it is found in low numbers within the innermost leaf sheath, feeding ectoparasitically around the apical meristem (Yoshii and Yamamoto, 1950b; Goto and Fukatsu, 1952; Todd and Atkins, 1958). The main stem is frequently more infected than subsequent tillers (Goto and Fukatsu, 1952). A rapid increase in nematode numbers takes place at later tillering (Goto and Fukatsu, 1952) and is associated with the reproductive phase of plant growth (Huang and Huang, 1972).

Nematodes are able to enter spikelets before anthesis, within the boot, and feed ectoparasitically on the ovary, stamens, lodicules and embryo (Dastur, 1936; Huang and Huang, 1972). However, A. besseyi is more abundant on the outer surface of the glumes and enters when these separate at anthesis (Yoshii and Yamamoto, 1950b). As grain filling and maturation proceed, reproduction of the nematode ceases, although the development of J3 to adult continues until the hard dough stage (Huang and Huang, 1972). The population of anabiotic nematodes is predominantly adult females (Huang et al., 1979). These nematodes coil and aggregate in the glume axis. More nematodes occur in filled grain than in sterile spikelets (Yoshii and Yamamoto, 1950b) and infected grain tends to occur more towards the middle of the panicle (Goto and Fukatsu, 1952).

A. besseyi aggregate in the glume axis of maturing grain and slowly desiccate as kernel moisture is lost. They become anabiotic and are able to survive for 8 months to 3 years after harvest (Cralley, 1949; Yoshii and Yamamoto, 1950b; Todd, 1952; Todd and Atkins, 1958). Survival is enhanced by aggregation and a slow rate of drying (Huang and Huang, 1974), but the number (Yoshii and Yamamoto, 1950b; Sivakumar, 1987a) and infectivity (Cralley and French, 1952) of nematodes is reduced as seed age increases. It is ironic that good seed storage conditions probably prolong nematode survival.

Togashi and Hoshino (2001) investigated relationships between the number of A. besseyi per seed, size of seed harbouring nematodes, and nematode mortality on 18 panicles collected from 12 paddy fields (in Japan) showing three different levels of white tip disease. The panicles were investigated after storage for 30-82 days at 4°C. There was no nematode mortality within seeds during storage. Mean nematode number per seed increased and mean degree of seed swelling decreased as the paddy field infestation of white tip disease increased. For paddy field means, there was a significant negative correlation between the mean nematode number per seed and the mean degree of rice seed swelling. Intriguingly, for individual seeds, the mean degree of swelling increased up to that typical of well-developed seeds with increasing nematode number per seed. Nematode mortality occurred in an inversely density-dependent fashion within individual seeds. The nematode exhibited a clumped distribution among seeds in each paddy field. Such ecological features of the nematode might contribute to its persistence in rice plant populations.

Effect on Seed Quality

Seeds infected with A. besseyi are capable of germination, but hot water treatment to control the nematode reduced germination of some varieties (Rahim, 1988). According to Tsay et al. (1998), 16% of A. besseyi survived under desiccation at 70°C for 12 h, and the germination of rice seed decreased to 44%. At 60°C, 40% of A. besseyi survived and there was no effect on rice seed germination.

Pathogen Transmission

Seed

The principal dispersal method for A. besseyi is seed. Nandakumar et al. (1975) showed that the nematodes were activated from their dormant state by soaking infested rice seeds in water at 28°C for 12 to 15 hours. They then feed on the tender primordium of the sprouting seeds but later migrate to the leaf as it unfurls. Symptoms of white-tip disease appear on leaves 16 days after germination. A. besseyi adapts to seed transmission through its capacity to remain quiescent upon dehydration and reactivate with rehydration. In northern Italy, where conditions were unfavorable for the overwinter survival of A. besseyi, phytosanitary measures such as the exclusion of infested seeds were effective in reducing the spread of the nematode in paddy crops (Bergamo et al., 2000).

The number of studies undertaken on crop losses due to the nematode is very limited, but an inverse correlation between the number of nematodes in seeds and crop performance has been convincingly demonstrated (Huang, 1983). Generally, population densities per seed number or weight are counted. Fukano (1962) determined an economic damage threshold density (300 live nematodes/100 seed). It has been shown that high seeding rates in infected seed beds facilitates local dispersal of the pathogen (Kobayashi and Sugiyama, 1977).

Other sources

A. besseyi can be transmitted in flood water in lowland rice (Tamura and Kegasawa, 1958) but the survival of nematodes in water decreases as temperature increases from 20 to 30°C (Tamura and Kegasawa, 1958). The nematode also lives in rice stubble left in the field after harvest which enables transfer of the inoculum from season to season (Sivakumar, 1987).

Seed Treatment

There are numerous variations of methods for the hot-water treatment of rice seed (Cralley, 1949, 1952; Yoshii and Yamamoto, 1950c, 1951; Todd and Atkins, 1958; Borokova, 1967). The most effective control of A. besseyi requires seed to be pre-soaked in cold water for 18-24 hours and immersed in water at 51-53°C for 15 minutes. Higher temperatures (55-61°C for 10-15 minutes) are required if seed is not pre-soaked. The temperature and duration of treatment must be closely monitored and, after treatment, the seed must be dried at 30-35°C, or sun-dried if stored, but otherwise can be sown directly in the field. Garcia et al. (2000) eradicated A. bessseyi by heat treatments, wet (60°C/10 minutes and 57°C/15 minutes) and dry (90°C/6 and 12 h and 95°C/6 and 12 h). Hoshino and Togashi (2000) showed that nematicides alone caused little nematode mortality within seeds. Most mortality occurred while seeds were being air-dried. Mortality caused by air-drying alone was 1.7 times greater than that caused by soaking seeds in water for 24 hours.

For quarantine purposes at the International Rice Research Institute (IRRI), seed is soaked in cold water for 3 hours followed by hot water at 55°C for 15 minutes. Tenente and Manso (1994) studied rice seeds infested with A. besseyi either immersed in water at 52, 54 and 57°C for 10 and 15 minutes, with and without shaking, or treated with water solutions at 32 and 33 ml/100 ml of thiabendazole (40%). Thiabendazole decreased the nematode population. Heat treatment with shaking at all temperatures and the 57°C/10 minute treatment without shaking also allowed eradication. Only the heat treatment at 52°C for 15 minutes with shaking gave eradication of A. besseyi without affecting germination, and is thus considered as the most appropriate control method. Giudici et al. (2004) found that seed immersion for 10-15 minutes in water at 55-61°C eradicated seedborne A. besseyi without reducing germination.

Various chemical seed treatments have been used for the control of A. besseyi including demeton, malathion and fensulfothion. Lee et al. (1972) reported effective control by treating water or by root dipping with diazinon and nemagon. A. besseyi control with carbosulfore sprays has been reported (Rao et al., 1986), but pre-harvest chemical treatment alone is only partially effective (Aleksandrova, 1981). The timing of application is important. 

Ultrasound and gamma irradiation have also been investigated for their potential in eradicating A. besseyi from seeds (Aleksandrova 1985; Nagy, 1987).

Seed treatment with dipotassium hydrogen phosphate, coupled with a foliar spray of salicylic acid effectively managed A. besseyi in the field in Karnataka, India, and increased the yield by 3.59% (Ramakrishnam and Raiendran, 2003a). Seed treatment with Pseudomonas flourescens, followed by foliar applications of P. flourescens at 45, 55 and 65 days after sowing, reduced chaffiness in rice (caused by A. besseyi) by 56.5% (Ramakrishnam and Raiendran, 2003b). Mohanty et al. (2004) recommend soaking rice seeds in 0.075% cartap hydrochloride for 12 hours, which resulted in 82.02% mortality of seedborne A. besseyi and a significant increase in grain yield.

Biocontrols were trailed in Ghana by Osei and Sackey-Asante (2006). Seed dressing treatment with ground pepper (Capsicum frutescens cv. Legon 18), ground neem seeds (Azadirachta indica) and wood ash, all at 15 g/kg, reduced seedborne populations of A. besseyi by 83.5%, 82.4% and 71.7%, respectively. However, the treatments also inhibited seed germination by 9.6-24.4%, with ground pepper resulting in the greatest reduction.

Seed Health Tests

Manual dehulling (Huang, 1983; Prot and Gergon, 1994)

- Soak seeds in water for 24 hours.
- Dehull seeds with a scalpel and needle.
- Transfer contents (kernels, hulls and water) to a Baermann funnel or a sieve.
- Recover and count nematodes after 48-72 hours.

Modified Baermann or Seive method (Gergon and Mew, 1991; Prot and Gergon, 1994)

- Place seeds over a 10 cm, 40 x 40 mesh steel wire dish in a funnel (12 cm in diameter) and filled with 250 ml water.
- Let it stand for at least 48 hours.
- After incubation, draw ca 20 ml of water into a test tube through a rubber tube attached to the bottom of the funnel.
- Allow the collected water to stand for 1 hour.
- Pipette out excess water and leave 10-15 ml in the test tube.
- Examine the remaining water for nematodes.

Notes on Methods

A mass extraction method has been described (Hoshino and Togashi, 2002). Common methods of nematode extraction applied to Panicum maximum seeds infested with A. besseyi have been compared by Bueno et al. (2002).

A diagnostic protocol for A. besseyi is given in OEPP (2004). The diagnosis of A. besseyi from abnormal rice grains in China is detailed in Lin et al. (2005).

A. Besseyi was identified in forage grasses in Brazil by observing morphological features under the light and scanning electron microscopes in Favoreto et al. (2011).

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.

Preventing the dispersal of A. besseyi in rice requires the elimination of nematodes from seed; hot water or chemical seed treatments are commonly used. Resistant cultivars and cultural methods have been used to reduce infection below damage thresholds, and tolerant cultivars avoid yield loss without nematode control. Stubble burning prevents transmission of A. besseyi in straw and chaff but would have to be used in conjunction with other control measures.

Hot water treatment

There are numerous variations of methods for the hot water treatment of rice seed (Cralley, 1949, 1952; Yoshii and Yamamoto, 1950c, 1951; Todd and Atkins, 1958; Borokova, 1967). The most effective control requires seed to be pre-soaked in cold water for 18-24 hours, then immersed in water at 51-53°C for 15 minutes. Higher temperatures (55-61°C for 10-15 min) are required if seed is not pre-soaked. The temperature and duration of treatment must be closely monitored and, after treatment, the seed must be dried at 30-35°C, or sun dried if stored, but otherwise can be sown directly in the field. For quarantine purposes at the International Rice Research Institute, seed is soaked in cold water for 3 hours followed by hot water at 55°C for 15 minutes.


Chemical Control

Various chemical seed treatments have been used for the control of A. besseyi including demeton, malathion and fensulfothion. Lee et al. (1972) reported effective control by treating water or by root dipping with diazinon and nemagon. A. besseyi control with carbosulfore sprays has been reported (Rao et al., 1986), but pre-harvest chemical treatment alone is only partially effective (Aleksandrova, 1981). The timing of application is important. 

Host-Plant Resistance

Resistance to A. besseyi is widespread. In the USA, A. besseyi has been controlled principally through the use of resistant cultivars. Resistance to A. besseyi has subsequently been reported from Japan (Nishizawa, 1953b); Yamada et al., 1953; Goto and Fukatsu, 1956), Korea (Park and Lee, 1976), India (Rao et al., 1986), Brazil (Oliveira and Ribeiro, 1980; da Silveira et al., 1982), USSR (Popova et al., 1980) and Italy (Orsenigo, 1954). Resistance to A. besseyi is thought to be genetically controlled and carried by the Japanese cultivar Asi-Hi (Nishizawa, 1953).

Symptomless but susceptible (i.e. tolerant) cultivars (Nishizawa, 1953; Goto and Fukatsu, 1956) are common. Symptom expression in the field is particularly variable (Atkins and Todd, 1959) and variations between plants of a cultivar also occur (Orsenigo, 1954). In Thailand, all local cultivars are considered tolerant of A. besseyi (Buangsuwon et al., 1971).


Cultural Control

Irrigating seed beds (Yamada et al., 1953) or direct seeding into water (Cralley, 1956) reduces infection by A. besseyi. In these conditions, nematodes emerge and lose vigour before seed germination. High seedling rates in the seed bed (Kobayashi and Sugiyama, 1977) and high numbers of seedlings/hill (Yamada et al., 1953) tend to increase infection by increasing the number of infection loci in the field. Such problems are thought to be responsible for the re-occurrence of A. besseyi in Japan (Inagaki, 1985). In the USA (Cralley, 1949) and Japan (Yoshi and Yamamoto, 1951; Yamada et al., 1953), early planting, presumably in cooler conditions, reduced or eliminated A. besseyi infection.

Summary of Control Measures

- Hot water treatment of seed. Probably the most effective and cheapest control measure

- Resistant or tolerant cultivars

- Early planting if rice season is preceded by a cooler period

- Low seed bed planting densities

In strawberry, control is possible by hot water treatment of planting stocks, or regeneration of plants from dormant excised axillary buds. Host weed control and planting healthy runners in clean soil is the basis for ensuring freedom from problems.