PURPOSE: To provide an antimicrobial agent for plants comprising the aqueous solution of a Melaleuca oil, not only controlling microbes living in plants such as field crops but also safe for handling men, the plants and the surrounding environment, especially due to the natural substance and good decomposability.

CONSTITUTION: The aqueous solution of a Melaleuca oil obtained by the steam distillation of the leaves of Melaleuca-altaniffolia is employed as an antimicrobial agent for plants such field crops. The antimicrobial agent is preferably diluted with a 300-600 times of water and employed, e.g. by immersing a plant such as the cut end of a cut flower 2 in the diluted solution 1, spraying the diluted solution on the plant such as a field crop or turf 3 or washing the plant such as a vegetable 4 with the diluted solution 1. The agent kills microbes living on the surfaces of the plants or the cut ends to exhibit the excellent effects such as the life prolongation of the cut flowers, the sterilization of the turf and the freshness retainment of the vegetable and the agent is safe for handling men, plants and surrounding environments.