Lettuce is an early vegetable listed in the early spring, and the cultivation technology is simple. In recent years, the planting area has been continuously expanded, but with the increase of the planting period, downy mildew and other diseases have also been increasing year by year, and attention should be paid to early prevention and treatment.
The downy mildew of lettuce mainly damages the leaves. In the early stage of disease, the lower leaves of the plant produce pale yellowish-yellow round or polygonal lesions. The back of the leaves has a white moldy layer, and the disease gradually develops and spreads. The later lesions turn brown, and when the disease is heavy, most of the lesions are connected to one another. Yellow and dry, the surface of the stem turns brown and black, and even the whole plant rots.
The downy mildew of lettuce is caused by Oomycetes, and the fungal spores are in the soil with the spores or the mycelium is in the seeds and become the source of infection in the early next season. Spores are produced in the following spring. Through wind and rain, insects spread, invade the host epidermis or stomata, it can infect repeatedly. Low-temperature and high-humidity conditions are favorable to the occurrence of downy mildew. Overweight plants, over-populations, over-application of nitrogen fertilizers, and poor ventilation and light-transmitting fields are serious.
To prevent downy mildew in lettuce, we should strengthen cultivation and management. The planting density should be reasonable to avoid the group's shelter, in order to facilitate ventilation and dampness, less watering in the shed or irrigation under the film; high ridge cultivation, full film coverage, combined application of nitrogen, phosphorus and potassium, and more organic fertilizer to enhance the plant Disease resistance; severe disease fields should be rotated with non-Asteraceae vegetables for more than 3 years, after the harvest, the diseased bodies will be cleared to reduce the source of bacteria. Before sowing, 0.3% to 0.4% of Formamide or 0.2% to 0.3% of metalaxyl seed dressing can kill germs on the seed surface and reduce the source of infection. In the early onset of the disease prevention and treatment, can be used Kelu 72% cream urea Mn Zn wettable powder 600 times, or anti-virus phlegm 64% Mn Zn wettable powder 500 times, or 69% methazide zinc WP wet 700 times Spray, once every 5 to 7 days, continuous control 2 to 3 times. In case of continuous rainy weather, the shed can be treated with 45% chlorothalonil smoke agent 350 grams per acre to reduce the humidity in the field.
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