Onion downy mildew is a significant plant disease that primarily affects green onions and onions, but can also impact other Allium species such as garlic and leeks. This disease spreads rapidly under favorable conditions, leading to substantial crop losses. Timely application of **pesticides** is crucial in managing the outbreak effectively.
The disease typically begins with infection on the leaves and stems. Infected stems develop yellowish-white or yellow-brown spots, which are spindle-shaped or oval in appearance. A white mold may appear on these lesions, later turning pale yellow or dark purple. As the infection progresses, the middle and lower leaves begin to droop, while the false stem becomes brittle and deformed. If the bulb is infected, it can lead to systemic infection, resulting in stunted growth, twisted or deformed leaves, and the development of a thick layer of white mold when humidity is high.
The pathogen responsible for onion downy mildew is a fungus belonging to the genus *Peronospora*. According to the **pesticide network**, the pathogen survives in the soil through oospores, often found in diseased plant residues. The mycelium can also remain dormant within the bulbs during winter. In the following growing season, rain splashes the spores onto the leaves, initiating the first infection. High humidity promotes the production of sporangia, which spread via wind, causing secondary infections. Rain and insects may also contribute to the spread, though whether seeds can transmit the disease is still under investigation.
The disease thrives in cool, wet nights and warm, foggy days, with moist soil conditions being particularly conducive to its spread. To prevent this disease, several effective measures can be taken:
- **Use disease-free seeds**: Choose seeds from healthy plots and treat them by soaking in 50°C warm water for 25 minutes before sowing.
- **Improve field management**: Plant in well-drained, elevated areas with good air circulation. Apply balanced fertilizers, focusing on phosphorus and potassium. Avoid overwatering and promptly remove any infected plants.
- **Proper post-harvest practices**: After harvesting, ensure all diseased plant material is removed, collected, and deeply plowed into the soil to reduce the inoculum source.
For **pesticide control**, the **China pesticide network** recommends spraying at the early stages of infection, applying treatments every 7–10 days for 3–4 times. Commonly used fungicides include:
- 70% Ethyl Phosphite (Mn) WP diluted 600–800 times,
- 25% Metalaxyl suspension 500–1000 times,
- 50% Anke wettable powder at 2500 times,
- 70% Dry Moisturizing Agent at 500–700 times,
- 72.2% Mefenoxam 700–1000 times.
By implementing these integrated management strategies, farmers can significantly reduce the risk of onion downy mildew and protect their crops from severe damage.
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