First, the symptoms of litchi downy mildew mainly harm young leaves, flowers, ears and fruits; Orchards with continuous rainfall, high air humidity and poor ventilation and light transmission conditions are prone to this disease. After the leaves are damaged, brown spots begin to appear, and then expand into yellow-green to brown irregular patches; At the beginning of the disease, the single flower and pedicel appeared brown spots, and soon the whole inflorescence became brown and dry, but the flowers did not fall off; Damaged fruit pads will cause a large number of fruits to fall and rot. When the humidity is high, white mold will grow on the diseased spots of leaves, flowers and fruits.

Second, the pathogenic mechanism of litchi downy mildew pathogen overwinters in diseased tissues with hyphae and oospores, and some oospores fall into the soil for overwintering. The swimming sporangium can be formed at 1 1-30℃, and the reproductive tube can be directly formed at 26-30℃. Swimming sporangium or reproductive duct became the initial source of infection. Under appropriate conditions, it takes less than 24 hours from pathogen invasion to symptoms. In hot and humid weather, diseases will occur quickly and become more serious.

Generally speaking, low-lying orchards with poor drainage are seriously affected; Trees with lush foliage and heavy fruits have serious diseases; On the same tree, in the shade under the canopy, the disease is early and serious; The incidence of nearly mature fruits is higher than that of immature fruits. Early and middle maturity varieties are prone to disease.

Iii. control method 1. Change the habit of extensive management of litchi in the past and adopt scientific cultivation and management measures. Strengthen orchard management, improve the growth environment of litchi, reduce the humidity of orchard, reduce the main infection source of diseases, and improve the disease resistance of plants. In field management, it is necessary to apply fertilizer scientifically, mainly using organic fertilizers such as chicken manure, pig manure and cow dung and soil miscellaneous fertilizers to improve the physical and chemical properties of the soil in litchi orchard and make the soil loose and breathable. Urea and other fertilizers can be used as topdressing to supplement various nutrients needed by plants in time. At the same time, do a good job of pruning and pruning fruit trees, cut off all branches of pests and diseases and branches that are too dense, make the crown ventilated and transparent, and reduce the sources of pests and diseases.

2. Clean up the orchard to reduce the initial source of infection. After the fruit is picked, combined with pruning, the dead branches and shady branches of trees are removed, and the leaves and fruits on the ground are burned. Then, spray the orchard with appropriate pesticides. When spraying, the ground under the canopy is generally sprayed.