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.本發明提供一種可共通地應用於各種作物或植物病害、於環境全域中長期確保安全性的植物病害防除劑及使用其之植物病害防除方法。本發明係一種植物病害防除劑，其特徵在於含有藉由以400~1000℃燒成氫氧化鎂所獲得之氧化鎂。又，本發明係一種植物病害防除方法，其特徵在於使用上述植物病害防除劑。上述方法較佳為將植物病害防除劑混合於土壤耕作層及混合於育苗培土、使用植物病害防除劑之水懸浮液進行葉面處理、以及使用植物病害防除劑進行根部浸漬及植株基部灌注中的任一者。
