Fuente: WIPO "tomato" To express a resistance-inducing active gene against a disease in order to reduce the disease without a synthetic agricultural chemical. A resin mixture obtained by combining a thermoplastic resin and a spent mushroom substrate is mixed with water, and the resultant is heated and filtered. In this manner, an extract is obtained. The extract is sprayed onto a plant, so that expression of a resistance-inducing active gene in the plant can increase and the resistance of the plant against a disease can increase.