Guar leaf curl disease from India is caused by Tomato leaf curl virus

Guar, Cyamopsis tetragonoloba, is cultivated in India and other tropical countries as a vegetable, fodder and green manuring crop. It is an economically important crop with its seed flour as the commercial source of gum. National Botanical Research Institute (NBRI), Lucknow, India, has a wide collection of guar germplasms. Since 2000, a leaf curl disease of guar with geminivirus-like symptoms has been consistently observed at the experimental plots of NBRI, during the rainy season. The symptoms consisted of curling, shortening and malformation of leaves, shortening of internodes and stems with overall stunting of diseased plant (Fig 1).

Previous studies demonstrated the association of an unknown begomovirus with this disease. The disease agent was whitefly (Bemisia tabaci) transmitted from diseased to healthy guar plant (C. tetragonoloba cv. RGC-936). Polymerase chain reaction (PCR) could amplify a ca. 578 base pairs (bp) DNA fragment from infected leaf tissues employing begomovirus specific primers representing the core region of the coat protein (CP) gene (Wyatt and Brown, 1996). In addition, Southern hybridization, under high stringency conditions, gave strong positive signals when hybridized to DNA probes prepared from the CP gene of other begomoviruses (Khan et al., 2002). To establish the identity of the unknown begomovirus, in present studies, the PCR amplicon was cloned and sequenced. A BLAST search of the sequence databases, revealed high nucleotide sequence identity (90% or more) with the corresponding regions of the CP genes of Indian isolates of Tomato leaf curl virus (ToLCV). It shared 95% sequence identity with ToLCV-BanII (accession number U38239); 91% with ToLCV-BanI (accession no. Z 48182), ToLCV-BaV (accession no. AF 295401) and ToLCV-Kolar (accession no. AF 428255); 90% with ToLCV-BanIV (accession no. AF 165098). Interestingly, the level of sequence identity with isolates from other Indian subcontinent countries was lower, being 84% with both ToLCV from Bangladesh (accession no. AF 188481) and Sri Lanka (accession no. AF 274349). This data indicates that the virus causing guar leaf curl disease in India is caused by an Indian isolate of ToLCV. This is the first report of ToLCV naturally infecting guar.