A new selective medium for isolation of Clavibacter michiganensis subsp. michiganensis from tomato plants and seed

A new selective and highly sensitive medium was developed for isolation of Clavibacter michiganensis subsp. michiganensis (Cmm), the causal agent of bacterial canker of tomato, from seed and latently infected plants. The new medium (BCT) proved to be superior to all published semiselective media for Cmm and is denoted as selective medium because of (i) its mean plating efficiency, amounting to ≤89% within 7 days for all 30 Cmm strains from different sources tested; (ii) the high selectivity, because accompanying bacterial species occurring on tomato plants and seed or bacteria obtained from culture collections were inhibited to an extent of 98 to 100%; and (iii) the remarkable detection sensitivity. Thus, 8 CFU of Cmm in field plant homogenates containing 12,750 CFU of accompanying saprophytes were detected on BCT. Under these extreme conditions, all of the published semiselective media (D2, KBT, D2ANX, SCM, mSCM, CMM1, mCNS, and EPPO) gave false-negative results. Either some media were rather toxic and Cmm growth was also inhibited or the other, less toxic media allowed growth of high numbers of saprophytes, so that Cmm growth was suppressed. Exclusively, BCT also supported growth of the closely related C. michiganensis subsp. insidiosus, nebraskensis, and tessellarius. The new medium is recommended for Cmm detection in tomato seed, and in symptomless tomato plantlets, to improve disease control of bacterial canker of tomato.