The natural occurrence of a begomovirus in sunn hemp (Crotalaria juncea) in India

Sunn hemp (Crotalaria juncea) is cultivated in India and other tropical countries as a fibre or green manure crop. During July/August 2001, a severe virus-like disease affected 80-90% of sunn hemp plants in experimental plots at the National Botanical Research Institute, Lucknow. The symptoms of the disease consisted of interveinal chlorosis, yellowing, curling and shortening of leaves (Fig. 1), and stunting of plants.

Whiteflies (Bemisia tabaci) transmitted the associated virus from diseased to healthy sunn hemp plants which developed symptoms similar to those occurring in naturally infected plants. Total DNA was isolated from the infected-leaf tissues and subjected to polymerase chain reaction (PCR) using begomovirus specific universal primers (AV494/AC1048) which were designed from the conserved sequences of the core region of the coat protein (CP) genes of several begomoviruses (Wyatt & Brown, 1996). As expected, a fragment of c. 500 bp was amplified from the DNA extracted from diseased, but not healthy, leaves. The authenticity of the PCR-amplified DNA fragment was confirmed by Southern hybridization in which DNA probes, prepared from the CP gene of Indian tomato leaf curl virus (IToLCV; Srivastava et al., 1995), cross hybridized with the PCR fragment under high stringency conditions. Although Sunn hemp mosaic virus and some strains of Tobacco mosaic virus infect sunn hemp (Brunt et al., 1996), a begomovirus has not hitherto been reported to naturally infect this important crop species. Transmission of the sunn hemp virus by whiteflies, PCR amplification of its DNA with begomovirus specific primers and its strong hybridization with a DNA-A probe of IToLCV strongly indicate that it is a begomovirus. The extent of its relationship to IToLCV (Family Geminiviridae; Genus Begomovirus) and other begomoviruses is now being further investigated.