Onion downy mildew is an important disease on green onions and onions. When conditions are appropriate, the disease spreads rapidly and causes serious damage. In addition to harming green onions and onions, it also harms other cash crops such as garlic and leeks.
Onion downy mildew is mainly harmful to leaves. The diseased lesions are pale green and long oval, and when it is severe, it affects the upper half of the leaves. The plants are yellow or dead, and the lesions are inverted "V" shape. The peduncle is infected with the same symptoms as the leaves, and it is easy to be broken by the disease. When the humidity is high, the disease minister has a white to purple gray mold layer, that is, a cystic stalk and a sporangia of the pathogen. The bulb becomes soft after infection and the outer scale is rough or shrunken. The plants are dwarfed and the leaves are twisted and deformed.
Symptoms: The downy mildew of the scallions is mainly harmful leaves and pedicels. The peduncle is yellowish white or milky yellow infective spots, spindle-shaped or elliptical, with white mold on it, and later turned pale yellow or dark purple. The middle and lower leaves are infected, and the diseased part is gradually drooping. False stem disease is more ruptured and curved. Infected by bulbs, it can cause systemic infection. The diseased plants are dwarfed, the leaves are deformed or distorted, and when the humidity is high, a large amount of white mold grows on the surface.
Pathogen and pathogenesis: The disease is caused by the fungal infection of the genus Genus. The pathogen mainly overwinters the oospore with the diseased body in the soil, and the mycelium can lurk in the bulb to overwinter. In the second year, it was splashed by rainwater and transmitted to the leaves for initial infection. When wet, the diseased part produced a large number of sporangia, which mainly spread with the airflow, causing reinfection. Rainwater and insects can also spread, and whether seeds can be transmitted remains to be studied. It is cool and wet at night, warm during the day, and heavy fog is exposed. The soil moisture condition is conducive to the occurrence and prevalence of the disease.
Prevention and treatment method:
1. Select disease-free seeds: select the disease-free plots to keep the seeds, and the newly purchased seeds are soaked in 50 °C warm water for 25 minutes, and then sown after cooling.
2. Strengthen the management of the field: choose the land with high terrain, good drainage and ventilation. Apply fertilizer to the fertilizer and increase the application of phosphorus and potassium fertilizer. Rational irrigation, do not make the onion too wet. The diseased plants were found and removed in time. After harvesting, the diseased bodies are completely removed, concentrated, and deep-plowed to reduce the source of bacteria.
3, chemical control: spraying in the early stage of the disease, spray once every 7-10 days, even spray 3-4 times, often 70% ethylphosphorus aluminum manganese zinc wettable powder 600-800 times, 25% The metalaxyl cream can be 500-1000 times, the 50% Anke wettable powder is 2500 times, the 70% oil dry suspension is 500-700 times, and the 72.2% is 700-1000 times.
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