A distinct begomovirus causes Indian dolichos yellow mosaic disease

Dolichos (Lablab purpureus) is a popular leguminous vegetable in India. Since the 1950s, cultivation of this vegetable has been affected by dolichos yellow mosaic disease (DYMD; Capoor & Varma, 1950), which is associated with a begomovirus tentatively named Dolichos yellow mosaic virus (DoYMV). Characteristic symptoms of the disease are bright yellow mosaic patches on leaves, with significant yield reductions (Fig. 1). A sample from a DYMD-affected dolichos plant was collected from Mysore, Karnataka, southern India in February 2004. The putative virus was transmitted by the whitefly, Bemisia tabaci from dolichos to dolichos cv. Mani.

Total DNA was extracted from the symptomatic leaves and used to PCR-amplify a begomovirus DNA-A component using degenerate primers (Muniyappa et al., 2003). Sequence generated from this PCR product was used to design virus-specific primers to obtain full-length DNA-A clones. The sequence of one clone was determined to be 2760 nucleotides in length (Accession No. AJ968370) and to be most similar to the sequences of DoYMV-[Bangladesh] (AY271891) and a further clone of DoYMV originating from India (AY309241) at 95.2% and 93.4% nt sequence identities respectively. The number and arrangement of open reading frames on DNA-A were similar to viruses of the genus Begomovirus (family Geminiviridae) originating from the Old World. Attempts to identify DNA-B and -β components by PCR (Muniyappa et al., 2003; Briddon et al., 2002) were unsuccessful.

Phylogenetic analysis (Fig. 2) shows the three DoYMV isolates to form a sister clade to the mungbean-infecting begomoviruses which together cluster separately from begomoviruses infecting other host on the Indian sub-continent. DoYMV has only 62.9-63.8% nt identity with mungbean viruses and lower sequence identities with other begomoviruses (61.0-63.9%). These results show that, on the Indian sub-continent, dolichos is infected by a distinct species of begomovirus. Further experiments are being carried out to identify additional viral DNA components and also obtain infectious DoYMV clones.