Downy mildew is one of the most severe diseases affecting grape production in Chongqing. It primarily affects the leaves, causing them to appear translucent at first, with yellowish-green, oil-like spots that gradually expand into irregular yellow to brown polygonal lesions. The size and severity of these lesions vary depending on the grape variety and the stage of infection. When humidity is high, a white mold layer appears on the underside of the leaves, and as the disease progresses, the affected areas turn brown, leading to leaf drying and falling off.
This disease typically peaks during the rainy season, from April to June each year. Preventing and managing downy mildew requires an integrated approach, focusing on early prevention and implementing multiple control strategies for effective results.
One effective method is to use rainproof cultivation techniques to reduce humidity within the vineyard. By covering grapevines with greenhouses or plastic scaffolding during the growing season, the plants can be protected from direct rainfall, which significantly reduces the spread of the disease.
Another key strategy is to implement the "high, wide, thin, vertical" training system, which promotes better air circulation and sunlight penetration. This involves planting grapes at lower densities—typically more than 3 meters between rows and over 1.5 meters between plants—with no more than 111 plants per acre. The trellis system should be designed with high branching points, ideally above 1.3 meters, and branches should be trained to spread out horizontally, allowing maximum exposure to sunlight and reducing the chances of new shoots developing.
Vineyard management plays a crucial role in preventing downy mildew. Ensuring proper drainage by digging deep ditches and regularly removing water helps keep the orchard dry. Pruning, tying vines, and weeding should be done promptly to maintain good airflow and light penetration. Applying organic fertilizers, especially in September to November, at a rate of over 2 tons per mu, along with balanced amounts of phosphorus, potassium, and micronutrients like boron, zinc, and calcium, supports plant health. Fruit bags should be used to protect clusters from infection, and after winter pruning, all pruned material should be removed and either buried or burned to prevent disease carryover.
Chemical control is also essential, particularly in the early stages of the disease. Spraying should begin in mid to late May and continue through early June, with additional applications in late August to early September. Recommended fungicides include 66.8% propineb wettable powder (such as Mold Grams) diluted 700–1000 times, 70% propineb wettable powder (Taisheng) at 400–600 times, 25% azoxystrobin suspension (Amisida) at 1000–2000 times, and 80% mancozeb wettable powder (Dasheng, Shan Desheng, etc.) at 600–800 times. These treatments are highly effective when applied correctly. It's important to avoid using powdered formulations in unsealed orchards. After harvest, a single spray of Bordeaux mixture at 200 times dilution can help protect the vines.
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