In March 2011, interveinal yellowing and necrosis symptoms on middle and lower leaves were observed in tomato (Solanum lycopersicum L., cv. Castle Rock) plants grown in three adjacent greenhouses of the Agricultural Research Corporation at Wad Medani (Gezira State, Sudan). These symptoms resembled those caused by Tomato chlorosis virus (ToCV) and Tomato infectious chlorosis virus (TICV) (4) (genus Crinivirus, family Closteroviridae). Whitefly (Bemisia tabaci) infestation was also observed in these greenhouses. Total RNA was extracted by TRIzol Reagent (Invitrogen, Carlsbad, CA) from symptomatic leaves and analyzed by dot-blot hybridization with digoxigenin-labelled RNA probes to the coat protein (CP) gene of ToCV and to the minor coat protein (CPm) gene of TICV. Positive signal was obtained only with the ToCV probe. Reverse transcription (RT)-PCR reactions were performed with two pairs of primers specific for the detection of ToCV, MA380(+) (5′-GTGAGACCCCGATGACAGAT-3′) and MA381(-) (5′-TACAGTTCCTTGCCCTCGTT-3′), specific to the CP gene (ToCV RNA 2) (3), and MA396(+) (5′-TGGTCGAACAGTTTGAGAGC-3′) and MA397(-) (5′-TGAACTCGAATTGGGACAGA-3′), specific to the RNA-dependent RNA polymerase (RdRp) gene (ToCV RNA 1) (1). DNA fragments of the expected sizes (436 and 763 bp, respectively) were obtained, thus supporting the presence of ToCV in the symptomatic samples. Amplified DNA fragments were cloned in pGEM-T Easy vector (Promega, Madison, WI) and one clone per amplicon was sequenced (Macrogen Inc., Seoul, South Korea). The highest nucleotide sequence identity of the CP gene fragment obtained (GenBank Accession No. JN411685) was 99.2% related with North American ToCV isolates from Florida (DQ234674), Colorado (DQ234675), and Georgia (HQ879842), while the RdRp gene fragment (JN411686) was more closely related (99.0%) to the Spanish AT80/99 isolate (DQ983480). Although yellowing symptoms similar to those reported here have been observed sporadically during the last few years in open-field tomato crops in the state of Gezira, additional studies are needed to determine the prevalence and economic impact of ToCV infections in tomato cultivation in Sudan. To our knowledge, ToCV has been found in continental Africa only in Morocco and South Africa, in the Mediterranean climate areas in the northern and southern edges of the continent, respectively (2). The finding of ToCV infecting tomato in Sudan raises the question of whether this virus is emerging also in other tropical areas of the continent and illustrates the need to monitor whitefly-infested areas within Africa for the presence of ToCV.
References: (1) G. Lozano et al. J. Virol. 83:12973, 2009. (2) J. Navas-Castillo et al. Annu. Rev. Phytopathol. 49:219, 2011. (3) H. P. Trenado et al. Eur. J. Plant Pathol. 118:193, 2007. (4) G. C. Wisler et al. Plant Dis. 82:270, 1998.