First report of a 16SrIII, X-disease phytoplasma affecting tomato plants in Mexico

Tomato (Solanum lycopersicum) is an important crop in Mexico, widely used for fresh consumption. Recently, symptoms of leaf yellowing and curling, little leaf, and severe stunting (Fig. 1) have been observed in tomato fields in the Yucatan Peninsula, with an incidence of about 50%. Leaf samples of fifty plants with and without symptoms were randomly collected in seven different municipal areas of the region. Total DNA was extracted and used as template in a nested PCR assay to amplify the phytoplasma 16S rRNA gene using universal phytoplasma primers P1/P7 (Deng & Hiruki, 1991) and R16F2/R16R2 (Gundersen & Lee, 1996). Bands of expected size (1.26 kb) were produced in all symptom-bearing samples, but not for the symptomless ones, which, after RFLP analysis, yielded identical AluI, HaeIII, HindIII, MseI, MspI, RsaI and TaqI restriction profiles, similar to those of phytoplasmas of group 16SrIII (X-disease) subgroup 16SrIII-F.

The nested PCR products of four samples were purified and directly sequenced. The resultant 16S rDNA sequences showed 100% identity with each other, and were deposited in GenBank (Accession Nos. FJ951626; FJ951627; FJ951628; FJ951629) representing municipalities of Calkini, Tekax, Oxkutzcab and Hopelchen, respectively. BLAST analysis showed the highest similarity (98.6%) with the milkweed yellows phytoplasma 16S rDNA, subgroup 16SrIII-F (AF510724), which was previously reported in the United States in Asclepius syriaca (Griffiths et al., 1994). However there are no records so far on this or other 16SrIII-related phytoplasmas in Mexico. To our knowledge, this is the first report of a 16SrIII phytoplasma in Mexico associated with a disease of a tomato crop.