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First Report and Molecular Characterization of DNA A of Three Distinct Begomoviruses Associated with Tomato Leaf Curl Disease in Ghana

Tomato leaf curl disease is reported to be widespread in Ghana and to cause severe yield losses (4). So far, the causal agent has not been identified. Thirty-three tomato (Solanum lycopersicum L.) samples with symptoms such as curling, yellowing, small leaves, and stunting were collected from the Ashanti Region, the main tomato-production area in Ghana, including three samples from Akumandan in the autumn of 2007 and 30 samples from Kumasi in the spring of 2008. The observed leaf curl disease incidence in the farmer's field in Kumasi was approximately 75%. Viral DNAs were extracted from the 33 samples and tested for the presence of begomoviral DNA-A, DNA-B, and associated satellite DNA by PCR with previously described primers (1,3). The expected 1.4-kb DNA-A begomovirus fragment was obtained from one of the samples from Akumadan and from 25 samples from Kumasi. DNA-B and DNA-beta were not detected by PCR. The 1.4-kb PCR products from all positive samples were cloned and sequenced. Sequence comparison by MegAlign software (DNASTAR, Inc., Madison, WI) showed three distinct virus groups. One isolate from each group was selected and specific primers were designed to complete the DNA-A sequence. The DNA-As of GH5-3 (group 1), GOTB2-2 (group 2), and GHK2 (group 3) isolates consisted of 2,803 (GenBank Accession No. EU350585), 2,794 (GenBank Accession No. EU847739), and 2,792 nt (GenBank Accession No. EU847740) respectively. All contain the geminiviral conserved nonanucleotide sequence TAATATTAC in the intergenic region and the six predicted open reading frames (ORFs V1, V2, C1, C2, C3, and C4). BLASTn analysis was conducted with geminivirus sequences available in the GenBank database at National Center for Biotechnology Information (Bethesda, MD). Further sequence comparisons were performed by Clustal V algorithm of MegAlign software with the representative isolates of begomovirus species reported by Fauquet et al (2) and the sequences that showed high scores in BLASTn search. The DNA-A sequence of isolate GHK2 from Kumasi showed highest sequence identity (96.5%) with Tomato yellow leaf curl Mali virus (TYLCMLV; GenBank Accesssion No. AY502934). The DNA-A sequence of GH5-3 and GOTB2-2 isolates had 87.5% sequence identity with each other. Both had highest sequence identities of 76.7 and 77.6%, respectively, with Tomato leaf curl Antsiranana virus, Madagascar (GenBank Accession No. AM701764). They constitute two distinct begomovirus species based on DNA-A sequence comparisons and the International Committee on Taxonomy of Viruses proposed species demarcation of 89% sequence identity. The names Tomato leaf curl Ghana virus for isolate GH5-3 and Tomato leaf curl Kumasi virus for isolate BOTB2-2 are proposed, respectively. To our knowledge, this is the first report of molecular characterization of begomoviruses associated with tomato leaf curl disease in Ghana and of the presence of three distinct tomato begomoviruses. This presence should be considered for recommending or developing stable begomovirus resistant tomato cultivars for Ghana.

References: (1) R. W. Briddon et al. Mol. Biotechnol. 20:315, 2002. (2) C. M. Fauquet et al. Arch. Virol. 153:783, 2008. (3) S. K. Green et al. Plant Dis. 85:1286, 2001. (4) D. Horna et al., eds. Online publication. Int. Food Policy Res. Inst. PBS Policy Brief 2, 2007.