Indian citrus ringspot virus.
Indian Citrus Ringspot is a serious disease of Kinnow mandarin in North India. The disease is characterized by vein clearing symptoms on young leaves followed by bright yellow rings on mature leaves. Affected trees often show decline symptoms. The incidence of ringspot disease was observed up to 100% in most of the kinnow mandarin orchards in North India especially in Punjab. Except bud transmission no other mode of natural spread of the disease could be established so far. The virus was mechanically transmitted from citrus to herbaceous hosts like Chenopodium quinoa, C. amranticolor, Phaseolus vulgaris var. saxa, singtamey, gheusami, and alapatri, Vigna unguiculata (Cowpea var. Pusa Komal.). Most of the commercial citrus cultivars and rootstocks used in India are susceptible to ringspot disease and develop variable symptoms upon inoculation. Virus particles of Indian citrus ringspot virus (ICRSV) are flexuous filamentous about 650 nm in length and 13 nm in diameter and have helical symmetry with surface pattern of cross banding. The genome of the virus is a ssRNA 7.6 kb in size and comprises six ORFs, encoding replication related protein, the putative MPs (TGB), the CP and a putative nucleic acid binding regulatory protein. The virus has Coat protein of 34 kDa in size. The natural host range of the single species is restricted to citrus. The International Committee on Taxonomy of Viruses (ICTV) has recently approved a proposal to create a plant virus family named Flexiviridae, which includes the existing genera Allexivirus, Capillovirus, Foveavirus, Carlavirus, Trichovirus and Vitivirus together with the new genus Mandarivirus for ICRSV. Four isolates of Indian citrus ringspot virus - ICRSV-DI, ICRSV-Ab, ICRSV-Ah and ICRSV-Pu, from Delhi, Abohar, Ahmedabad and Pune respectively have been studied with a view to determine the variability in the pathogen. The virus was successfully detected in RT-PCR with specific primers. The primers ICRSV-1096/ICRSV-1420 detected all the four isolates of ICRSV, but primers ICRSV-518/ICRSV-951 and ICRSV-518/ICRSV-1420 specifically detected the isolates ICRSV-DI and ICRSV-Ab showing variability in genome of ICRSV. The probe made from 350 bp amplicon of the PCR product from primers ICRSV-1096/ICRSV-1420 also detected all the four isolates in NASH tests. Virus-free nucleus planting material of Kinnow mandarin and Mosambi sweet orange through shoot-tip grafting (STG) was developed. A shoot tip size of two leaf primordia was found free from ICRSV and other viruses such as citrus tristeza virus (CTV), Citrus Yellow Mosaic badnavirus (CYMV), and Citrus yellow vein clearing Virus (CYVCV). Indian citrus ringspot virus can be managed by an integrated approach using virus-free planting material, sanitation, cultural practices and regulatory measures. A sound certification programme, which includes quarantine, clean stock and certification, is required for elimination of viral pathogens in citrus in the country, which is yet lacking. Efforts are required to identify resistant or tolerant rootstocks and to develop virus resistant transgenic plants.