1Plant Virology Unit, Division of Plant Pathology, Indian Agricultural Research Institute, New Delhi-110012.
2Plant Virus Laboratory and Biotechnology Unit, Division of Crop Protection, Central Research Institute for Jute and Allied Fibres, Barrackpore, Kolkata, India.
Abstracts of the papers presented at the International Conference of Indian Virological Society on “Emerging and Re-emerging viral Diseases of the Tropics and Subtropics” at Indian Agricultural Research Institute, New Delhi, India, December 11–14, 2007.
In India, the nature of viroid diseases could not be established due to paucity in the availability of detection methods. Exocortis disease symptoms, like scaling and splitting of bark, were recorded on Mosambi rootstock from Maharashtra in 1968, much before the viroids became known as plant pathogens. Similar symptoms were also observed on different citrus species grafted on Rangpur lime and mandarins, Cachexia- like symptoms in sweet lime and tangelo and stunting in several rootstocks were reported by biological indexing. In 1992, the molecular evidence for association of viroid-like low molecular weight RNA (LMW-RNA) with exocortis like symptoms was reported using electrophoretic technique. By end of the century, the Indian tomato bunchy top disease was reported to be caused by a citrus exocortis viroid variant (CEVd-t) by sequencing the viroid. Recently, the yellow corky vein disease prevalent on various citrus species has been shown to be associated with at least two viroids, named CEVd-ycv and HSVd-ycv (Hop stunt viroid -yellow corky vein variant). In silico analysis of cloned components showed that CEVd-ycv shared more sequence identity with a mild variant of CEVd while HSVd-ycv with severe variant of HSVd from other parts of the world. Symptoms of bark scaling, bark splitting and leaf yellowing seen on Nagpur mandarin and mosambi grafted on rough lemon and Rangpur lime rootstocks were also found associated with HSVd (HSVd-RL, rough lemon variant). Presence of these viroids indicates the need for the testing of citrus plants for CEVd and HSVd in budwood certification and germplasm evaluation.
