Abstract
Integrated management of Rice Tungro Disease (RTD) by combining the appropriate planting time and cultivar rotation has successfully controlled the disease in South Sulawesi, Indonesia. By the early 1990’s, about 10 years after the management was implemented, tungro disease incidence was so low that the integrated management was no longer strictly implemented and eventually abandoned. Surprisingly however, the tungro incidence steadily decreases in the area ever since. Therefore, the purpose of the study was to elucidate why the rice tungro disease incidence decreases even when the integrated management is no longer in place. A series of studies was conducted to determine the adaptability of rice Green Leafhopper (GLH), Nephotettix virescens, colonies on different cultivars with different resistance genes against the leafhopper, tungro transmission efficiency in laboratory and in field and tungro viruses (RTBV and RTSV) detection in weeds. The results indicated that all GLH colonies had adapted to all cultivars carrying different GLH resistance genes. For each colony, there were no significant differences between the resistant cultivars and TN-1 (no GLH resistance gene) in: (1) Percent of first instars developing to the second instar nymphs, (2) nymphal development indices, (3) tungro virus transmission efficiency and (4) GLH populations. The PCR results showed that none of the weeds tested contained the rice tungro viruses. Therefore, the study results suggested that the lack of rice tungro inoculums in the weeds was the main reason why tungro incidence decreases from time to time. However, if the rice tungro viruses are reintroduced to the region, the RTD can become devastating because the GLH colonies have already adapted to all available resistance genes. Further studies should be conducted to find and incorporate new resistance genes into commercial cultivars.
Highlights
Rice Tungro Disease (RTD) is a major limiting factor of rice production, widely distributed in South and Southeast Asia (Bunawan et al, 2014)
There were no significant differences in the percentages of the first instar nymphs developing to the second instar nymphs for all colonies on different rice cultivars with different resistance genes against the Green Leafhopper (GLH) (Fig. 1)
For Pinrang colony, the percentage of the first instar nymphs developing to the second instar nymphs on TN-1 was 100% while the percentages on other cultivars ranged from 96-98%
Summary
Rice Tungro Disease (RTD) is a major limiting factor of rice production, widely distributed in South and Southeast Asia (Bunawan et al, 2014). In Indonesia, during the period of 1969-1983, 168,000 ha of rice was damaged by the disease throughout the country, of which about 100,000 ha occurred in South Sulawesi from 1972-1975 (Hibino, 1987). The viruses are semi-persistently transmitted by several species of the rice Green Leafhopper (GLH) (Hemiptera: Cicadellidae) (Hibino et al, 1978) and among which Nephotettix virescens (Dist.) is the most efficient vector species (Yuliani, 2014). The presence of a less efficient vector species of the viruses, N. nigropictus Stal., has been reported in South Sulawesi (Widiarta, 2005; Nasruddin et al, 2016). During the fallow period between planting seasons when rice plants are not available in the field, the tungro viruses and their vectors survive on different weed species.
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