1. Injury Symptoms
One of the primary symptoms of this disease is leaf blight, which typically starts from the lower parts of the plant and progresses upward. Initially, small, light yellow lesions appear, often with a water-soaked appearance and distinct edges. As the disease progresses, these lesions become irregular in shape. The color of the spots changes from grayish-yellow to pale yellow, and small black spots form on the surface—these are conidia of the pathogen. Lesions on the leaf petioles and stems are oval, darker in color, and slightly sunken. A yellow halo is commonly seen around the affected areas. Conidia tend to cluster around the lesions. In severe cases, the infected leaves may dry out and die.
2. Transmission Routes and Conditions
The pathogen can survive in the seed coat, attached to seeds, or overwintering on the roots of infected plants. The mycelium within the seed coat can remain viable for over one year, while the pathogen on the seed surface can survive for more than two years. Conidia on diseased tissues can persist for 8 to 11 months. Under favorable environmental conditions, the pathogen produces conidiospores, which are spread by wind, rain, human activity, animals, and farm tools. When conidia germinate in the presence of moisture, they produce germ tubes that invade through stomata or directly through the plant's epidermis. This disease thrives in cooler climates, particularly when temperatures range between 20°C and 25°C, and when there is heavy rainfall or high humidity. Large temperature fluctuations between day and night, along with prolonged condensation at night, can lead to rapid disease development and severe outbreaks.
3. Control Methods
a. Use seeds obtained from healthy, disease-free plants. It is recommended to use seeds within two years of harvest. Before sowing, soak the seeds in warm water at 48–49°C for 30 minutes, stirring continuously. Afterward, transfer them to cool water to stop the heating process and allow them to dry thoroughly before planting.
b. Implement crop rotation for 2 to 3 years. Apply sufficient base fertilizer and ensure timely top-dressing. Avoid flooding irrigation and drain excess water after rainfall. Maintain good air circulation in the growing area by ensuring proper ventilation.
c. Apply fungicides such as 75% chlorothalonil wettable powder diluted at 600 times, or a 1:0.5:200 Bordeaux mixture. Alternatively, use 50% sulfur suspension diluted 200–300 times, or a lime sulfuric acid copper sulfate powder (30 parts lime, 10 parts copper sulfate, 10 parts sulfur). Spray the solution every 7–10 days, applying it 2–3 times for effective control.
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