1. Injury Symptoms
The primary symptom of the disease is leaf blight, which typically begins on the lower leaves and progresses upward. Initially, small, round, light yellow lesions appear, often with a water-soaked appearance and distinct margins. As the disease advances, these lesions become irregular in shape. The affected areas turn pale yellow or grayish-yellow, and small black spots develop on them—these are the conidia of the pathogen. Lesions on the leaf petioles and stems are oval, darker in color, and slightly sunken, often surrounded by a yellow halo. Conidia tend to cluster around the lesions. In severe cases, the infected leaves may dry out and fall off, leading to significant yield loss.
2. Transmission Routes and Conditions
The pathogen can survive in the seed coat, on the surface of infected seeds, or on the roots of the host plant during winter. Mycelium inside the seed coat can remain viable for over one year, while the pathogen attached to the seed surface can survive for more than two years. Conidia on infected lesions can stay active for 8 to 11 months. Under favorable environmental conditions, the pathogen produces conidiospores that spread through wind, rain, human activity, animals, and agricultural tools. When conidia come into contact with moisture, they germinate and produce germ tubes that penetrate the plant through stomata or the epidermis. The disease thrives in cooler climates where temperatures range between 20°C and 25°C, especially in areas with heavy rainfall or high humidity. Large temperature fluctuations between day and night, along with prolonged dew periods, can accelerate disease development and lead to early outbreaks.
3. Control Methods
a. Use seeds from healthy, disease-free plants. It is recommended to use seeds within two years of harvest. Soak the seeds in warm water at 48–49°C for 30 minutes, stirring continuously, then transfer them to cool water and allow them to cool down before drying thoroughly before sowing. This helps reduce the risk of introducing the pathogen into the field.
b. Implement crop rotation for 2–3 years, apply base fertilizers, and provide timely top-dressing. Avoid flooding irrigation and ensure proper drainage after rainfall. Maintain good air circulation in the growing area to reduce humidity levels, which can inhibit pathogen growth and spread.
c. Apply fungicides such as 75% chlorothalonil wettable powder diluted at 600 times, or a Bordeaux mixture (1:0.5:200), or 50% sulfur suspension diluted at 200–300 times. Alternatively, use lime sulfuric acid copper sulfate powder (30 parts lime, 10 parts copper sulfate, and 10 parts sulfur) or other approved formulations. Spray the crops every 7–10 days, applying a total of 2–3 treatments to effectively manage the disease.
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