Downy mildew is a serious and widespread disease affecting onions, causing significant economic losses due to its increasing prevalence in recent years. Effective **pesticide** application is crucial for managing the disease. The infection can be categorized into two main types based on environmental conditions and plant growth stages: the first (systemic infection) and the second (local infection). The first infection typically occurs during the autumn seedbed or early spring planting. The fungal hyphae develop during winter, and symptoms become visible in the following spring. Infected seedlings show poor growth, with dull, distorted leaves. As temperatures rise in spring, the lesions spread rapidly and may damage new foliage. Under moist conditions, white or purple mildew appears on the lesions, which can lead to re-infection as the disease spreads from the center outward.
The disease affects both the leaf buds and seed-producing plants. There are five distinct symptom types: 1) the affected leaf surface is covered with a lavender velvet-like coating; 2) yellowish-green, elongated oval or elliptical lesions appear, with white or gray-purple mildew on the surface, which washes away after rain, eventually turning the leaves gray and dying; 3) some lesions appear yellow without any mold; 4) small, oval, slightly sunken, gray-white spots about 2–3 mm wide; 5) under dry conditions, the lesions appear as small, grayish-white spots. In later stages, saprophytic bacteria such as gray mold and black spot often infect the lesions, producing gray or black mildew.
Once the bulb is infected, the outer layers become soft and wrinkled, sometimes leading to soft rot. The disease is characterized by large, long oval, yellowish-white lesions that turn grayish-white after rain. When wet, the lesions are covered with sparse white mold, while gray-purple mold develops at higher temperatures.
The disease spreads mainly through oospores in the soil and infected plant residues. In autumn, the mycelium from infected seedlings or bulbs can cause systemic infections. In southern regions, where the climate is warmer, the pathogen can survive over winter in diseased plants. Sporangia then grow on lesions and are carried by wind and rain to infect new plants through stomata. The disease tends to be more severe in cold, rainy, or foggy weather. It is also more prevalent in heavy soils, low-lying areas, and fields with excessive planting density.
Control measures include crop rotation for 2–3 years and thorough removal and burning of infected plant debris. Selecting healthy, vigorous seedlings and eliminating diseased ones before planting is essential. Proper spacing, controlled watering, and improved drainage during the rainy season can help reduce the risk. For early-stage treatment, fungicides like 75% chlorothalonil WP (diluted 600 times), 58% metalaxyl-manganese zinc, 70% mancozeb wettable powder, or 60% fosetyl-aluminum wettable powder (diluted 500 times) can be used. Alternatively, a 1:1:20 Bordeaux mixture with 5–10g of neutral detergent per 10kg of solution can be applied as a spreader. Spraying should be done every 5–7 days, with 2–3 applications for effective control. Additional treatments can be applied depending on the severity of the outbreak.
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