The prevention and treatment steps of onion downy mildew

Onion downy mildew is a serious fungal disease that primarily affects green onions, onions, garlic, and leeks. It spreads quickly under favorable conditions, leading to significant crop losses. Timely application of **pesticides** is crucial in managing this disease effectively. The disease mainly targets the leaves and stems of the onion plant. Infected stems develop yellowish-white or yellow-brown spots, which are spindle-shaped or elliptical, and often covered with white mold. Over time, these lesions turn pale yellow or dark purple. The middle and lower leaves become drooping, while the false stem may become brittle and twisted. When the bulb is infected, it can lead to systemic infection, resulting in stunted growth, deformed or curled leaves, and the development of a thick layer of white mold on the surface when humidity is high. The symptoms on the leaves include pale green, elongated oval lesions that can spread up the leaf. Severe infections cause the leaves to turn yellow or die, forming an inverted "V" shape. The stem shows similar symptoms to the leaves and is prone to breaking due to the disease. High humidity promotes the formation of a white to purplish-gray mold layer, which consists of the pathogen’s sporangia and cysts. Infected bulbs become soft, with rough or shriveled outer scales, and the plants remain stunted with twisted, deformed leaves. The pathogen responsible for onion downy mildew is *Peronospora destructor*, a fungus that survives in the soil through oospores, especially in the presence of diseased **residues**. The mycelium can also overwinter in the bulbs. In the following growing season, rain splashes the spores onto the leaves, initiating the infection cycle. Under wet conditions, the infected parts produce large numbers of sporangia, which are spread by wind, causing further reinfection. Rainwater and insects may also contribute to the spread of the disease, although the transmission through seeds is still under investigation. Environmental conditions favoring the disease include cool nights, warm days, and heavy dew. High soil moisture levels significantly increase the risk of disease occurrence and spread. To prevent and control onion downy mildew, the following measures are recommended: - Use **disease-free seeds** and treat them before planting. Soak the seeds in 50°C warm water for 25 minutes, then allow them to cool before sowing. - Implement proper field management practices: choose well-drained, elevated plots with good ventilation. Apply balanced fertilizers, focusing on phosphorus and potassium. Avoid overwatering and remove infected plants as soon as they are detected. After harvest, ensure all diseased plant material is removed, collected, and deeply plowed to reduce the pathogen's survival. For **pesticide** applications, the **China Pesticide Network** recommends spraying at the early stages of the disease, repeating every 7–10 days for 3–4 times. Commonly used fungicides include: - 70% Ethyl Phosphate (WP), diluted 600–800 times - 25% Metalaxyl (emulsion), diluted 500–1000 times - 50% Anke Wettable Powder, diluted 2500 times - 70% Dry Moisturizing Agent, diluted 500–700 times - 72.2% Mefenoxam (Mildew), diluted 700–1000 times By combining cultural practices with timely pesticide applications, farmers can effectively manage onion downy mildew and minimize its impact on yield and quality.

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