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GLH control for the management of rice tungro disease
T. Ganapathy, N. Subramanian, and M. Surendran
Green leafhopper (GLH) Nephotettix virescens is one of the major sucking pests of rice and is capable of transmitting rice tungro disease (RTD), a devastating virus disease of rice. GLH is considered to be more important as a virus vector than as a direct pest of rice. In a seedbed protection trial, application of neem cake in the seedbed followed by spraying of 5% neem seed kernel extract (NSKE) at 30 days after transplanting reduced the incidence of tungro disease by more than 50% and increased rice grain yield. GLH abundance and the presence of a higher percentage of transmitters resulted in high RTD incidence. A significant positive linear relationship was observed between RTD incidence and both log transmitters and percent transmitter population during an RTD epidemic in the northern districts of Tamil Nadu, India, during 1991-93.
Rice tungro disease (RTD) is caused by two morphologically, chemically, and serologically unrelated virusesrice tungro bacilliform virus (RTBV) and rice tungro spherical virus (RTSV). The rice green leafhopper (GLH), Nephotettix virescens (Distant), is the most efficient vector and can transmit both viruses either together or separately (Hibino et al 1979). RTBV, however, is acquired by GLH only after prior acquisition of RTSV. GLH does not usually cause any significant feeding damage to the rice crop unless its numbers are extremely high. GLHs that carry tungro viruses, however, may cause extensive damage and crop loss (Anjaneyulu et al 1994). Where an overlapping rice cropping pattern occurs, GLH prefer young seedlings and migrate from older crops to seedbeds which can then become infected (Mukhopadhyay et al 1984). Tungro spreads very fast during the early growth stage of the crop (Shukla and Anjaneyulu 1981). Hence, it is essential that rice seedlings be adequately protected from tungro infection. This can be achieved by applying insecticides in seedbeds to kill immigrant vectors to prevent virus transmission. A seedbed protection trial was therefore conducted to study the effect of nursery protection on the incidence of GLH and RTD.
The onset of the disease depends on the presence of a susceptible host, a virus source, and the vector. Anjaneyulu (1975) observed that adult GLH plays an active role in introducing primary inoculum to the field, whereas both nymphs and adults help in further secondary spread. The availability of virus inoculum, a high population of GLH, and early growth stage of the crop are responsible for a disease outbreak (Chen and Othman 1991). High vector populations (Vidhyasekaran and Lewin 1986) and a large proportion of viruliferous vectors (Savary et al 1993) also play an important role in disease outbreaks. A direct correlation between vector population and disease incidence has been observed at different locations (Lim 1972, Hibino 1986, Shukla and Anjaneyulu 1981). The presence of viruliferous GLH is one of the most important factors that cause RTD incidence.
Materials and methods
Two field trials were conducted to study the effect of seedbed protection with pesticides on the incidence of RTD and yield. A susceptible variety, IR64, was used in the trials, which relied on naturally occurring sources of virus inoculum. Seedlings were raised in a nursery and transplanted into 8 8-m plots at 24 d after sowing (DAS). There were five treatments. Each treatment was replicated three times in a randomized complete block design: T 1 = carbofuran at 6.25 g m -2 at 20 DAS, T 2 = carbofuran in seedbed at 20 DAS and monocrotophos (0.2%) at 25 d after transplanting (DAT), T 3 = neem cake at 1.5.5 g m -2 at 20 DAS, T 4 = neem cake in seedbed and 5% neem seed kernel extract (NSKE) at 25 DAT, T 5 = control. Numbers of GLH adults were estimated at 14, 28, and 56 DAT using 10 sweeps of a 30-cm-diameter insect net in each plot. Nymphal numbers were counted on 10 hills in each plot. Rice plants were scored visually for tungro incidence on the same dates. After harvest, grain yield was recorded.
A rice tungro epidemic occurred during 1991-93 in the northern districts of Tamil Nadu. Using data collected at the Rice Research Station (RRS) in Tirur, the relationship between tungro incidence, monthly numbers of GLH collected in a light trap, and the proportion of virus transmitters was examined. The percentage of transmitters in the GLH population was calculated by vector indexing. This was done as follows:
Individual GLHs were placed in a test tube containing 7-10-day-old TN1 seedlings for a 24-hours inoculation feeding period. Seedlings were subsequently planted in pots and grown in insect-proof cages. One week after inoculation, the number of seedlings showing typical symptoms of RTD was counted. The percentage of seedlings infected was considered to be equal to the percentage of viruliferous GLH. The following formula was derived:
Log transmitters (Lt) = log 10 (Tg Pvv)
where Tg = total monthly light trap GLH collection and Pvv = proportion of viruliferous vectors (% vv/10) where % vv = (no. of transmitters/no. of GLH tested) 100. The relationship of RTD incidence with log transmitters and percent transmitters was examined using regression analysis.
Results and discussion
Protection of the seedbed with insecticides and biopesticides reduced tungro incidence compared with the control (Table 1). Adult GLH numbers at 14 and 28 DAT were lowest in plots where carbofuran and neem cake were applied in the seedbed, but nymphal numbers were similar in all treatments. The highest grain yield was recorded in T4, where neem cake and NSKE were applied.
These results are consistent with previous findings. Protection of rice seedlings grown in soil incorporated with 150 and 250 kg neem cake ha -1 was effective against rice tungro (Saxena 1987). Also, NSKE and neem cake powder mixed with carbofuran at 1.0 kg ai /ha reduced tungro incidence similarly as applying a higher rate of carbofuran alone (Abdul Kareem et al 1988).
At RRS, Tirur, viruliferous GLH were first observed in August 1991 and initial disease symptoms appeared in September when 16% of rice plants were affected. The disease was prevalent up to December 1991, with a peak incidence of 35% during October, which corresponded to the highest percentage of transmitters. Even though the GLH population increased from January to March 1992, no RTD incidence occurred because of the absence of transmitters. Again, the GLH population increased in May 1992 and, because of the presence of transmitters, symptoms reappeared during June 1992. Numbers of viruliferous GLH and numbers of GLH per hill peaked in August and November 1992. Because of the presence of a high percentage of transmitters.
RTD incidence reached a maximum of 96% during August 1992. RTD incidence and the percentage of transmitters started declining at the end of 1992. No further disease was recorded in February 1993. An analysis of the relationship between log transmitter population and RTD incidence using linear and exponential models revealed that the linear regression model provided the best fit to the data (r = 0.831, error mean square = 400.4) (Table 2). The relationship between the percentage of transmitters (percent viruliferous GLH) and RTD incidence was also studied. A highly significant positive relationship between the percentage of transmitters and percent RTD incidence was observed in the linear model (r = 0.978) (Table 2). Suzuki et al (1992) demonstrated a relationship between the percentage of infective GLH and the percentage of RTD-infected hills in rice fields at 5-7 wk after transplanting. They developed an infective vector index and found it useful in predicting cumulative infection at early stages of crop growth in Indonesia.
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Authors address: T. Ganapathy, N. Subramanian, and M. Surendran, Department of Plant Pathology, Tamil Nadu Agricultural University, Coimbatore 641003, Tamil Nadu, India.
Citation: Ganapathy T, N. Subramanian, and M. Surendran. 1999. GLH control for the management of rice tungro disease. p. 152-157. In: Chancellor TCB, Azzam O, Heong KL (editors). Rice tungro disease management. Proceedings of the International Workshop on Tungro Disease Management. 9-11 November 1998, International Rice Research Institute, Los Baos, Philippines. 166 p.