Mustard black spot disease is commonly observed during the rainy season and can significantly impact both the quality and yield of crops when left unchecked. Effective **pesticide** application at an early stage is crucial for managing this issue.
The pathogen responsible for the disease is *Alternaria brassicae*. Symptoms typically begin on the outer leaves, forming circular, brown to dark brown spots ranging from 4 to 6 mm in size. These lesions often display distinct concentric patterns and may be surrounded by a yellow halo. On stems and petioles, the lesions appear as longitudinal stripes, dark brown and slightly sunken. The pods and flower stalks may show irregular brown spots, and under moist conditions, black mold can develop on the lesions. In severe cases, the lesions merge, leading to leaf death or desiccation.
The pathogen overwinters in infected plant debris, seeds, or stored materials, and can also survive on plants through the winter. Spores are spread by wind and rain. The fungus thrives between 10°C and 35°C, with optimal growth around 17°C. High humidity, above 80%, is essential for disease development. If relative humidity drops below 75%, the disease tends to be mild or absent.
According to the **pesticide network**, the early growth phase of autumn-sown cabbage usually starts in late August to early September, with rapid growth occurring between September 19 and October 7. After October 10, growth generally stabilizes. The disease outbreak is not directly linked to total rainfall but rather to the number of rainy days. If there are more than four days of continuous rain, especially during wet periods, the disease is likely to spread rapidly. High humidity, heavy rainfall, and cooler temperatures are key factors in disease onset.
The incidence of the disease is influenced by cultivation practices. Early planting, high density, low-lying fields, poor management, water shortages, and nutrient deficiencies can weaken plant resistance and increase the likelihood of infection.
To manage mustard black spot disease:
1. **Use resistant varieties**: Such as Xiaoqingkou, Qingqing, Beijing New No. 1, Beijing New No. 5, Beijing 88, Zhengza No. 2, Luoyang Tokyo No. 3, Zhengbai No. 4, Jinqing No. 9, Jin No. 3, Shuangqing 156, and Shuangqing.
2. **Seed disinfection**: Plant seeds from disease-free sources, or soak them in 50°C warm water for 25 minutes, then transfer to cold water and dry before sowing. Alternatively, use 0.3% chlorhexidine wettable powder (0.3% of seed weight) or 0.4% acesulfame powder.
3. **Improve field management**: Implement crop rotation with non-cruciferous vegetables for at least two years. Avoid planting cabbage near cruciferous crops. Apply well-decomposed organic fertilizer and increase phosphorus and potassium levels. Remove and destroy infected plant parts promptly to prevent spread.
4. **Apply fungicides early**: Use 50% phorhin WP diluted 1000 times, 50% carbendazim WP diluted 800 times, 50% thiram double WP diluted 500 times, 40% sterilized Dan WP diluted 400 times, 64% anti-virus WP diluted 500 times, or 50% keeling WP diluted 1000 times. According to the **China Pesticide Network**, 70% mancozeb WP diluted 400 times is also recommended. Spray every 7 days, and repeat up to 3–4 times if necessary.
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