The present invention relates to the use of a combination comprising at least one host defense inducer and at least one biological control agent in a synergistically effective amount for controlling bacterial harmful organisms in useful plants. The biological control agent is selected from specific microorganisms and/or a mutant of these strains having all the identifying characteristics of the respective strain, and/or a metabolite produced by the respective strain. In a preferred aspect of the invention the host defense inducer is isotianil or a combination of isotianil and acibenzolar-S-methyl. The present invention also relates to a method for controlling bacterial harmful organisms in useful plants by treatment with a combination comprising at least one host defense inducer and at least one biological control agent in a synergistically effective amount. A further aspect of the present invention is directed to method for controlling bacterial harmful organisms in useful plants by subjecting the plants to be protected against attack by bacterial harmful organisms to two or more sequential treatment blocks.
