A plant disease control agent contains a magnesium oxide obtained by firing magnesium hydroxide at 400 to 1000°C and a nonionic surfactant that contains a polyoxyalkylene group and has an HLB value of 14.5 or less. Alternatively, a plant disease control agent contains a magnesium oxide that generates radical species and nonionic surfactant that contains a polyoxyalkylene group and has an HLB value within a range of 14.5 or less.
