Symptoms
Onion black spot disease has become one of the most prevalent diseases affecting green onions in recent years. It primarily affects the leaves and stems, with certain varieties being more vulnerable to infection. 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 shuttle-shaped lesions. The lesions turn dark brown, often surrounded by a yellow halo. Over time, the affected areas develop a fusiform shape, and distinct concentric rings or streaks become visible on the lesions. In the later stages, a dense layer of black fungal spores, along with conidial stalks and conidia, can be observed on the infected tissue. The upper parts of the diseased leaves may show chlorosis, wilting, or even complete drying, while the stems might break easily, leading to significant yield loss.Control Methods
(1) **Cultural Control**: Apply organic fertilizers to improve soil health and increase the use of phosphorus and potassium to enhance plant resistance. Avoid over-crowding by maintaining proper planting density. Remove and destroy infected plants promptly to prevent spread. Implement a crop rotation system of 2 to 3 years, avoiding planting onions or related crops in the same area consecutively. Proper field management is essential—ensure good drainage after heavy rains and promote strong, healthy plant growth to boost natural immunity against disease. (2) **Chemical Control**: In the early stages of the disease, apply 70% chlorothalonil wettable powder diluted at 600 times, or 50% mancozeb wettable powder at 1000 times. Alternatively, 70% copper oxychloride wettable powder at 600 times or 14% copper oxychloride at 300 times can also be effective. A Bordeaux mixture of 0.5:200 (copper sulfate to lime) can be used as a preventive measure. Spray every 5 to 7 days, applying the treatment 3 to 4 times for best results. Always follow the manufacturer’s instructions and safety guidelines when handling chemical products. Regular monitoring of the crop is recommended to detect symptoms early and respond quickly.fresh frozen pacific markerel
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