Winter is the season of high incidence of downy mildew in lettuce. The disease mainly damages the leaves. In the early stage of disease, the lower leaves of the plant produce yellowish-yellow lesions, which are later restricted by the veins and appear polygonal, and the diseased leaves develop from bottom to top. When wet, white mold grows on the back of the leaves. In severe cases, it can also spread to the front of the leaves and also grows white mold. Late lesions turn yellowish-brown, and most of the lesions become contiguous when the disease is severe. The leaves are yellow and dry. Finally, the surface of the stem turns brown and dark, and the entire plant rots. 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. The prevention of downy mildew in lettuce should adopt comprehensive measures based on agricultural control and supplemented with chemical control.
1, agricultural control. (1) Select resistant varieties according to local conditions. Roots, stems, and leaves with purple or dark green varieties are more resistant to disease. (2) Implement rotation. Serious disease fields should be planted with non-Agronomy crops for more than three years to reduce the source of field bacteria. (3) Strengthen cultivation management. When the seedlings are planted and colonized, the diseased plants are eliminated. Reasonable dense planting, diligent cultivator, more loose soil, strengthen ventilation and humidity, scientific watering, pay attention to drainage. Remove the lower sick leaves in time. Appropriate increase in phosphorus and potassium fertilizers.
2, pharmaceutical control. Before the onset of disease, 75% chlorothalonil wettable powder can be sprayed 500-600 times; at the beginning of the disease, 80% or 90% schisopazol WP can be sprayed 500-600 times, or 70% phospine manganese zinc can be Wet powder or 58% metalaxyl WP or 76% wet mildew WP 400-500 times, or 72% EM WP 600-800 times, or 72% downy may Wet powder 600-800 times, or 72% Kelu wettable powder 600-800 times, or 72.2% Propamoide Wei water 600-800 times, or 72.2% Preclosure 600-800 times, every Spray 7 times a day, spray 4-5 times. 30% or 45% chlorothalonil smoke agent can be used before the onset of lettuce in the protected area. 25% of downy mildew smoke can be used at the beginning of the disease. 250 grams per acre is used. In the evening, 4-5 points are placed to light the smoke. After the closed room smoking prevention. 7 days smoked once, smoked 4-5 times, can not be interrupted. Before the onset of the disease, 5% chlorothalonil dust can be used. When the disease just happened, use 7% anti-mildew spirit dust agent. Use 1 kilogram per acre, spray sooner or later. 7 days, spray 4-5 times.
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