Prevention and treatment of lettuce downy mildew :
First, the characteristics of the disease:
Lettuce downy mildew mainly harms the leaves. In the early stage of the disease, the leaves of the lower part of the plant produce pale yellow near-circular or polygonal lesions, and the white back of the leaves has a white mildew layer. The disease gradually develops upward and spreads. The lesions turn yellow-brown in the later stage, and most of the lesions are connected into one piece when the disease is heavy. The yellowing is dry and the surface of the stem becomes brown and black, and even the whole plant rots.
Second, the disease conditions:
Lettuce downy mildew is caused by the fungus of the genus Oomycetes, which is in the soil with the oospores, or the mycelium in the seed, becoming the source of infection at the beginning of the next season, and the spores are produced in the spring of the next year. By wind and rain, insect spread, intrusion from the host epidermis or stomata, can repeatedly infect the hazard. The low temperature and high humidity conditions are conducive to the occurrence of downy mildew, and the field is too dense, the population is too large, the application of nitrogen fertilizer is too much, and the field with poor ventilation and light transmission is seriously affected.
Third, prevention and treatment methods:
1. Strengthen cultivation management. Planting density should be reasonable, avoid group depression, to facilitate ventilation and dampness, less watering in the shed or under-film irrigation. High ridge cultivation, full mulch film coverage, application of NPK fertilizer, and application of organic fertilizer to improve disease resistance of plants. Severely ill fields should be rotated with non-Compositae vegetables for more than 3 years, and the diseased bodies should be cleared after harvest to reduce the source of bacteria.
2. Seed dressing before planting. Seed dressing with 0.3%-0.4% thiram or 0.2%-0.3% metalaxyl before planting can kill the pathogen on the surface of the seed and reduce the source of infection.
3. Fungicide control. In the early stage of the disease, timely use of drugs can be used, such as Kelu 72% cream urea • manganese zinc wettable powder 600 times liquid, or anti-virus 矾 64% mold • manganese zinc wettable powder 500 times liquid, or 69% enoyl • manganese zinc wettable powder 700 times liquid spray, once every 5-7 days, continuous control 2-3 times. In the case of continuous rainy weather, the shed can be controlled with 45% of 5% chlorothalonil smoker per acre to reduce the humidity in the field.
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