The prevalence, aggressiveness and survival of Clavibacter michiganensis subsp. michiganensis strains associated with different genetic groups in Israel

Clavibacter michiganensis subsp. michiganensis (Cmm), the causal agent of bacterial wilt and canker of tomato, is one of the most important bacterial diseases of tomato in all major tomato-growing areas worldwide. Cmm strains may be divided into various distinct genetic groups using macrorestriction pulsed-field gel electrophoresis (PFGE) analysis. The objectives of this study were: (i) to document changes in Cmm population structure in the main tomato production areas of Israel, and to reveal (ii) differences in aggressiveness of the various strains and (iii) differences in survival capability among strains associated with different genetic groups. Of the 38 strains sampled between 1998 and 2008, 37 (97.3%) were associated with group B and only one strain (2.7%) was associated with group A. However, in the four years that followed, the prevalence of strains associated with group B declined gradually and that of other genetic groups, A, E and Z, increased; the latter was the most common in 2012, making up ~42% of the population. The aggressiveness of strains associated with different genetic groups was examined in two artificially inoculated experiments: the first included strains of groups A, B, E and Z and the second strains of groups A, B, K, L and Z. Two to four strains from each genetic group were included in each experiment. Differences in aggressiveness among the genetic groups were insignificant. There were, however, some differences in aggressiveness between individual strains within each genetic group. In another set of experiments, the survival of four bacterial strains associated with each of the A, B, E and Z genetic groups was determined in roots of infected plants or in infested soil (three soil types). Differences in survival capabilities were recorded among strains associated with the same genetic groups, but not among those associated with different genetic groups.