Lettuce is one of the earliest and most popular vegetables cultivated in spring. Due to its relatively low input requirements, the area under cultivation has been increasing steadily over the years. However, as planting continues, the incidence of pests and diseases has also risen. Among these, downy mildew is one of the most serious and widespread diseases affecting lettuce. This disease causes significant damage to the leaves, stunts plant growth, reduces product quality, and ultimately lowers economic returns for farmers.
**1. Symptoms of Downy Mildew**
The symptoms of lettuce downy mildew primarily affect the leaves. In the early stages, small, yellowish, circular or polygonal spots appear on the lower leaves. A white, mold-like layer develops on the underside of the leaves and gradually spreads. As the disease progresses, the lesions turn brown and may merge together. In severe cases, the entire leaf turns yellow, dries up, and eventually becomes dark brown. The plants may become completely wilted and rot, leading to significant yield losses.
**2. Disease Pathogenesis**
The causal agents of lettuce downy mildew are oomycetes belonging to the genera *Peronospora* and *Bremia lactucae*. These pathogens produce oospores that can survive in the soil or within infected plant debris. They can also persist in the mycelium present in seeds or on winter-sown lettuce plants, serving as a primary source of infection for the next growing season. The spores are spread by wind, rain, and insects. Once they come into contact with the host, they typically invade through the epidermis or stomata, causing repeated infections. The disease development is strongly influenced by environmental conditions, particularly temperature and humidity. Sporangia germinate best at temperatures between 6°C and 10°C, while optimal growth occurs at 15°C to 17°C. High humidity (over 85%) and prolonged dew periods further promote disease development. Overcrowded planting, excessive nitrogen fertilizer use, and poor air circulation also contribute to more severe outbreaks.
**3. Control Methods**
**3.1 Crop Rotation**
To reduce disease pressure, it is recommended to rotate lettuce with non-Asteraceae crops for at least 2 to 3 years. Infected fields should be thoroughly cleaned after harvest, removing all diseased plant material to minimize the pathogen’s survival in the field.
**3.2 Cultivation Practices**
Proper spacing is essential to ensure good air circulation and prevent excessive humidity inside the plant canopy. In greenhouse settings, proper management of irrigation—especially under mulch—is crucial. Raised bed cultivation, combined with the use of organic and balanced fertilizers (including nitrogen, phosphorus, and potassium), can enhance plant resistance and reduce susceptibility to disease.
**3.3 Chemical Control**
Before sowing, seeds can be treated with fungicides such as 0.3% to 0.4% Funginex seed dressing or 0.2% to 0.3% metalaxyl seed treatment to reduce the risk of infection from contaminated seeds. During the early stages of the disease, timely application of fungicides is essential. Options include 75% Miconazole diluted at 500 times, 72% mancozeb at 600 times, 64% mefenoxam at 500 times, or 69% dimethomorph at 700 times. Spraying should be repeated every 5–7 days, possibly up to two or three times. For indoor or greenhouse production, using 45% chlorothalonil smoke agents at 350g per 100m² can effectively control the disease.
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