The invention provides a germicide. The germicide contains the following ingredients in percentage by mass: 25% of ingredient A, 7.79% of ingredient B and the balance of water, wherein the ingredient A contains difenoconazole and kresoxim-methyl; the ingredient B contains a dispersant, a defoamer, an antiseptic, an antifreezer and a thickener. Through the reasonable proportioning ratio and synergistic effect of all the ingredients, the germicide has the characteristics that prevention and treatment effects are good, the suitable dosing period is long, the dosage is low, the harm caused by the germicide is lowered, the drug tolerance and drug resistance of germs are delayed, the cost of investment is low, the cycle of research and development is short, the economic value is high, and the like. The invention further discloses application of the germicide in prevention and treatment of cucumber powdery mildew.