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

Clavibacter michiganensis subsp. michiganensis ( C mm), 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 2 years wilted and died earlier in the spring (21–24°C) and autumn (18–23°C) than in the winter (15–18°C) and summer (28–31°C): T 50 (the time taken for 50% of the plants to wilt or die) was 2 and 3–4 months 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 (70 days) was observed for an average temperature of 26°C. Expression of virulence genes ( pat‐1 , cel A , chp C and ppa A ) by C mm 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 2 weeks. Subsequently, they were transplanted and maintained in commercial‐type greenhouses for 4–5 months. The temperatures prevailing in the first 48 h after inoculation were found to affect C mm population size and virulence gene expression and to have season‐long effects on bacterial canker development.