Symptoms
Onion black spot disease has become one of the most prevalent diseases affecting green onions in recent years. This fungal infection primarily affects the leaves and leaf sheaths, with certain onion varieties being more vulnerable to the disease. Initially, small yellow or pale green elongated spots appear on the leaves. As the disease progresses, these spots expand rapidly both upwards and downwards, forming characteristic oval or spindle-shaped lesions. The center of the lesions turns dark brown, while a yellowish halo forms around the edges. Over time, the lesions continue to grow and merge, maintaining a fusiform shape. A distinct ring-like pattern may also develop within the lesions, making them clearly structured. In the later stages of the disease, a dense layer of black fungal spores, along with conidial stalks and conidia, becomes visible on the affected areas. The upper parts of the infected leaves may show chlorosis, drying, or even complete breakage of the stems.Control Methods
(1) **Agricultural Practices**: Apply organic compost as a base fertilizer and increase the use of phosphorus and potassium-rich fertilizers. Avoid overcrowding by maintaining proper planting density. Remove and destroy infected plants to prevent the spread of the disease. Implement a crop rotation system of 2–3 years, avoiding planting onions or related crops in the same area. Improve field management by ensuring proper drainage after heavy rains to promote healthy plant growth and enhance resistance to diseases. (2) **Chemical Control**: During the early stages of the disease, you can use 70% chlorothalonil wettable powder diluted at 600 times, or 50% acetamiprid wettable powder at 1000 times. Alternatively, 70% wettable powder at 600 times or 14% copper oxychloride at 300 times can be applied. Another effective option is a 0.5:200 Bordeaux mixture, which should be sprayed every 5–7 days for 3–4 applications. Regular monitoring and timely application of fungicides are essential to control the spread of the disease effectively.Additional Tips:
It's important to ensure good air circulation in the field, as high humidity can accelerate the development of the fungus. Always follow the manufacturer’s instructions when using chemical treatments, and consider rotating different fungicides to avoid resistance. By combining cultural practices with timely chemical interventions, farmers can significantly reduce the impact of onion black spot disease and maintain healthy, productive crops. (Word count: 548)good frozen pacific mackerel
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