Further Characterization and Detectionof Indian Citrus Ringspot Virus

Abstract

Indian citrus ringspot virus (ICRSV) from a Kinnow mandarin orchard at the IARI, New Delhi, India (isolate K1) has flexuous particles 650 nm long, with clearly visible cross- banding. It can be mechanically transmitted to and multiplied in Phaseolus vulgaris cv. Saxa. The genome consists of one ssRNA of about 7.5 kb and the coat protein (CP) is 34 kd in size. The virus has been purified and an antiserum prepared suitable for DAS ELISA. Partial sequencing of the genome, revealing the CP ORF near the 3' end and further sequences upstream, indicates that ICRSV, while morphologically resembling a capillovirus, has no homology with this genus. Absence of serological relationship confirms this. Short amino acid motifs in the CP sequence reveal limited similarities with the genera Potexvirus, Carlavirus, Foveavirus and Allexivirus , but no strong similarity to any one of these, so that the virus does not fall into any known genus. RNA and cDNA probes that detect K1 are available, and also primers suitable for RT-PCR. EM decora- tion tests with an antiserum to K1 revealed the presence, in a Kagzi Kalan lime-lemon hybrid of a virus (KK) with similar morphology but differing consistently from K1 by one serological step. Moreover, in the original Kinnow orchard, we detected a further virus (K2), again with similar morphology, serologically related to K1 but differing by five steps. K2 now in turn appears, in pre- liminary results, to be a mixture containing yet a further morphologically similar virus, K3. Thus, Indian citrus ringspot disease may be caused by a series of viruses, and while diagnostic methods are now available for K1, these may not immediately solve the problem of field diagnosis.