Lettuce is a relatively common vegetable, you can eat the leaves and stems (Ming bamboo shoots), nutritious, but also can be used in medicine, beneficial to the five organs, meridians, clearing stomach heat. All over the country are planted, is one of the important vegetables in spring, summer, fall and winter. Winter and spring are the seasons when downy mildew is common. When it develops seriously, many leaves will turn yellow or brown and wither, affecting the growth of plants and product quality, and bringing great losses to production. This disease mainly affects the leaves. At the beginning of the disease, light yellow source-type spots appear on the lower part of the plant, which are later restricted by the leaf veins when expanding and become polygonal, and the diseased leaf develops from the bottom to the top. When wet, white mold grows on the bottles on the back of the leaves.

In severe cases it can spread to the leaf adaxial surface, where white mold also grows. The later spots become yellowish brown, and when the disease is severe most of the spots are connected together, the leaves turn yellow, and finally the surface of the stem turns brown, blackens, and even rots a tree. The disease develops to adapt to the temperature of 15-17, 1-19 can develop, the beginning of winter or the rainy season continues in late spring is most likely to occur in the low temperature and high humidity, the growth of fall and winter stubble is growing in the group is larger, ventilation is not good, the light is not strong, it is easy to be reported to the disease. At the same time, the water after the main meal is too early, give too much, soil wet or poor drainage, nitrogen fertilizer application too much field management, easy to cause disease aggravation. Select disease-resistant or resistant varieties. According to the resistance to downy mildew there are some differences, usually roots, stems and leaves with purplish red or dark green varieties are more resistant to the disease.

Practice crop rotation. Crop rotation with eggplant, cruciferous vegetables or grass crops for 2-3 years. Strengthen field management. Reasonable dense planting, increase site ventilation and light transmission rate; avoid partial application of nitrogen fertilizer, increase phosphorus and potassium fertilizer. Take monopoly or ridge cultivation, strictly prohibit large water flooding, keep the ditch open, reduce field humidity. "Winter plastic greenhouse frost mold is not easy to burn, thick leaves, leaves attached to the floor, bottles attached to the back, resistance to suction is not good agents hit the front, the back of the suction can not be hit under the leaves, it is difficult to control." Wang Heng studied the experience of downy mildew control with his own planting experience. Timely removal of the lower part of the diseased leaves, spraying the new high fat film 600 times at night, can automatically form a polymer protective film invisible to the naked eye, blocking pest predation signals, weakening the medium of transmission, resistance and defense against natural environmental disasters.

In winter, we should pay attention to strengthening the plastic greenhouse against wind and moisture, scientific watering, discharge stains, while regularly cleaning the plastic greenhouse film to increase light transmission. Lettuce before the onset of the disease can be used 30% or 45% white fungus clear fumigant, the onset of the early stage of the disease can be used 25% frost fungus clear fumigant, 250 grams per acre at a time, at night, divided into 4 ~ 5 points, lit smoke, smoking prevention in the airtight room. 7 days a time, Yanhun 4 ~ 5 times, shall not be interrupted. Or start spraying from the early stage of the disease, 2 ~ 3 consecutive preventive sprays in about the interval of 7 ~ 10 days. Pharmacies can choose 80% dasheen zinc wet powder 500-800 times liquid or 58% methyl bed, manganese zinc wet powder 600 times liquid, 75% white fungus green wet powder 700 times liquid, 72% cream zinc wet powder 800 times liquid and so on. Winter is the high incidence of plastic greenhouse downy mildew season, it is recommended that vegetable farmers and friends like Wang Heng timely preparation for prevention, selection and prevention of fungicides with good attractiveness, to prevent downy mildew outbreaks on a large scale, to reduce unnecessary losses.