Peanut leaf spot disease is a serious fungal infection that affects peanut crops, causing significant damage and yield loss. In severe cases, the disease can infect 100% of the plants in a field. During an outbreak, the disease can rapidly cause large numbers of leaves to die, leading to substantial economic losses for farmers.
Symptoms of the disease typically appear in two forms: focal spot and pepper spot. The focal spot type usually begins at the tip or edge of the leaf, forming wedge-shaped lesions that spread toward the petiole. As the disease progresses, the lesions turn from green to yellow and eventually brown. The edges often become dark brown with a yellow halo surrounding them. Early lesions are grayish-brown and may contain numerous small black spots. These lesions frequently overlap with those caused by other leaf spot diseases. The pepper spot type, on the other hand, produces small, irregularly shaped lesions, often less than 1 mm in diameter. These lesions are commonly found on the upper surface of the leaf. Under wet and rainy conditions before harvest, the disease can develop acutely, resulting in large, water-soaked, dark brown spots that quickly spread across the entire leaf, leading to wilting and damage to the petiole, stem, and even the fruit.
The pathogen responsible for peanut leaf spot is a fungus that survives the winter in infected plant debris through hyphae and spore structures. Spore production peaks during morning dew and the early stages of rainfall. The optimal temperature for disease development is between 25°C and 27°C, with relative humidity ranging from 70% to 74%. These conditions create an ideal environment for the fungus to thrive and spread.
To manage this disease, several control methods can be employed. Agricultural practices such as crop rotation, deep plowing, and proper disposal of infected residues can significantly reduce the risk of infection. Sowing seeds early, reducing plant density, and using plastic mulch also help in controlling the disease. For chemical control, fungicides like Bordeaux mixture (1:2:200 ratio of copper sulfate, lime, and water), 75% chlorothalonil diluted 500–800 times, or 70% mancozeb WP mixed 300–400 times are effective. It's recommended to apply these treatments when the disease incidence reaches 10–15% of the leaves, with a disease index of 3–5. If the disease develops further, spraying should be done every 10–15 days, with 2–3 applications required in severe cases. Regular monitoring and timely intervention are crucial for effective disease management.
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