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. michigan ensis and its transmission via roots of tomato in hydroponic culture was st udied 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 wi th C. m. ssp. michiganensis through wounds on the stems, the bacteria moved downward from the inoculation site to the roots and infectious bacteria we re released from the roots into the nutrient solution. Of two pH regimes te sted in greenhouse nutrient-film technique (NFT) culture, the C. m. ssp. mi chiganensis 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 int roduced by transplanting two root-inoculated plants, significantly more pla nts 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 hydr oponic culture is discussed.