Cabbage Black Spot Control Technology
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In recent years, the occurrence of cabbage black spot disease has been increasing in vegetable-growing areas of Guangdong. This disease commonly affects varieties such as Chinese cabbage, regular cabbage, Shanghai green, and black leaf white, especially during autumn and winter. Outbreaks often occur during the transition between winter and spring, when the weather is overcast, rainy, warm, and humid. This has significantly impacted both the yield and quality of the crops.
Cabbage black spot primarily affects the leaves, petioles, and cotyledons. In the early stages, small chlorotic spots appear on the leaves, which gradually enlarge into brown spots with a diameter of 5 to 10 mm. These spots exhibit clear concentric rings, with edges ranging from light green to dark brown, and some may have a yellow halo. When humidity is high, a dark brown moldy layer forms on the affected areas. Under high temperature and humidity, the diseased parts may become perforated. In severe cases, the lesions merge into large patches, causing partial or complete leaf death, and eventually leading to the entire plant's leaves drying from the outside inward. Lesions on the petioles and stems are typically spindle-shaped and dark brown.
The disease is caused by the fungus *Streptomyces* species. The mycelium can remain hidden in seeds and infected fields. When conditions are favorable, it produces spores that spread through wind, rain, and agricultural tools, infecting plants via stomata or directly through the epidermis. Symptoms usually appear within 3 to 5 days after infection. The bacteria can grow and develop at temperatures ranging from 0°C to 35°C, with an optimal range of 17°C to 25°C, allowing the disease to persist throughout the year in Guangdong. Factors contributing to outbreaks include prolonged rainy weather, warm temperatures, dense planting, excessive watering, high field humidity, and low resistance in certain crop varieties.
To effectively control cabbage black spot, several techniques can be applied. First, select locally adapted, disease-resistant varieties. Rotate crops with non-cruciferous plants for 2–3 years, prepare the soil deeply, apply organic base fertilizers, and use balanced fertilizers combined with organic trace elements and compound nutrients to enhance plant resistance. Also, manage planting density and water supply appropriately.
Additional specific measures include: (1) Soaking seeds in 50°C warm water for 25 minutes before sowing, then cooling and re-sowing. Alternatively, treat seeds with 0.4% Bao Ning (60% wettable powder), Fome Double (50% wettable powder), or 0.3% acetaminophen (50% wettable powder). (2) During the mid-seedling stage—especially if lesions are observed—apply fungicides promptly. Recommended treatments include: 500–600 times dilution of Bao Ning; 600–800 times for Bai Kong Ning or chlorothalonil (40% suspension); 800 times for Calcium Chloride (64% wettable powder); 700–800 times for Propogon (70% wettable powder); 1500 times for Pentocaine; and 500 times for Green Cape (70% WP). Apply every 7 days for 3–4 times to ensure effective control.