Int.J.Curr.Microbiol.App.Sci (2019) 8(9): 749-757 749 Original Research Article https://doi.org/10.20546/ijcmas.2019.809.090 Biological Indexing and Molecular Approaches in Detecting a Mild Strain ‘CRS 4’ against Citrus tristeza Virus in Khasi Mandarin (Citrus reticulata) Borsha Rani Baruah 1* , R. K. Kakoti 2 , A. Borbora 2 , S. Singh 3 , S. Saikia 4 and P. D. Nath 1 1 Department of Plant Pathology, 3 Department of Agricultural Biotechnology, Assam, India Agricultural University, Jorhat-13, India 2 Citrus Research Station, Tinsukia, 4Horticultural Research Station, Kahikuchi, Assam, India Agricultural University, Assam, India *Corresponding author ABSTRACT International Journal of Current Microbiology and Applied Sciences ISSN: 2319-7706 Volume 8 Number 09 (2019) Journal homepage: http://www.ijcmas.com Citrus tristeza virus, the most important viral disease of citrus, is reported in Assam and other NE states of India to infect Khasi mandarin (Citrus reticulata), the most economically important citrus crop of the region. For effective management, an attempt was made to identify a potential mild isolate against virus. Leaf samples were collected from Khasi Mandarin plants expressing differential symptoms from three different locations viz., Tinsukia, Golaghat and Mariani of Upper Brahmaputra Valley Zone of Assam. These were then grouped into three categories based on ELISA OD 405 values. Biological indexing with CTV positive samples from these three serological categories on Mexican lime (Citrus aurantifolia) seedlings resulted in symptom expression within three months post grafting. Visible symptoms of CTV infection were observed in some of the graft successful indicator plants whereas, in Khasi mandarin selection ‗CRS 4‘, no visible symptom development took place within this period. Based on the results, the plants were grouped into two groups- symptom producing and non- symptom producing, and were confirmed through Bi-directional RT-PCR with mild and severe strain primers. PCR products for ‗CRS 4‘ were sequenced. Consensus sequences showed a single nucleotide difference at position 371 for mild isolates (CRS 4), thereby confirming the identity. Keywords Citrus tristeza virus, Khasi mandarin, mild strain, biological indexing, BD/RT-PCR, Assam Accepted: 10 August 2019 Available Online: 10 September 2019 Article Info