Disclosed is a novel means whereby plant diseases can be controlled. An antibacterial agent against plant disease-causing bacteria and a plant disease controlling agent according to the present invention comprise at least one member selected from a group consisting of amino acids, derivatives thereof and oligomers and polymers of the same, are substantially free from any antibacterial active ingredient against plant disease-causing bacteria, and contain cyano acrylate polymer particles having an average particle size less than 1000 nm as the active ingredient. These agents according to the present invention have no risk of toxicity to humans, livestock, fishes and birds and place little load on the environment. The antibacterial agent has a novel function mechanism that is completely different from the function mechanisms of conventional agents and, therefore, can be used even in the case where a drug-resistant strain appears.
