*vgmalathi@rediffmail.com

Accepted for publication 08/11/04

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 (Fig. 1A). 

Figure 1a: Leaf curl and yellow mosaic symptoms in cowpea: downward leaf curl

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, others showed a yellow mosaic without enation (Fig. 1B); the majority of the leaves showed a yellow mosaic with severe leaf curl and enations (Fig. 1C). In Southern analysis, total nucleic acid extracted from symptomatic plants hybridised 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, hybridisation was also performed with a radiolabelled probe to DNA β associated with cotton leaf curl disease (CLCuD). A positive signal obtained with extracts from leaves showing leaf curl symptoms indicated the presence of DNA β. However, no hybridisation 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).

Figure 1: Leaf curl and yellow mosaic symptoms in cowpea: b (left), typical yellow mosaic; c (right), enation

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.

Briddon RW, Bull SE, Amin I, Idres AM, Mansoor S, Bedford ID, Dhawan P, Rishi N, Siwatch SS, Abdel-Salam AM, Brown JK, Zafar Y, Markham PG, 2003. Diversity of DNA β, a satellite molecule associated with some monopartite begomoviruses. Virology 312, 106-121.

The British Society for Plant Pathology