Effects of nutrient solution pH on the survival and transmission of Clavibacter michiganensis ssp michiganensis in hydroponically grown tomatoes

The effect of pH on the survival of Clavibacter michiganensis ssp. michiganensis and its transmission via roots of tomato in hydroponic culture was studied in laboratory and greenhouse experiments. In a laboratory experiment, C. m. ssp. michiganensis could not survive for 24 h in nutrient solutions with a pH of 4.0 or 4.5, while 1, 14, 51 and 62% of inoculum survived at pH 5.0, 5.5, 6.0 and 6.5, respectively. When tomato plants were inoculated with C. m. ssp. michiganensis through wounds on the stems, the bacteria moved downward from the inoculation site to the roots and infectious bacteria were released from the roots into the nutrient solution. Of two pH regimes tested in greenhouse nutrient-film technique (NFT) culture, the C. m. ssp. michiganensis population was significantly lower in pH 5.0 than in pH 6.5 in most sampling data. In treatments in which C. m. ssp. michiganensis was introduced by transplanting two root-inoculated plants, significantly more plants developed canker at pH 6.5 (34 out of 48 plants) than at pH 5.0 (11 out of 48 plants). When the bacterium was introduced by transplanting two stem-inoculated plants at pH 6.5, seven out of 24 plants developed canker. The potential of pH manipulation in controlling tomato bacterial canker in hydroponic culture is discussed.