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Cucumber downy mildew, commonly known as "race horse" and "black hair," is a common disease in the cultivation of cucumber in protected areas in early spring. Its onset characteristics are ferocious, spread quickly, and endanger large cucumbers. The reduction of production is 30% to 50%, which leads to severe destruction and pull. The disease usually occurs in March and April in spring, and the disease is even more severe in the sheds where many years are planted. Now introduce the symptoms, conditions of the disease and comprehensive prevention and treatment methods. First, the symptoms manifested in the cucumber seedling stage, adult stage disease, mainly damage the leaves. Seedling disease leaves yellow, after the whole plant withered; adult disease mostly from the lower leaves began after the disease, the leaves showed water spots after the initial spot, after the emergence of uneven chlorosis yellow, due to limited vein lesions presented polygonal In the wet, a black layer of mold is formed on the back surface, and later sick leaves are fragile and dry. In severe cases, the whole plant of cucumber is browned, leading to premature pull. Second, the onset of conditions The pathogen is a fungal parasite, can be parasites perennial parasitic on the host plant in the northern greenhouse, greenhouse cucumber planting winter, by wind, rain spread to spread around. Germs prefer conditions of temperature and humidity, germination and invasion require the presence of water droplets in the leaves. The suitable temperature for disease onset is 15°C～22°C. Less than 15°C or higher than 28°C is not conducive to onset. When the relative humidity of air is above 85%, it is conducive to disease. Rainy, foggy, dreary, and large temperature difference between day and night, alternating rain and sunny weather, the incidence of early and heavy. The temperature and humidity of the cucumber in the protected area are improperly controlled, improperly vented, or watered in the evening so that the surface of the leaf is hung with water drops for a long time, which is conducive to the occurrence and spread of diseases. When the conditions are appropriate, the bacteria invade the disease from the stomata, and the incubation period is only 3 to 5 days. The disease will worsen when the plant grows weak. Third, prevention and control methods First, select disease-resistant varieties. In early spring greenhouse protection cultivation, it is possible to select good resistant varieties such as Jinchun 3, Zhongnong 13 and Jinyou 3. The second is to strengthen the cultivation and management. Pay attention to reasonable rotation, watering can be carried out on a sunny morning, pouring small water, to prevent flooding, and sprinkler irrigation and drip irrigation can be used for conditions. Promote balanced fertilization techniques, pay attention to increase phosphorus, potassium, calcium and trace fertilizer. When a diseased plant occurs, diseased leaves and old leaves must be removed in time to widen the row spacing, and high-riding mulching film should be used to reduce the humidity in the shed. The third is high temperature stuffy shed. Select sunny noon, the greenhouse temperature was sealed to 44 °C ~ 46 °C, high temperature treatment for 2 hours, repeated every 3 to 5 days, you can control the sudden increase of bacteria (this method applies to cucumber plants grow more robust conditions) . The fourth is the smoke agent fumigation. In the central greenhouse, the cucumber in the greenhouse is ignited in an amount of 0.5 to 1 kg per acre, and then ignited and smoked overnight in the drenching booth before and after the evening to effectively control the occurrence of disease. When the number of diseased plants is large, it can be repeated every 3 to 4 days. Fifth, chemical control. Silver spray 800 times solution, spray every 7 to 10 days, continuous spraying 2 to 3 times, pay attention to the front and back of the spray to spray evenly.