Detection of Clavibacter michiganensis subsp. michiganensis in Symptomless Tomato Transplants. R. D. Gitaitis, Associate Professor, Department of Plant Pathology, University of Georgia, Coastal Plain Experiment Station, Tifton 31793-0748. R. W. Beaver, and A. E. Voloudakis. Assistant Research Chemist; and Former Graduate Assistant, Department of Plant Pathology, University of Georgia, Coastal Plain Experiment Station, Tifton 31793-0748. Plant Dis. 75:834-838. Accepted for publication 18 February 1991. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/PD-75-0834.

Clavibacter michiganensis subsp. michiganensis, the cause of bacterial canker, was isolated from symptomless tomato (Lycopersicon esculentum) seedlings that were surface-disinfested with 100% ethanol, sectioned, and pressed onto the surface of a semiselective medium (mCNS). In greenhouse tests, movement of bacteria down stems was variable 1–3 days after inoculation of the apical end of the stem, but by 7 days, C. m. michiganensis was isolated from sections at least 10 cm below the inoculation site of all plants. However, symptoms were not observed for 17 days. Inoculated plants were infectious within 2 days because the pathogen was transmitted when the tops of plants were clipped with a razor blade. C. m. michiganensis was isolated on mCNS from only three of 24,000 transplants sampled from 24 commercial fields in southern Georgia. Color changes in ELISA wells that had been stamped with tomato stem pieces excised from symptomless transplants provided presumptive evidence for bacterial presence when compared with positive controls. The presumption of C. m. michiganensis was confirmed when the bacterium was recovered on mCNS and identified. Bacterial fatty acids of C. m. michiganensis were detected in extracts from stem tissues by gas-liquid chromatography. A minimum threshold of 1 × 10⁸ colony-forming units (cfu) of bacteria per milliliter of saponification solution was necessary for identification of bacteria in plant tissues. The mean population of C. m. michiganensis in symptomless transplants by 7 days after inoculation was 1.7 × 10⁸ cfu/cm of stem. In comparison, populations of Pseudomonas syringae pv. syringae and P. s. pv. tomato in lesions were determined to be 9.5 × 10⁵ and 8.1 × 10⁶ cfu/cm² of leaf disk, respectively, and were not detected by gas-liquid chromatography. In addition, several fatty acids common to all three bacteria are constituents of healthy tomato tissue, thus complicating the use of gas-liquid chromatography for routine detection of phytopathogenic bacteria in host tissues.

Keyword(s): bacterial speck, latent infections, syringae leaf spot.