Types and prevention of onion downy mildew

Downy mildew is a major disease that afflicts onions. In recent years, the epidemic has been highly prevalent and has occurred in various places, causing great losses. Timely pesticide control is the key.
According to different environmental conditions and age, it can be divided into the first dip dye (system infestation) and the second dip stain. The first infestation occurred in the autumn seedbed or in the early spring field. The hyphae developed in winter and the lesions appeared in the spring of the following year. The seedlings grow poorly after the disease, the leaves are dull, the leaf body is distorted; the spot spreads rapidly after the warming in spring, and can damage the new leaves. When the air is wet, the lesions produce sparse white or purple mildew. The diseased plant continues to spread as a disease center, forming a re-infection. It is mainly harmful to the flower buds of the leaves and seed collection plants. There are 5 types of symptoms: 1 the surface of the damaged part of the leaf is covered with lavender velvet; 2 leaves have long oval or elliptical yellowish green lesions, and the surface grows white or gray-purple mildew, washed by rain. After the lesion becomes grayish white leaves die; 3 produces yellow lesions of different shapes, but no mold; 4 oval lesions around 2 - 3mm wide, slightly concave gray-white circle; 5 in continuous drought Under the conditions, it presents a grayish white small lesion. In the later stage, it is often infested with saprophytic bacteria such as gray mold and black spot to produce gray or black mildew. After the bulb is damaged, the outer scales become soft and wrinkled, sometimes mixed with soft rot. The disease is characterized by large lesions, long oval shape, yellowish white, and the lesions turn grayish white after the rain. When wet, the lesions are covered with sparse white mold, and gray purple mold grows at high temperature.
Incidence conditions:
According to the Chinese Pesticide Network , the spread of this disease mainly occurs in the soil with oospores and disease residues , and in the autumn, the mycelium in the seedlings or bulbs of the plant is infiltrated to form a systemic infection. In the south, due to the warm climate, the bacteria can survive in the field with the diseased plants. Thereafter, the pathogen can survive in the field with the diseased plant. Thereafter, the sporangia grow on the lesions by the wind and rain, and invade from the stomata. When the disease is cold, rainy or often in heavy fog, it is popular. Under the conditions of heavy land, low-lying land and flooding, excessive dense planting, etc., the pathogenesis is also relatively serious: the flagellum is a genus of the genus Aspergillus.
Control method:
1. Carry out 2 - 3 years of rotation, and pay attention to cleaning up and burning the sick tissue and its stubble in the field.
2. Strictly select robust seedlings before planting and eliminate diseased seedlings.
3. Reasonable close planting, proper amount of watering, and strengthening drainage work in the rainy season in the field.
4, timely application of prevention and treatment, in the early stage of the disease, available 75% fungicide chlorothalonil WP powder 600 times liquid; 58% metalaxyl manganese zinc, 70% manganese manganese wet powder or 60% succinylphosphine Aluminum wettable powder is sprayed on water 500 times; or 1:1:20 Bordeaux mixture, 5-10g of neutral laundry is added as a spreading agent per 10kg of liquid. Spray once every 5 to 7 days, and continue to prevent 2 to 3 times. After that, you can spray again according to the condition.

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