The rapid diagnosis and control measures of cucumber downy mildew

Downy mildew is one of the most devastating diseases affecting cucumbers, both in greenhouses and open fields. Its severity can vary from year to year, depending on management practices and the resistance level of the cucumber varieties. The disease can occur throughout the entire growth cycle of the plant. Primarily affecting the leaves and stems, it spreads quickly, causing significant damage. On average, it can reduce yields by 20% to 30%, and in severe cases, crops may be completely destroyed within just a few days. After years of research and field testing, we have developed an effective and practical method for managing downy mildew. This approach is simple, cost-effective, and highly efficient, making it very popular among farmers. **1. Symptoms** **1.1 Typical Symptoms** Cucumber downy mildew typically begins on the lower leaves, particularly on the underside. The initial signs are small, water-soaked lesions that appear in a polygonal shape, confined by the leaf veins. As the disease progresses, these spots become more defined, and under high humidity, a black mold layer develops on the back of the leaves. The front side of the leaves may remain relatively unaffected at first. In later stages, gray or black molds appear on the leaf underside, while the upper surface shows irregular yellowish or chlorotic spots that eventually form distinct lesions. **1.2 Other Symptoms** Depending on the variety's resistance and environmental conditions, symptoms can vary significantly. In some cases, the entire leaf may show multiple yellow spots without visible mold. In others, round spots may appear, with no black mold on the underside. These variations make accurate identification challenging. However, there is a reliable method to quickly determine if the disease is downy mildew. **2. Rapid Identification Method** To identify downy mildew quickly, collect infected cucumber leaves and place them in a cup. Add a small amount of water to increase humidity, then seal the cup. Leave it at around 20°C overnight. If black mold appears on the back of the leaves, it is likely downy mildew. If a sticky liquid forms, it could be bacterial leaf spot. If yellow, sticky substances are present, it might be anthracnose. **3. Disease Conditions** Downy mildew is caused by a fungal pathogen that spreads through the air. High humidity is a critical factor in its development. The spores require moisture on the leaf surface to germinate and infect the plant. The optimal temperature range for disease occurrence is 16–24°C, with relative humidity above 85%. When humidity drops below 50–60%, the disease is less likely to develop. Winter is often the most vulnerable season for this disease. **4. Control Methods** **4.1 Selecting Resistant Varieties** Different cucumber varieties have varying levels of resistance. Choosing locally adapted, resistant varieties can significantly reduce the risk of infection. **4.2 Field Management** Use healthy seedlings and proper fertilization to strengthen plants. Plastic mulch and drip irrigation help control humidity and raise soil temperature. Non-drip plastic films can also help reduce greenhouse humidity and improve light penetration, which helps suppress disease. **4.3 Ecological Control** Ventilate greenhouses in the morning for about 30 minutes to lower humidity. Keep temperatures between 25–30°C and ensure humidity stays below 75%. In the afternoon, maintain temperatures around 20–25°C and humidity below 70%. If nighttime temperatures outside are above 12°C, you can leave the greenhouse open overnight. Proper ventilation is essential even on cloudy or rainy days to prevent prolonged leaf wetness. **4.4 Fumigation** Fumigation is an effective and time-saving method. Before any signs of disease, apply 15% epidemic smoke or 45% chlorothalonil at 250 grams per acre in the evening. Seal the greenhouse and distribute the fumigant evenly. Smoke once every 7 days, and repeat up to 2–3 times. At the early stage of infection, use 45% chlorothalonil aerosol (safe type) at 200–300 grams per acre, close the greenhouse, and fumigate overnight. Ventilate the next day and repeat every 7 days. **4.5 Spraying** For prevention, spray 80% mancozeb WP diluted 600 times every 7–10 days. Once the first signs of disease are detected, apply 72% DuPont Kocide 500 times, ensuring thorough coverage. Spray twice within three days. Rotate pesticides to avoid resistance. **4.6 Dust Application** Dusting is a low-humidity method that’s easy to apply. Use 5% chlorothalonil dust or 7% anti-mildew dust at 1 kg per acre. Apply in the evening using a duster, keep the greenhouse sealed overnight, and ventilate the next day. Repeat every 7 days, up to 4–5 times. Avoid spraying directly onto the plants; instead, spray over them so the dust falls naturally.

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