Why are cucumber leaves yellow in the greenhouse?

If the disease is ruled out, it is necessary to consider whether the soil is sticky. Sticky soil will cause the roots to turn yellow due to poor drainage, and the shed is in a state of high humidity, so pay attention to ventilation. The cotyledons of cucumber growing normally at seedling stage opened at 75 degrees about 4 days after sowing, and the wheelbase of the lower embryo of the seedling was 3 ~ 4 cm on the ground. On the 5th to 6th day, the leaves are flat and full, dark green in color, and slightly curled with spoon-shaped edges. Under unfavorable environmental conditions, the seedlings will be abnormal. According to the seedling performance, it is the key to ensure the normal growth and late yield of cucumber seedlings to determine the reasons and take effective measures.

1. After sowing, the radicle becomes thicker without binding, the root system rusts, the cotyledons are small and twisted, the leaf edges turn yellow, and then droop, which is caused by low ground temperature and high soil humidity.

2. The seedlings grow slowly, the cotyledons are small, and the volume of the lower part of the leaf margin is inverted spoon, which is caused by low temperature.

3. After the cloudy day suddenly cleared up, the uncovered seedlings wilted and recovered quickly after turning green, which was caused by insufficient light, disharmony between ground temperature and air temperature, and failure to replenish evaporated water in time, resulting in physiological hunger of seedlings.

At noon, the seedlings wilted and the leaves were burnt, which was caused by the high temperature and strong sunshine in the shed.

5. The seedlings are thin, the cotyledons are thin, the color is light and the hand feels soft, which is caused by the high night temperature.

6. Seedling atrophy is not long, leaves are aging and stiff, and leaves are dark green, which is caused by insufficient soil moisture or excessive fertilization. In addition, the cotyledons of cucumber downy mildew and virus downy mildew turned green at seedling stage, showing yellow irregular spots, and there was a gray-black mold layer on the back of leaves when the humidity was high. In the adult stage, light green water spots appeared on the leaves. After expansion, the spots are polygonal yellow-green, and then turn light brown. In the later stage, the spots become patches and the whole leaves atrophy. When the humidity is high, a gray-black mold layer will also appear on the back of the leaves, and the diseased leaves will develop rapidly from bottom to top. Control method 1. Strengthen cultivation management and control field humidity: The field cultivation management of cucumber downy mildew mainly controls field humidity. Take measures such as plastic film mulching and drip irrigation to reduce the humidity in greenhouse fields; Timely removal of old diseased leaves can ventilate, transmit light and reduce humidity, and also reduce the amount of bacteria in the field; Greenhouse cultivation should be ventilated in time; Apply more phosphorus and potassium fertilizer to improve plant disease resistance. 2. Chemical control: the chemical control of cucumber downy mildew should focus on prevention, and prevention should be carried out under the condition of continuous rainfall. Methods Spraying protective agent or fumigant was used for prevention, among which "Yi Xunling" was the best fumigant. Light it in the greenhouse at night, then suffocate and smoke it all night, and ventilate it the next morning, 4-6 days 1 time. Once cucumber downy mildew occurs, chemical control should be carried out in time at the early stage of the disease. When spraying, the front and back of the leaves should be sprayed evenly and thoughtfully, with emphasis on the back of the diseased leaves. Plants should be sprayed from bottom to top. The chemicals that can be used are: 25% metalaxyl wettable powder, 64% antiviral alum wettable powder, 72.2% Prick aqueous solution, 50% metalaxyl wettable powder, 47% Jialenong wettable powder, etc. Spray 500-800 times liquid, and pay attention to the safe interval after picking cucumbers. Cucumber seedlings infected with virus are susceptible to diseases, cotyledons turn yellow and wither, young leaves are uneven, mottled with dark green and light green, and gradually develop into dark green and light green mosaic. The diseased leaves are small, shriveled and tend to roll down, the leaves become hard and brittle, and the plants are short. In the later stage, the lower leaves gradually turned yellow and died. Prevention and control methods: In the prevention and control of virus diseases, especially in the seedling stage, attention should be paid to the prevention and control of aphids. When aphids occur in the field, 10% imidacloprid wettable powder can be used for prevention and control with 2000 ~ 3000 times solution. At the initial stage of the disease, 500 times of 20% virus A wettable powder or 6 00 times of 20% virus ling wettable powder can be sprayed. However, when selecting varieties, it is best to pay attention to selecting varieties with strong disease resistance.