Severe leaf curl disease of cowpea – a new disease of cowpea in northern India caused by <i>Mungbean yellow mosaic India virus</i> and a satellite DNA β

Abstract

In the 2004 crop season, severe leaf distortion symptoms were observed in cowpea (Vigna unguiculata) plants in agricultural fields in northern India. Affected plants showed a yellow mosaic with downward leaf curling, vein swelling and vein enations. The virus causing this leaf curl disease was easily transmitted to cowpea seedlings by whitefly (Bemisia tabaci), resulting in a combination of symptoms. Typically, a few leaves showed only mild leaf curl symptoms, while others showed a yellow mosaic without enation. The majority of the leaves showed a yellow mosaic with severe leaf curl and enations. In Southern analysis, total nucleic acid extracted from plants with symptoms hybridized to radiolabelled probes specific to DNA A and DNA B of Mungbean yellow mosaic India virus (MYMIV-[Cp]). As veinal enation is a typical symptom associated with the presence of satellite DNA β, hybridization was also performed with a radiolabelled probe to DNA β associated with cotton leaf curl disease. A positive signal obtained with extracts from leaves showing leaf curl symptoms indicated the presence of DNA β. However, no hybridization was seen with an extract from leaves showing only a yellow mosaic. Satellite DNA β was amplified with universal DNA β primers β01/β02 (Briddon et al., 2003) and the ∼1·3 kb amplicon was cloned and sequenced (accession no. AY728263). The nucleotide sequence of DNA β showed 59% identity with DNA β associated with Cotton leaf curl Rajasthan virus (accession no. AY083590). While MYMIV-[Cp] alone causes yellow mosaic symptoms, in the presence of DNA β more severe-symptom phenotypes of vein enation and leaf curl occur. Thus, the pathogenicity of the virus is increased by the presence of satellite DNA β. This is the first report of a satellite DNA β associated with a bipartite begomovirus infecting a leguminous host.