Temperature at the early stages of Clavibacter michiganensis subsp michiganensis infection affects bacterial canker development and virulence gene expression

Clavibacter michiganensis subsp. michiganensis (Cmm), the causal agent of bacterial canker and wilt, causes severe economic losses in tomato net-houses and greenhouses worldwide. In this study, seedlings which were transplanted and inoculated monthly over 2years wilted and died earlier in the spring (21-24 degrees C) and autumn (18-23 degrees C) than in the winter (15-18 degrees C) and summer (28-31 degrees C): T-50 (the time taken for 50% of the plants to wilt or die) was 2 and 3-4months after inoculation, respectively. A highly significant correlation was found between the average temperatures during the first month after inoculation and T-50; the shortest T-50 mortality (70days) was observed for an average temperature of 26 degrees C. Expression of virulence genes (pat-1, celA, chpC and ppaA) by Cmm was higher in plants inoculated in the spring than in those inoculated in the summer. In another set of experiments, seedlings were inoculated and maintained in controlled-environment growth chambers for 2weeks. Subsequently, they were transplanted and maintained in commercial-type greenhouses for 4-5months. The temperatures prevailing in the first 48h after inoculation were found to affect Cmm population size and virulence gene expression and to have season-long effects on bacterial canker development.