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How to Control Lettuce Brown Spot
Lettuce brown spot is a fungal disease caused by the infection of *Cercospora lentis* or *Cercospora maculata*, both belonging to the subgenus *Demenomyces*. This disease primarily affects the leaves, leading to visible symptoms that can significantly reduce crop quality and yield. The first symptom observed is water-soaked spots on the leaf surface, which later develop into circular or irregularly shaped lesions. These lesions range in color from brown to dark gray and typically measure between 2 to 10 mm in diameter. The second type of lesion appears as deep brown spots with irregular edges and a water-soaked halo around the perimeter. Under high humidity conditions, dark gray mold may develop on the affected areas, and severe infections can cause the lesions to merge, resulting in entire leaves turning brown and drying out.
The first type of brown spot is mainly caused by *Cercospora maculata*, while the second is often attributed to *Cercospora verticillata*. The pathogen survives the winter in the form of mycelia and conidial masses on infected plant debris. Spores are spread through wind and rain splash, leading to initial infections and subsequent re-infections. Factors such as prolonged cloudy weather, poor plant growth, or excessive use of nitrogen-based fertilizers can accelerate disease development.
To manage lettuce brown spot effectively, several control measures are recommended. First, it's essential to maintain good field hygiene by removing and destroying infected plant material, ideally by burning it. Second, proper fertilization practices should be followed—using balanced fertilizer formulas that include increased amounts of organic matter, phosphorus, and potassium while avoiding overuse of nitrogen. This helps promote strong plant growth and improves resistance to disease. Third, chemical control options are available at the early stages of infection. For example, spraying with 40% polyoxon suspension diluted at 500 times, 75% chlorothalonil wettable powder at 1000 times, 70% thiophanate-methyl wettable powder at 1000 times, or 50% carbendazim wettable powder at 1200–1500 times can be effective. Other options include 60% ethyl aluminum phosphite wettable powder at 500 times. Applications should be repeated every 10 days, with a total of 2–3 treatments. It is important to stop all chemical treatments 5–7 days before harvest to ensure food safety.