Resistances in Rice to Tungro-Associated Viruses

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Abstract
Tungro is a composite disease associated with rice tungro bacilliform virus (RTBV) and rice tungro spherical virus (RTSV). Over a period of 23 yr, 40,000 rice germ plasm accessions were evaluated for resistance to tungro by greenhouse mass inoculation using the vector green leafhopper (GLH), Nephotettix virescens. From those, 119 cultivars that showed low levels of infection were selected. Each seedling of the selected cultivars, was exposed in a test tube to five GLH that had fed on plants infected with either both RTBV and RTSV or RTSV alone. Inoculated seedlings were indexed by enzyme-linked immunosorbent assay (ELISA). Seedlings inoculated with both viruses combined were also scored for symptom severity on a scale of 1-9. Nine cultivars had low levels of overall infection with RTBV and RTSV, 40 cultivars had low or no infection with RTSV, and 16 cultivars were tolerant to RTBV and developed very mild symptoms even when infected with both viruses. Some cultivars had more than two types of resistance in combination. Some of these cultivars with resistances or tolerance to the viruses also had resistance to GLH. The resistance to virus infection or tolerance to RTBV can be used in the breeding program for tungro resistance. The method adopted would be useful in evaluating breeding lines to tungro and in the analysis of inheritance for tungro resistance.