Purple spot disease primarily affects onions and green onions, but it can also infect garlic and leeks. The disease mainly damages the leaves, stems, and bulbs of the onion. Lesions typically start from the tips of the leaves or at sites where the plant has been damaged by thrips. Initially, the lesions are small, ranging in color from gray to light brown, with a slightly purple center and black or gray mold. As the disease progresses, the lesions expand into elliptical or spindle-shaped areas, featuring a dark purple, concentric ring pattern and a yellow halo around them. When humidity is high, the lesions may spread across the entire leaf, causing the lower parts of the onion leaves to turn yellow, wilt, and eventually die or break. This significantly reduces both the yield and quality of fresh onions.
To prevent and manage purple spot disease, it's important to clean the fields and remove any infected plant material. Enhancing crop management practices, such as applying sufficient base fertilizer and top dressing, can help strengthen the plants and improve their resistance to disease. Once the disease appears, irrigation should be carefully controlled to reduce soil moisture and avoid further damage to the plant's vascular system.
For chemical control, seeds can be soaked in a 30% formaldehyde solution diluted 300 times for three hours, followed by immediate rinsing. At the early stage of infection, several fungicides can be applied, including 75% chlorothalonil wettable powder diluted 600 times, 64% antivirus WP diluted 500 times, 80% Green Hunter Mancozeb 500 times, 40% Da Fu 500 times, 50% Dicofol 1500 times, 2% Polyoxin D at 30 ppm, and 50% Fast-King at 30-50 grams per 10 liters. One or more of these products can be used, alternating between them every 7–10 days for a total of three applications. To enhance the spray’s adhesion, adding 5–10 grams of neutral laundry detergent per 10 kg of solution is recommended. Consistent monitoring and timely application of these measures are essential for effective disease control.
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