Characterization of atypical Clavibacter michiganensis subsp. michiganensis populations in greenhouse tomatoes in Italy

The quarantine bacterium Clavibacter michiganensis subsp. michiganensis (Cmm) is the agent of tomato bacterial canker (TBC), one of the most destructive bacterial diseases of this crop, that causes severe economic losses worldwide. During a recent outbreak of TBC in Sicily (insular Italy), it was impossible to amplify the pat-1 gene by PCR with CMM5/CMM6 primers from affected greenhouse-grown plants from three farms, although Cmm-like colonies were consistently isolated. Microlog metabolic profile, 16S rDNA sequencing and the positive amplification of other Cmm DNA targets supported the identification of the strains as Cmm, suggesting the occurrence of a population lacking the virulence gene pat-1. A detailed phenotypic and molecular characterization of these Cmm strains and their virulence to tomatoes, as compared with Cmm reference strains is reported in this study. Pathogenicity tests revealed the strains under study were able to induce TBC, although delayed wilting symptoms were observed. Strains with or without pat-1 were undistinguishable, based on disease indexes.