Control of cucumber downy mildew in winter greenhouse
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Cucumber downy mildew is a highly contagious and rapidly developing disease that poses a serious threat to greenhouse cucumber production. It is one of the most critical challenges faced by growers, often leading to significant yield losses if not managed properly. Effective control requires an integrated approach that combines the use of resistant varieties, ecological management, and chemical protection.
The level of resistance varies widely among cucumber varieties. Generally, early-maturing types tend to be more susceptible compared to late-maturing ones. For greenhouse cultivation, varieties such as Jinyan No. 2, 4, and 6; Jinza No. 1 and 2; and Jinchun No. 2 and 3 are recommended for their better performance in terms of both growth and disease resistance. However, it's important to monitor and manage other diseases, like wilt, when planting these resistant varieties.
Ecological control plays a key role in preventing the spread of downy mildew. By carefully managing temperature and humidity levels within the greenhouse, it's possible to create an environment that is unfavorable for pathogen growth while supporting healthy plant development.
Temperature and humidity should be managed in four distinct phases throughout the day:
1. After sunrise, raise the greenhouse temperature to 20–30°C and dehumidify when it exceeds 30°C. Maintain relative humidity between 60% and 70%, allowing the temperature to reach 30°C for optimal photosynthesis and disease prevention.
2. In the afternoon, keep the temperature at 20–25°C to support nutrient transport and metabolism. At this time, reduce humidity to around 60% to limit pathogen activity.
3. After sunset, insulate the greenhouse to prevent rapid temperature drops and sudden increases in humidity. Keep the temperature between 15–20°C before midnight and maintain humidity below 85%.
4. During the night, as temperature decreases, humidity rises. If it exceeds 90%, lower the temperature to 10–13°C to suppress disease development and reduce leaf moisture.
Watering should be done on sunny days, preferably in the morning, using drip irrigation under mulch. Avoid watering during cloudy or rainy periods. After watering, close the greenhouse to warm up, and once the temperature reaches 30°C, ventilate the area.
When severe outbreaks occur, high-temperature fumigation can be effective. Water the plants the day before, then raise the greenhouse temperature to 45°C at noon. After one hour, gradually release the air once the temperature drops to 25°C. Repeat this process every 10 days, up to 2–3 times, to effectively control the disease.
Proper cultivation practices also help. Use disease-free seedlings, select well-drained plots, apply sufficient base fertilizer, and avoid excessive nitrogen. Foliar sprays of 1% urea can enhance plant resistance. Limit watering before flowering and increase soil temperature through frequent cultivation.
Chemical control methods include dusting, smoking, and spraying. Dusting with 5% chlorothalonil powder at 1000g per 667m² is effective, applied evenly using a duster. Spraying should be done every 8–10 days, especially during dry conditions.
Smoking with 20% chlorothalonil smoke tablets (250g per 667m²) is ideal when the first signs of disease appear. Apply it in the evening after sealing the greenhouse, and ventilate the next morning. This method can be repeated every 7 days, depending on the severity.
Spraying with a combination of protective and systemic fungicides, such as 72% DuPont Kocide at 600x, 75% chlorothalonil at 600x, and 64% virus inhibitor at 400x, can significantly improve control. Spray every 5–7 days, with 2–3 applications as needed.
By combining these strategies, growers can effectively manage cucumber downy mildew and ensure healthier, more productive crops.