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How to Control Lettuce Brown Spot
Lettuce brown spot is a fungal disease caused by the infection of *Cercospora lentis*, *Cercospora maculata*, or species within the subgenus *Demenomyces*. This disease primarily affects the leaves, causing visible damage that can significantly reduce crop yield and quality. The symptoms typically appear in two distinct forms. Initially, small water-soaked spots develop on the leaf surfaces. As the disease progresses, these spots expand into circular or irregularly shaped lesions, ranging from 2 to 10 mm in diameter, with a brown to dark gray coloration. In some cases, the lesions may appear as deep brown spots with irregular edges and a water-soaked halo around them. When humidity is high, a dark gray mold often develops on the affected areas, and severe infections can lead to merging lesions, resulting in the entire leaf turning brown and drying out.
The first type of symptom is usually associated with *Cercospora maculata*, while the second form is commonly caused by *Cercospora verticillata* or similar pathogens. These fungi survive the winter in the form of mycelium and conidial masses on infected plant debris. The spores are spread through wind and rain splash, leading to new infections. Factors such as prolonged rainy weather, poor plant growth, or excessive nitrogen fertilizer application can worsen the disease.
To manage lettuce brown spot effectively, several control measures should be implemented:
1. **Sanitation**: Remove and destroy infected plant material promptly. Burning the diseased parts can help prevent the spread of the pathogen.
2. **Fertilization Management**: Apply balanced fertilizers using formula-based techniques. Increase the use of organic matter, phosphorus, and potassium while avoiding excessive nitrogen. This helps promote strong plant growth and enhances resistance to diseases.
3. **Chemical Control**: Early intervention is key. At the first sign of infection, apply fungicides such as 40% polyoxon (500 times dilution), 75% chlorothalonil (1000 times), 70% thiophanate-methyl (1000 times), 50% mancozeb (1200–1500 times), or 60% ethephon (500 times). Spray every 10 days for 2–3 applications. Ensure to stop spraying 5–7 days before harvest to meet safety standards.
By combining cultural, preventive, and chemical methods, farmers can effectively manage lettuce brown spot and protect their crops from significant losses.