Onion downy mildew is an important disease on green onions and onions. When conditions are right, the disease spreads quickly and causes serious losses. In addition to harming green onions and onions, it also harms other cash crops such as garlic and leeks. Onion downy mildew mainly damages the leaves. The lesions with light onset were pale green and long oval, and when severe, they affected the first half of the leaves. The plants were yellow or withered, and the lesions showed an inverted "V" shape. Pedicel infected with the symptoms of the leaf, easy to die off by the disease. When the humidity is high, the disease minister produces a white to purple gray mold layer, that is, the cysts and sporangia of the bacteria. The bulb becomes soft after infection and the outer scales are rough or shrunken. The plants are dwarfed and the leaves are distorted. Symptoms: Onion downy mildew mainly damages leaves and peduncles, primordial yellowish white or milky yellow larger infestation spots, spindle-shaped or oval-shaped, on which produce white mold, later turned pale yellow or dark purple. The middle and lower leaves became infected, and the diseased parts gradually dangled. The pseudostem is more broken and bent. Bulb disease, can cause systemic infection, this disease strains dwarf, deformed or distorted leaves, humidity, the surface grow a lot of white mold. Pathogen and Pathogenesis: The disease is caused by fungal infection of the flagellate of the flagellate genus Aspergillus. The pathogenic bacteria are mainly borne by oospore in the soil with the diseased body, and it is also possible for the mycelia to lurk in the bulb for wintering. In the second year, the water was splashed back and spread to the leaves for initial infection. During the wet period, a large number of sporangia were generated in the diseased area, which mainly spread with the airflow and caused re-infection. Rainwater and insects can also spread, and whether seeds can be transmitted remains to be studied. It is cool and humid at night, warm during the day, heavy fog re-exposed, and soil moisture conditions are conducive to the occurrence and epidemic of the disease. Control methods: 1. Use disease-free seeds: select disease-free plots to retain seed, soak seeds for 25 minutes in fresh water at 50°C, and sow seeds after cooling. 2. Strengthen the management of farmland: Choose high-lying, well-drained and well-ventilated plots. Apply fertilizer and increase phosphorus and potassium fertilizers. Reasonable irrigation, not too wet onion. Discover the diseased plants and remove them in time. After the harvest, the diseased bodies were completely removed, concentrated treatment, and deep plowing were performed to reduce the source of bacteria. 3, chemical control: spraying the beginning of the disease prevention and control, spray once every 7-10 days, even spray 3-4 times, often the agent is 70% phosporium manganese zinc wettable powder 600-800 times, 25% Metalaxyl preperoxide can be 500-1000 times wettable powder, 50% ank wettable powder 2,500 times, 70% product dry dry suspension 500-700 times, 72.2% pomegranate 700-1000 times liquid.
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