How to prevent downy mildew of broccoli

The downy mildew of broccoli was obvious in the process of flower head development. The old leaves first fell ill, and then spread to the young leaves. At the initial stage of the disease, there was a lack of green and the leaves turned yellow. When the leaf is dry, the mildew layer appears on the back of the leaf when it is wet.

Prevention and control methods. The resistant varieties were planted in ①, and the seeds were mixed with chemicals before sowing. ② has carried out crop rotation for more than two years to avoid continuous cropping. The soil should be turned deep into the sun and sprayed with tree protection generals to disinfect and sterilize. When ③ is cultivated in off-season, the suitable sowing date should be determined according to local conditions. After the previous harvest of ④, the diseased leaves should be removed and turned deeply in time, and it is best to adopt high ridge cultivation, which is beneficial to drainage and drought prevention, and prevent the surface soil from being too high or dry, so as to avoid burning young roots.