In recent years, tomato plants (Lycopersicon esculentum Mill., cultivar MST 32/1) grown in Mauritius were observed with symptoms characteristic of a phytoplasmal infection. Young plants exhibited stunted growth, with a bunchy top symptom on one or more stems. The leaves were small with a curled margin, sometimes were purplish, and were clustered as a rosette. On older plants, other leaf symptoms, such as necrotic vein, mosaic, and vein clearing, that were typical of viral infection were observed. In March to June 1998, research was initiated to determine whether a phytoplasma or a virus was associated with the problem. Virus screening was carried out by double antibody sandwich enzyme-linked immuno-sorbent assay (DAS-ELISA) with the following antisera: tomato mosaic virus (TMV), tomato leaf curl virus (TLCV), tomato spotted wilt virus (TSWV), cucumber mosaic virus (CMV), potato virus X (PVX), potato virus Y (PVY), and potato leaf roll virus (PLRV). The presence of a phytoplasma was assayed by the polymerase chain reaction (PCR) with universal primers that amplify the 16S rDNA sequences. Oligonucleotide pairs RU3 and FU5 (1) and P1 and P6 (2) were evaluated separately. Template DNA was prepared from young leaves with the Nucleon Phyta-Pure Plant DNA extraction kit (catalog no. RPN 8511, Amersham Life Science, Buckinghamshire, U.K.). Amplification of a 880-bp or a 1,400-bp product with the two primer pairs confirmed the presence of a phyto-plasma in the tomato plants with the bunchy top symptoms. No amplification was obtained from plants that had both bunchy top and necrotic symptoms on the leaves and that were found to be infected with PVY by ELISA. To determine whether phenolics and other inhibitors from the virus-induced necrotic tissue interfered with the detection of phytoplasmas, plant DNA from these tissues was spiked with known phytoplasmal DNA. Amplification was not successful, confirming the presence of PCR inhibitors. Further purification of the DNA with phenol/chloroform before precipitation helped to obtain DNA of a purer quality from which a positive band representing the sequence of a phytoplasma was amplified. This is the first report of a phytoplasma in tomato in Mauritius.
References: (1) U. Ahrens and E. Seemüller. Phytopathology 82:828, 1992. (2) S. Deng and D. Hiruku. J. Microbiol. Methods 14:53, 1991.
