The pathogen of downy mildew is a living specific parasitic fungus, and the seeds are sterile. Pathogens mainly spread through airflow and invade from the stomata of leaves. The occurrence of downy mildew is closely related to the temperature and humidity environment around the plant. The initial temperature is about 65438 06℃, the suitable epidemic temperature is 20 ~ 24℃, and the relative humidity is above 85%. The spread of this disease is very fast, and some people call it "flying dry". Once the central disease virus strain appears, it only needs 3 ~ 4 times of expansion and reinfection, which can lead to great disaster. Therefore, the key to prevent and treat the disease is to find the central disease strain or ward as soon as possible. The disease is the main disease of cruciferous vegetables, and all cruciferous vegetables can get sick. Its main hosts are Chinese cabbage, green vegetables, cabbage, rape, cauliflower, radish and so on.

At seedling stage or from flowering stage to pod setting stage of seed plants, mold is prone to diseases and harm. The diseased cotyledons or tender stems at seedling stage turn yellow and die. The diseases of true leaves mostly start from the back of the lower leaves, and the yellow spots around them are not obvious. Soaking in water for a long time, the affected area will grow white mold when the humidity is high. Generally, pale green spots appear on the leaves first, and gradually turn yellow-brown. The diseased spots are not shaped or polygonal because of the veins. Pedicels are damaged, some are slightly bent, some are deformed and swollen, twisted like faucets, seed pods are infected with white sparse mold layer, and severely damaged seed pods are small and bent, with little or no fruit.