Review: Virus-Indexing of Citrus Tristeza Virus for Screening Quality Planting Material in Citrus spp.

Citrus is one of the most popular fruit crops of the world. It is cultivated in around 140 countries, including many in the Asia-Pacific. During recent years, there has been significant increase in the citrus production mainly on account of its increased use as nutritious and healthy drink, but with increasing area production has not been increase because of several reasons including diseases and pest. Among these organisms causing diseases viruses causes major losses in citrus field. Citrus is infected by more than 15 virus or virus-like diseases, including citrus greening, citrus tristeza, citrus tatter leaf and citrus exocortis that severely affect its production as well as quality. In India, Philippines, Thailand and Taiwan, the reduction in yield due to these diseases could range up to 60 to 80 percent, sometimes wiping out the whole orchard. Control of these diseases has become possible through the use of disease-free nursery seedlings, removing diseased trees, eliminating insect vectors, rejuvenating infected trees by chemotherapy, thermotherapy etc. Use of pathogen-free planting material is of primary importance in raising healthy trees for which the technique of shoot-tip micro grafting (STG) has proved very successful. There are two major virus indexing techniques implemented in the plant industry since 1980s. The first one is “serological techniques”, which include many different formats of protocols but basically they depend entirely on specific antibodies against viruses that is monoclonal antibodies. ELISA detects the protein presents in the sample by antibody-antigen interaction. The second technique for virus indexing is DNA-based technologies including DNA probe hybridization, Polymerase chain reaction (PCR) and biochips (DNA micro-array). DNA-based techniques detect the genome of viruses, instead of the gene products such as coat protein; therefore, these techniques can theoretically detect virus infection earlier. Results of biological indexing by using cuttings were confirmed by using serological and molecular assays. It is generally accepted by the scientific communities that DNA-based techniques and serological ones are more sensitive for detection of virus sample in early stage.