Suqian City District, Zhongyang Town, Fan Junguo telephoned on December 13th, said that his 6-mu land lettuce recently had downy mildew, yellow spots on the leaves of diseased plants, white frost on the back of the lesions, including a greenhouse The rate of diseased plants reached 70% to 80%. The rate of other diseased plants in greenhouses ranged from 5% to 10%. The control effects of methotrexate MnZn and thiophanate-methyl were poor. He asked which agents are effective in controlling downy mildew in lettuce. Downy mildew is one of the most common diseases of stem lettuce. It mainly damages the leaves. Plants can develop from the seedling stage to the adult stage, and the incidence of disease is heavier in the middle and late stages of growth. The first leaf of the lower part of the disease develops, with pale yellow near-circular lesions on the surface of the leaf. The lesion gradually expands into an irregular shape or is limited by the veins and is polygonal, the color turns to yellow-brown, and the frosty mold grows on the back of the lesion when it is wet. Layers, many lesions connected to dry leaves. The disease gradually developed from the lower leaves to the upper leaves. Although the stems of the damaged leaves could grow taller for a long time, they could not grow thick, resulting in a decline in yield and quality. The disease is a low-temperature, high-humidity disease with high field density, poor drainage, and heavy wet ground. Chemical control can be used 72% cream urea manganese zinc wettable powder 800 to 1000 times liquid, 52.5%? f ketone cream urea cyanide water dispersible granules 2000 to 2500 times liquid, 64% manganese zinc methacrylate wettable powder 1000 Diluent, 50% enoyl morpholine wettable powder 3000 times, 58% fine amazan MnZn wettable powder 800 times, 72.2% Promazine hydrochloride salt water 800 times spray, every 7-10 days Once, continuous control 2 to 3 times. All of the above-mentioned agents are systemic and have a therapeutic effect, and they can be effectively used to control diseases during the initial or prosperous period. In addition, before the onset can be used 1.5% polymycin WP 200 times, 70% mancozeb wettable powder 600 times, 77% copper hydroxide powder 600 times, 70% fentanyl manganese Zinc wettable powder 600 times spray prevention. Greenhouses and other protected areas to prevent the disease can also be used 45% chlorothalonil smoke agent, standard shed 100 grams per shed, in the evening after the closed shed smoke, the next morning ventilation, smoked 7 days, depending on the condition of continuous smoking 3 ~ 6 Times. According to Fan Junguo, there is a greenhouse disease strain rate of 70% to 80%, the incidence is more serious, that is, the use of drugs control is also very poor. In the greenhouse in the initial stage of disease, the above-mentioned agents can achieve better control effect. The thiophanate-methyl, metalaxyl and other drugs have been used for many years in production. The germs may have a high resistance to these drugs, resulting in a decrease in control efficacy. In areas where the effectiveness of metalaxyl and other drugs is reduced, metalaxyl can be mixed with other pesticides and used alternately. In addition to good chemical control, field management should also be strengthened to control the humidity in the shelter. Open the ditch around the shed and close the shed in case of low temperature and rainy days. When the humidity in the shed is high, the shed is ventilated and dehumidified at noon. When the soil is dry, use drip irrigation or artificial watering, do not flood irrigation.
