SOME BIOLOGICAL AND GENOMIC PROPERTIES OF RICE TUNGRO BACILLIFORM BADNAVIRUS AND RICE TUNGRO SPHERICAL WAIKAVIRUS FROM NEPAL

A survey of rice fields during the main growing seasons in 81 location s from 21 districts of the Southern Terai region of Nepal indicated th at rice tungro was primarily restricted to the Hardinath (Janakpur) an d Parwanipur (Bara) regions. The tungro incidence in Hardinath ranged from 17% to 51% and in Parwanipur from 6% to 61% causing about 89% gra in yield loss in Hardinath. Both rice tungro bacilliform badnavirus (R TBV) and rice tungro spherical picornavirus (RTSV) were found in tungr o isolates collected from Hardinath and Parwanipur. These isolates wer e transmitted by Nephotettix virescens and leaf extracts reacted to an tisera against RTBV and RTSV. In a dot blot hybridisation assay, leaf extracts of 12 weed species collected from the tungro-affected area in Hardinath and Parwanipur also reacted with RTBV DNA probes. On mass i noculation of 15 popular rice cultivars most became more than 50% infe cted and only cv. Radha 9 had low (22.2%) infection. RTBV DNA and the coat protein region of RTSV from the Hardinath isolate were cloned and partially characterised. A comparative analyses by restriction endonu clease digestion, cross hybridisation, the polymerase chain reaction a nd partial sequencing indicated that the Nepalese RTBV DNA clone and t he cDNA clones of the RTSV RNA were more similar to the various tungro isolates from the Indian subcontinent than to those from the Philippi nes.