[Problem] To provide a method for controlling diseases in a rice plant, which is highly safe to a rice plant, is simple and efficient, has little labor load, and can exhibit a sufficient effect. [Solution] An ungerminated rice seed is subjected to at least one treatment selected from an immersion treatment, a powder coating treatment, a smearing treatment and a spraying treatment using an agrochemical composition containing 3-(3,4-dichloroisothiazole-5-ylmethoxy)-1,2-benzothiazole- 1,1-dioxide as an active ingredient. In this manner, it becomes possible to keep a disease control effect for a long period from the time point when the rice seed germinates to the time point when the rice seed is harvested without causing any harmful effect on the rice plant.
