Induction of resistance to Clavibacter michiganensis subsp. michiganensis

Tomato plants pre-inoculated with the avirulent strain NCPPB 3123 of Clavibacter michiganensis subsp. michiganensis (Cmm) were protected largely against challenge infection by virulent strains of Cmm. Effectiveness of this protective effect was mainly dependent on the inoculation sites, the bacterial cell concentration used for pre- and challenge inoculations, and the time interval between both inoculations. This defence reaction was systemic and stable throughout the whole growing season. Resistance can also be induced by pre-inoculation of heat-killed bacteria or application of isolated EPS of the strain 3123. Strain 3123 spreads out in tomato plants in the same manner as virulent Cmm isolates, but its colonization of tomato fruits and seeds was substantially lower. Papillary to spherical electron dense particles were observed at the tonoplast in parenchyma cells of the vascular system of tomato plants inoculated with the strain 3123. Numerous investigations carried out to examine the ability of 3123 to induce resistance in other host/pathogen-systems showed that it was only specific for tomato/Cmm.