Disclosed is a method of preventing diseases in plants which can bring about a redox reaction in the inside of plant cells thus inducing the expression of pathogen-resistance genes in the plant cells. There is no possibility that residual contents remain in soils. It is also possible to cultivate plants which are strong against diseases. By bringing water which contains reactive oxygen species and allows the reactive oxygen species to be held and to function for a long period into contact with plants, the expression of the pathogen-resistance genes which the plants possess is induced. Accordingly, the water containing reactive oxygen is absorbed into the plants to prevent diseases in plants.
