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Onion Purple Spot Prevention and Control
Onion purple spot disease is a fungal infection caused by the pathogen *Onychococcus*, which can also affect garlic, leeks, and other allium crops. The fungus survives the winter in infected plant debris or on the host itself, and in the following growing season, it produces conidia that are spread by wind and rain. The disease tends to be more severe during warm, wet summers, as these conditions favor rapid infection and spread.
The symptoms of the disease typically appear on the leaves and stems of onions, and can also damage the bulb. Lesions usually begin at the tip of the leaf or along the stem, gradually spreading downward. Initially, the spots appear as small, pale white areas, which then expand into oval or spindle-shaped lesions. The size and color of the lesions vary depending on the host plant. Onions and leeks often show dark purple-brown spots that are relatively large, while garlic infections tend to have smaller, yellowish-brown lesions.
When humidity is high, the affected areas may develop a powdery, brown to dark brown mildew. This consists of conidial stalks and spores, often arranged in concentric rings. The infected tissue becomes fragile and breaks easily, leading to further damage.
To manage the disease, it's important to start with disease-free seeds. Seeds can be treated by soaking them in a 40% formalin solution diluted 300 times for 3 hours, followed by thorough rinsing. After planting, remove any infected plants and deeply till the soil to reduce overwintering inoculum. Choosing well-drained, nutrient-rich soil and applying sufficient base fertilizer, especially phosphorus, can help improve plant resistance.
During storage, bulbs should be kept in a cool, dry environment (around 0°C and 65% relative humidity), with good ventilation to prevent moisture buildup and continued disease development.
If the disease appears early, it can be controlled with fungicides such as 75% chlorothalonil wettable powder or 58% metalaxyl-MnZn WP diluted to 500 times. Spraying should be done every 7 to 10 days, and up to three applications may be necessary depending on the severity of the outbreak. Regular monitoring and timely intervention are key to preventing widespread damage.