The disease mainly harms leaves, occasionally stems, tendrils and pedicels. Cucumber can be damaged at seedling stage, and the newly unearthed cotyledons of seedlings are prone to disease. After infection, the front surface appears uneven chlorosis and yellowing, and irregular withered yellow spots are gradually produced, and the black mold layer can be produced on the back surface of leaves under wet conditions. With the development of the disease, the cotyledons quickly turn yellow and dry.

Diseases of adult plants usually occur after plants enter flowering and fruiting. At the early stage of the disease, water-stained pale green spots appeared on the leaves, which became polygonal after expansion, changed from yellow-green to yellow, and finally turned brown, but the edge of the disease spots was still yellow-green, and the junction between the diseased part and the healthy part was not obvious. In a humid environment, a purple-black mold layer will grow on the diseased spots on the back of leaves. The humidity in the plastic greenhouse is high, and thick black mold is formed on the diseased spot, which is easy to disappear when it is dry. When the disease is serious, the diseased spots heal into pieces, which leads to the premature death of leaves and directly affects the growth of plants. The disease spots on resistant varieties are small, round and brown, and rarely expand, and there are few mildew spots on the back of leaves.