Disclosed is a novel means by which plant diseases can be controlled. The antibacterial agent for a phytopathogenic bacterium/bacteria and the agent for plant disease control according to the present invention comprise as an effective ingredient cyanoacrylate polymer particles having an average particle diameter of less than 1000 nm, which particles contain at least one selected from the group consisting of amino acids, amino acid derivatives, and oligomers and polymers thereof, and substantially do not contain an active antibacterial ingredient effective against a phytopathogenic bacterium/bacteria. The agents according to the present invention have no risk of toxicity to humans, livestock, fishes and birds, as well as have a lower impact on the environment. Since this novel antibacterial agent is completely different from existing antibiotics, the agent can be used even in the case where a drug-resistant strain has emerged.