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. The severity of the disease varies from year to year, depending on management practices and the variety of cucumber being grown. It can occur at any stage of the plant's growth cycle, making it a persistent threat. The primary damage occurs on the leaves and stems, with symptoms that appear quickly, spread rapidly, and cause significant losses. In many cases, the disease can reduce yields by 20% to 30%, and in severe situations, entire crops may be destroyed within just 3 to 5 days. After years of research and field trials, we have developed an effective, low-cost, and easy-to-apply method for managing downy mildew, which has been widely accepted by farmers due to its practicality and high success rate. **1. Symptoms** **1.1 Typical Symptoms** Cucumber downy mildew typically begins on the underside of the lower leaves. Early signs include small, water-soaked lesions that are confined by leaf veins, forming polygonal spots. Under high humidity conditions, a black mold layer develops on the affected areas. On the upper surface of the leaf, there may be little visible change initially. As the disease progresses, gray or black mold appears on the underside, and later, irregular yellowish spots may develop on the top side, eventually turning into distinct, polygonal lesions. **1.2 Other Symptoms** Depending on the variety’s resistance and environmental factors, the symptoms can vary significantly. In some cases, the entire leaf may be covered with yellow spots without any visible mold. In others, round spots may appear without the characteristic black mold layer on the back of the leaves. This variability makes identification more challenging, but there are reliable methods to quickly confirm the presence of downy mildew. **2. Rapid Identification Method** To identify downy mildew quickly, collect infected leaves and place them in a cup with a small amount of water to increase humidity. Seal the cup and keep it at around 20°C overnight. If the leaves show a black mold layer on the back, it is likely downy mildew. If a sticky liquid appears, it might be bacterial infections like bacterial leaf spot. If yellow, sticky substances are present, it could indicate anthracnose. **3. Disease Conditions** Downy mildew is caused by a fungal pathogen that spreads mainly through the air. High humidity is essential for the disease to develop, as spores need moisture on the leaf surface to germinate. The optimal temperature range for infection is between 16°C and 24°C, with relative humidity above 85%. When humidity drops below 50–60%, the disease is less likely to occur. Winter is often the most challenging season for this disease. **4. Control Methods** **4.1 Resistant Varieties** Choosing cucumber varieties with good resistance to downy mildew is a key preventive measure. Local adaptation and genetic resistance can significantly reduce disease pressure. **4.2 Field Management** Using strong seedlings, applying balanced fertilizers, and using mulch films can help improve plant health. Plastic film covering and drip irrigation under the film help lower humidity and raise soil temperature. Non-drip films offer additional benefits such as heat retention, light transmission, and disease suppression. **4.3 Ecological Control** Ventilating the greenhouse for 30 minutes in the morning helps control humidity. Maintain temperatures between 25°C and 30°C, with humidity below 75%. In the afternoon, keep temperatures between 20°C and 25°C, and humidity around 70%. If night temperatures outside are above 12°C, ventilation can be left open overnight. Proper ventilation on cloudy or rainy days also helps reduce leaf moisture and prevent disease development. **4.4 Fumigation** Smoke treatments are efficient and require minimal labor. Apply 15% epidemic smoke or 45% chlorothalonil at 250 grams per acre in the evening. Ensure the greenhouse is sealed and the smoke is evenly distributed. Repeat every 7 days, up to 2–3 times. At the early stages of infection, use 45% chlorothalonil aerosol (safe type) at 200–300 grams per acre, fumigate overnight, then ventilate the next day. **4.5 Spraying** For prevention, spray 80% mancozeb WP diluted 600 times every 7–10 days. Once the first signs of infection appear, spray 72% DuPont Kocide 500 times, ensuring thorough coverage. Apply twice within three days. Rotate pesticides to avoid resistance buildup. **4.6 Dusting** Dusting is a simple and effective method that does not increase indoor humidity. Use 5% chlorothalonil dust or 7% anti-mildew dust at 1 kg per acre. Apply in the evening with a duster, and leave the greenhouse sealed overnight. Repeat every 7 days, up to 4–5 times. Avoid spraying directly onto the plants—spray over the crop so the dust naturally settles on the foliage.

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