Potato late blight is a fungal disease, which is a devastating disease worldwide. It happens in almost all potato growing areas. Late blight is easy to occur under the condition of low temperature and high humidity. When the air relative humidity exceeds 85% and the temperature is 18-25℃, late blight is most likely to occur. In the epidemic year of late blight, plants will die early, and the loss can reach 20-49%. If the prevention and control measures are not appropriate, it will cause a serious loss of 70-80% reduction in production. 1, symptoms of late blight The late blight mainly harms potato leaves, stems and potato pieces. First of all, there are brown spots on the tips and edges of leaves, which usually form pale yellow and green edges around diseased leaves. When it is dry, the lesion is dry, and when the air humidity is high, the lesion expands rapidly, which can extend to most or all leaves. Lesions are black or brown stripes on petioles and stems; When the disease is serious, the whole plant dries up; When the stem is damaged, brown stripes of different lengths will appear. When the weather is wet, white mold will grow on the surface, but it is sparse. The diseased seedlings grown from diseased seed potatoes are connected with underground stems to form a central diseased plant in the field. When potato is damaged, small brown or purplish spots will be formed in the early stage of the disease, and the spots can be enlarged after being slightly depressed. When the diseased part is cut open, we can see that the potato meat under the skin is brown and gradually develops around and inside. When the soil is dry, the affected part becomes hard and dry; In sticky soil, miscellaneous bacteria often invade from diseased spots, resulting in soft rot of potato pieces. Potato pieces will rot in the field after being sick in the field, and will also rot in the cellar during storage. 2. Epidemic law of late blight Late blight is a typical epidemic disease, and its incidence is closely related to variety resistance and weather conditions. Because its physiological race is easy to produce splint, resistant varieties will become susceptible varieties after years of planting. Late blight needs high humidity and cool climate conditions. The formation of sporangium stalk requires that the relative humidity of air is not lower than 85%, and the relative humidity of sporangium formation is above 90%, and the saturation humidity is the most suitable. After the sporangium falls on the leaf, there must be water film or water droplets on the leaf to germinate and invade. After the invasion of the pathogen, at 20-30℃, the hyphae spread fastest in the host and the incubation period was the shortest. Generally, it shall not exceed 24℃ during the day and not lower than 10℃ at night. Rain or fog, dew and relative humidity are conducive to the epidemic of late blight. On the contrary, if it is dry and rainy, the temperature is high and the illness is mild. 3. Prevention and control measures of late blight. Late blight is a very destructive disease. Once it happens and begins to spread, it is difficult to control. Therefore, the prevention and control of potato late blight should follow the principle of "prevention first". Based on the promotion of disease-resistant varieties and virus-free seed potatoes, focusing on seed potato treatment and chemical emergency prevention and control, supplemented by fitness cultivation, the whole process of comprehensive prevention and control technology was adopted. Control the loss rate of potato late blight within 10%. (1) Selecting improved potato tubers is an important way to spread late blight. Once the tuber with bacteria meets the suitable conditions, it is easy to form a central diseased plant, which may lead to diffusion. Improve the virus-free seed potato breeding system and establish a disease-free seed reserve base. Take strict management measures and go it alone. Promote and cultivate varieties with relatively strong resistance and tolerance according to local conditions. Advocate small whole potato sowing. When the seed potato is cut into pieces for sowing, the cut pieces are soaked in 75% alcohol, 0. 1% potassium permanganate solution or formalin for disinfection. 68% metalaxyl manganese zinc wettable powder and 72% cymoxanil manganese zinc wettable powder can be used for seed dressing. Add appropriate amount of plant ash or gypsum powder according to 0.3% of seed potato weight and mix well before seed dressing. (2) High Ridge Cultivation Promote high ridge cultivation, especially in areas with much rain and good soil moisture. We can use the method of sowing on the ridge and sowing flat and then ridging to thicken the soil layer, prevent the late blight fungus from infecting the potato pieces and reduce the bacteria-carrying rate of the potato pieces. (3) Scientific planting, reasonable close planting, and control the excessive growth in bud stage. Sow in time, rotate crops reasonably, avoid continuous cropping or interplanting with solanaceae and cruciferous crops, and prohibit continuous cropping with tomatoes. In the process of potato planting, if the plants on the ground are infected with late blight, the diseased seedlings should be harvested in advance and the plots should be cleared, and the plots should be harvested on a sunny day after two days of exposure to prevent the potato blocks from contacting with germs. (4) Timely chemical control of potato late blight can only be prevented, but not treated. Therefore, spraying pesticides regularly in late blight season can achieve remarkable disease prevention effect. Before the emergence of potato buds or when it rains (cloudy days with high humidity), the central diseased plants were investigated, the fixed-point system investigation and large-area field investigation were carried out. It was found that the central diseased plants were removed in time and taken out of the field for destruction, and the spraying of chemicals within 50 meters around the diseased plants was blocked and controlled, every 7- 10 day 1 time, 3-4 times in a row. After entering bud stage, field investigation was carried out, and it was found that the disease strain from systemic fungicide Center for Prevention and Control could be used. The first is to use protective drugs for prevention. When the emergence rate reaches 95%, the protective drug, namely 70% mancozeb wettable powder, should be sprayed in time, and the dosage per mu is175 ~ 225g; Or 70% Antaisheng, the dosage per mu is150 ~ 200 g; Or 75% kill animals violently, and the dosage per mu is125 ~150g; It is still 68.75% easy to protect, and the dosage of 75 ~ 94 grams per mu is very good. Second, timely use of therapeutic drugs for prevention and treatment. When the disease plant in the center of late blight is found in the field, spray 800 times of 25% metalaxyl or 50% mancozeb or 500 times of 64% antiviral alum per mu, or mix 500 grams of copper sulfate, 500 grams of quicklime and 50 kilograms of water into the same amount of Bordeaux solution, once every 10 day, and spray continuously for 2-3 times. Use interchangeably. It is important to note that the sporangium of late blight is produced on the back of potato leaves, so late blight has occurred. When spraying chemicals, spraying chemicals on the back of leaves and the ground at the same time can achieve the effect of comprehensive prevention and control of large-scale epidemic situation.