In the summer of 2008, tomato (Solanum lycopersicum) plants in a large greenhouse tomato facility located in Delta, British Columbia, Canada exhibited general stunting, chlorosis, and purple-leaf symptoms that were distinct from those of Pepino mosaic virus (PepMV) (1). Diseased plants were localized mainly in two rows in a section of the greenhouse and produced no fruits or only fruits with reduced size. Leaf samples were collected from four individuals among numerous diseased plants in this greenhouse. Screening samples by ELISA, PCR, or reverse transcription (RT)-PCR for PepMV, Tomato spotted wilt virus, Tomato yellow leaf curl virus, Tomato torrado virus, Tomato apex necrosis virus, and Begomovirus, Tobamovirus, and Pospiviroid species showed that all four plants had a mixed infection of both PepMV and a pospiviroid. RT-PCR with the pospiviroid-specific primers Pospil-RE and Pospil-FW (3) amplified the expected 196-bp products from these four samples. Each amplicon was cloned into the pCR4-TOPO vector (Invitrogen, Carlsbad, CA) and one individual cDNA clone from each isolate was sequenced. BLASTN analyses of nucleotide sequences of these clones showed 97 to 99% identity to Mexican papita viroid (MPVd) isolates currently in the NCBI Genbank. These four newly identified MPVd isolates were not identical; seven nucleotide substitutions or indels were identified in this region. The full viroid genome was obtained by RT-PCR in isolate VF2 with a new reverse primer MPVd-RE (5′ GATCCCTGAAGCGCTCCT 3′) in combination with the forward primer Pospil-FW (3). Using the same approach as stated above, this amplicon was cloned and sequenced. The nucleotide sequence of the 196-nt amplicon previously amplified and cloned from the isolate VF2 genome was identical to this region in the genomic clone. BLASTN analysis showed that the VF2 genome (GenBank Accession No. FJ824844) had >98% sequence identity to each of nine MPVd isolates (GenBank Accession Nos. L78454 and L78456--L78463), 94% identity to Tomato planta macho viroid (TPMVd) (GenBank Accession No. K00817) and ~80% identity to Tomato chlorotic dwarf viroid (GenBank Accession Nos. EF582392--EF582393). Prior to this find, MPVd had been identified only in papita (Solanum cardiophyllum) in Mexico and is considered a possible ancestor of TPMVd, Potato spindle tuber viroid (PSTVd), and possibly of other PSTVd-group viroids now infecting crop plants (2). The origin of MPVd in this greenhouse facility in Delta, British Columbia is unknown. The infected plants were destroyed by the grower. The pathogenicity of MPVd isolates characterized in this study was not evaluated on tomato because of quarantine regulations governing this viroid in the United States. The identification of MPVd infecting an important agricultural crop (tomato) outside its center of origin in Mexico indicates a potentially important major shift in the epidemiology of MPVd. To our knowledge this is the first report of MPVd from tomato in Canada.
References: (1). K.-S. Ling et al. Plant Dis. 92:1683, 2008. (2) J. P. Martinez-Soriano et al. Proc. Natl. Acad. Sci. U.S.A. 93:9397, 1996. (3) J. Th. J. Verhoeven et al. Eur. J. Plant Pathol. 110:823, 2004.