Downy mildew of cabbage can occur in spring (March-May) and autumn (9 ~11month), but it is the most harmful in spring. The diseased spots on the diseased leaves are light green at first, and then gradually turn black to purple-black, slightly concave, irregular or polygonal due to the veins, and white frosty mildew layer appears on the diseased spots on the back of the leaves. When the disease is serious, the disease spots are contiguous and the leaves turn yellow and die.

Occurrence regularity: The pathogen of cabbage downy mildew mainly overwinters in the soil with oospores. Cabbage seedlings can be infected in late February in spring. Sporocysts produced by diseased parts spread in the field by wind and rain, and were reinfected many times. The epidemic of this disease is most likely to occur when the temperature is slightly lower (below 18℃), the temperature difference between day and night is large, and it is rainy, wet or foggy. Continuous cropping, water accumulation in the field, fertilizer removal and other conditions are also conducive to the occurrence of this disease.

Prevention and control methods:

① Agricultural control. Select disease-resistant varieties. Rotation with non-cruciferous vegetables every other year, and paddy-upland rotation can be implemented if conditions permit. Rational fertilization, timely topdressing, improve plant disease resistance.

② Chemical control. At the initial stage of the disease, you can choose 500 times solution of 64% antivirus alum, 600 times solution of 75% chlorothalonil, or Bordeaux solution of 1∶2∶400 times, and spray 1 time every 5 ~ 7 days and * * * for 2 ~ 3 times.