The present invention provides a control agent that is extremely effective in controlling bacterial diseases. The quorum sensing inhibitor and bacterial disease control agent for agricultural and horticultural use of the present invention contain a pyrimidinone compound represented by General Formula (1) : wherein R 1 represents a C 1-5 alkyl group or a phenyl group, and R 2 represents a hydrogen atom or a C 1-5 alkyl group, provided that R 1 cannot be a methyl group when R 2 represents a hydrogen atom.
