By treating a Cucurbitaceae plant such as cucumber, melon, pumpkin or watermelon with an agent for enhancing resistance to a disease of the Cucurbitaceae plant containing an amino acid selected from the group consisting of glutamic acid, threonine, serine and proline, a disease such as cucumber powdery mildew, cucumber anthracnose, melon powdery mildew and melon anthracnose is controlled.