Although the use of bacteria belonging to the genus Bacillus is a promising sustainable alternative in crop protection. Many questions remain on their modes of action in the biocontrol of plant pathogens. This work aims to evaluate and characterize the modes of action of the biocontrol agent B. subtilis 30B-B6 against early and late blight diseases on Solanaceae. First, we investigate the potential of 30B-B6 to manage the late blight disease on potato crop. Our results highlighted that 30B-B6 significantly reduces the disease severity caused by Phytophthora infestans. This study also investigates the potential of 30B-B6 to control the necrotrophic fungus Alternaria solani on tomato. Our findings indicate that 30B-B6 can control the pathogen by direct antagonism, but also by the stimulation of plant defenses, since the two microorganisms were applied on different plant parts and remain physically separate. This work is a first insight into the molecular pathways involved in the induction of systemic resistance by 30B-B6 in tomato. We used RT qPCR and hormone (salicylic acid) quantification to understand the action of 30B-B6 on plant defenses, and to characterize the active ingredients produced by the bacterium. We showed notably that salicylic acid is involved in the response of the plant against the infection by A. solani after treatment with 30B-B6. Our results also indicated that lipopeptides and siderophore produced by 30B-B6 significantly reduced symptoms of early blight. Taken together, this thesis confirms the interest of 30B-B6 in plant protection against early and late blight and reveal the multiple modes of action of the bacterium

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