Begomovirus related to Tomato leaf curl Pakistan virus newly reported in Mentha crops in India

Spearmint (Mentha spicata cv. Viridis; family Lamiaceae), an important essential oil-bearing crop, is cultivated on 0.15 million ha land of India. Approximately 80 % of its planting consists of identical genetic material released by CIMAP, Lucknow. Mentha spp. have been reported to be affected by many viral diseases (Samad et al.1994, 2000). In a field of M. spicata cv. Viridis at CIMAP, Lucknow, and in adjoining areas the occurrence of a begomovirus-caused disease was for the first time observed during May and June 2005. Typical symptoms were foliar rugosity, light yellow mosaic, upward curling, crinkling and retarded growth leading to drastic reduction in the herb's yield (Fig. 1). In 2007, the disease incidence was recorded to be in the range of 50 to 60 %. The disease also appeared in the Tarai region of Uttaranchal province. Infected plants were used for electron microscopy, whitefly transmission tests and polymerase chain reaction (PCR)-mediated begomovirus-specific diagnostics, but neither virus particles (in leaf dip), nor cytopathic effects or inclusion bodies were detected. However, from the naturally infected plants, the disease was transmitted to healthy seedlings of M. spicata cv. Viridis by whiteflies (Bemisia tabaci), but not by aphids or mechanical inoculation. Viruliferous whiteflies did not transmit the infection to other mint species (M. arvensis; M. citrata or M. piperita) in glasshouse tests, confirmed through PCR. Total DNA was extracted from symptomatic and asymptomatic Mentha samples from different locations. The presence of a begomovirus was detected by PCR using begomovirus coat protein gene-specific primers (forward 5’ –ATGGCGAAGCGACCAG- 3’ and reverse 5’ –TTAATTTGTGACCGAATCAT- 3’; Hallan, 1998). A product 771 bp in size was amplified from symptomatic, but not from asymptomatic plant samples (Fig. 2). Amplicons were cloned into PCR-TOPO TA cloning vector V2.0 (Invitrogen, USA). Selected clones were sequenced in both orientations and data submitted to GenBank (Accession. No. EU263016).

Sequence analysis (Fig. 3) showed the highest levels of sequence identity (93%) with the begomovirus Tomato leaf curl Pakistan virus (isolate [-Pakistan:Rahim Yar Khan 1:2004]; DQ116884, nomenclature according to Abhary et al., 2007).

To our knowledge, this is the first report of a begomovirus associated with an economically relevant disease of spearmint in India, and the second finding of a geminivirus in a species of Labiatae worldwide, the other one consisting of a partial sequence from a putatively bipartite begomovirus collected from the weed Leonurus sp. in Brazil (Faria & Maxwell, 1999).