In the middle and lower reaches of the Yangtze River, there are many rainy days in early spring. Soybeans in greenhouses are often exposed to low temperature and high humidity and are susceptible to downy mildew. The symptoms, occurrence rules, and prevention and control measures are described below. Hazards Symptoms Maize downy mildew can occur from the seedlings to the harvesting period. Symptoms mainly manifest on leaves and beans. At the seedling stage, chlorotic spots often appear along the sides of the veins, and sometimes the whole leaves become light yellow, and when the humidity is high, the gray downy layer (spore cysts and sporangia) on the back of the leaves. The affected seedlings are short and thin, and the leaves are shrinking. They often die after ridge closure. In the adult stage, the disease usually begins with the lower leaves and spreads upward. At the initial stage, there were yellowish-green lesions on the surface of the leaves that were not round or irregular, and the lesions turned brown in the late stage. When the humidity is high, the grayish-white to grayish purple mildew layer is also densely stained on the leaf spot. In severe cases, the lesions are connected into a piece, causing the leaves to fall off and yellow, the plant premature decline, the whole plant withered. There is no obvious symptom on the surface of the affected soybean pods. There is a layer of yellow-white hyphae on the surface of the soybean pods in the pods, which contains a large amount of pathogenic oospore. The occurrence of the downy mildew disease bacterium belonging to the flagellin subfamily fungus, oospore with the diseased body in the soil or latent in the seed overwintering. When the environmental conditions in the shed were suitable the following year, the zoospores infested the embryos with oospores, causing initial infestation and subsequent re-infestation by airflow and rainwater. The occurrence and prevalence of the disease are closely related to temperature and humidity. The optimal temperature for the disease is 20～22°C and the relative humidity is above 90%. The main onset of downy mildew in the greenhouse in the middle and lower reaches of the Yangtze River is from mid-March to mid-May. In early spring, the period of long rain and heavy rainfall is heavy. The continuous cropping plots and low-lying plots are early and heavy, and the plots with partial nitrogen fertilization, dense planting, flood irrigation, and poor transparence are seriously affected. Prevention and control measures (1) Select early-middle-maturing varieties such as Taiwan 75, Qingsu No. 1, Qingsu No. 2, and Liaoxian No. 1 that are resistant to low temperature, low light, and disease resistance. (2) Select seed before sowing, dressing with metalaxyl, triethylphosphonate, thiram, carbendazim and other drugs. (3) It is advisable to use non-legume vegetables such as onion, garlic and leaf vegetables. (4) Remove the diseased body immediately after harvesting, bring it out of the field or burn it deeply, and plunge the soil deeper, soak it in the cold. (5) Scientific management. Increase organic fertilizer to avoid partial application of nitrogen fertilizer. Covering the membrane reduces watering and reduces air humidity in the shed. Properly dense planting to prevent field cover. Open a ditch to avoid picking up after the rain. Remove diseased seedlings in time. Rational control of greenhouse temperature, no ventilation before emergence, when the temperature is low, use multi-layer covering warming and heat preservation, keep the greenhouse temperature at 25 ~ 30 °C; Qimiao appropriate ventilation and cooling, greenhouse temperature control during the day at 18 ~ 25 °C, control at night 10～15°C; during the growth period, the shelf temperature is maintained at 20-25°C during the day and not less than 14°C during the night; the temperature rises; when the temperature of the shed exceeds 26°C, the ventilation volume is gradually increased; the plants enter the flowering stage, and the outside temperature is suitable. Remove the film. (6) Chemical control. In case of continuous rainy weather, it can be treated with 45% chlorothalonil smoke agent 250 grams per acre. In the evening, it is divided into 5 to 6 heaps in the greenhouse. After being ignited with dark fire, it will be smoked overnight and the next morning will be ventilated. Every 8 to 10 days, smoked continuously for 2 or 3 times. Early onset of disease prevention and treatment, can be used 25% metalaxyl WP 500 times, or anti-virus 64% Mn Zn wettable powder 500 times, or the world's highest 10% difenoconazole water dispersible granule 1000 Double liquid, or Kelu 72% cream urea manganese zinc wettable powder 800 times, 40% chlorothalonil suspension 600 times liquid, or 69% manganese zinc water dispersible granules 600 times spray, every 5 ~ 7 days once, continuous control 2 to 3 times, pay attention to alternate medication. The spray should be even and thoughtful, the front and back of the blade should be sprayed, with the focus on the diseased leaf blade.
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