To provide a plant disease controlling agent and the method for controlling plant diseases using the same, the plant disease controlling agent being commonly applicable to various crop and soil conditions, and ensures safety over a long period throughout the environment.Provided is a plant disease controlling agent including magnesium oxide obtained by baking magnesium hydroxide at 400 to 1000° C. Further, provided is a method for controlling plant diseases using the plant disease controlling agent. The method is preferably any of mixing the plant disease controlling agent with the surface soil, mixing the plant disease controlling agent with the nursery soil, treating foliage with an aqueous suspension of the plant disease controlling agent, immersing plant roots in the plant disease controlling agent, and irrigating plant feet with the plant disease controlling agent.
