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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 related plants. The disease cycle begins when the fungus overwinters in infected plant debris or on the host itself. In the following growing season, spores are released through wind and rain, leading to new infections. Warm, humid conditions during summer typically favor the spread and severity of the disease.
The symptoms of the disease primarily appear on onion leaves and stems, though it can also damage the bulb. Lesions usually start at the leaf tips or along the stem and gradually move downward. Initially, they appear as small, white spots that expand into oval or spindle-shaped lesions. The color and size of these spots vary depending on the host plant. On onions and leeks, the lesions tend to be purplish-brown and larger, while on garlic, they are more yellowish-brown and smaller. Under high humidity, the lesions may develop a powdery, brown to dark brown coating composed of conidia and conidial stalks, often arranged in concentric rings. Affected tissues become weak and prone to breaking.
To manage the disease, it's important to use disease-free seeds and treat them with a 40% formalin solution diluted 300 times for 3 hours. After washing, remove any diseased parts and deeply till the soil to reduce inoculum levels. Choose well-drained fields with proper fertilization, and apply sufficient phosphorus-rich base fertilizer. During storage, keep bulbs in cool, dry conditions (around 0°C and 65% relative humidity) with good ventilation to prevent further development of the disease.
In the early stages of infection, fungicides such as 75% chlorothalonil wettable powder or 58% metalaxyl-MnZn wettable powder can be applied at a dilution of 500 times. Treatments should be repeated every 7 to 10 days, and up to three applications may be necessary for effective control. Regular monitoring and timely intervention are key to minimizing losses from this disease.