Occurrence and Control of Grape Downy Mildew in Southern China
Grape downy mildew is one of the most significant diseases affecting grape cultivation in southern China. In recent years, with the increasing use of greenhouse and rain-sheltered cultivation systems, the conditions have become more favorable for the development of this disease. The disease tends to occur rapidly after periods of exposure, leading to weakened vine vigor, reduced fruit quality, early leaf fall, and impaired flower bud formation. These factors can significantly lower yields in the following season, making it a critical issue for grape growers. The initial symptoms on infected leaves appear as translucent, oily spots, ranging from pale yellow to reddish-brown. Within 4 to 5 days, a dense layer of white, powdery growth forms on the underside of the leaves. As the disease progresses, the leaves become dehydrated, scorched, and eventually fall off. The leaf tips may become swollen and curled, and the white growth turns brown before the leaves die. Young fruits also show discoloration and are covered with a layer of mildew. The pathogen causing downy mildew belongs to the genus *Plasmopara*, which is classified under the class Oomycetes. It is an obligate parasite that reproduces within grape tissue cells. The primary survival stage is the oospore, found mainly in diseased leaves and capable of surviving for 1–2 years. In spring, these oospores germinate, forming zoosporangia that release motile zoospores. These spores are splashed by rain onto nearby leaves, where they penetrate through stomata and lenticels. The hyphae then grow between cells, forming specialized structures to extract nutrients. Eventually, sporangia develop and are released into the air, spreading the infection further. Environmental conditions play a crucial role in disease development. Cool, moist climates are ideal for the spread of the disease. Oospores germinate best at temperatures between 13–25°C, requiring adequate moisture. Sporangia germinate at 5–10–15–27°C, and mycelium grows well between 7–29°C. High humidity (95–100%) is essential for sporangia formation. The optimal relative humidity for infection is 70–80%. Under favorable conditions, the incubation period lasts 4–7 days, though resistant varieties may extend this to 20 days. Several factors contribute to disease occurrence. First, weather conditions such as cloudy, rainy days with little wind in spring and autumn create a perfect environment for the disease. Rainfall is the main driver of epidemics. Second, poor orchard management, including excessive plant density, low trellis height, and crowded foliage, reduces air circulation and light penetration, creating a humid microclimate conducive to disease. Third, grape varieties vary in resistance. European and American species tend to be more resistant than Eurasian types. Resistant varieties often have fewer and smaller stomata, along with white deposits around them, which help reduce infection. Prevention and control strategies focus on integrated approaches combining cultural practices and chemical treatments. First, during winter, it's important to clean up the orchard by removing fallen leaves, pruning diseased branches, and burning them. Deep plowing helps bury pathogens in the soil. A lime sulfur solution (Baume 3–5 degrees) mixed with sodium pentachlorophenol is applied during winter pruning to eliminate overwintering inoculum. Another spray is done during the bloom period to reduce initial infections. Second, improving the orchard’s microclimate is essential. This includes spacing plants appropriately, orienting trellises parallel to wind directions, avoiding overly low scaffolding, and reducing nitrogen fertilizer while increasing phosphorus, potassium, and lime. Proper irrigation and drainage, along with maintaining open canopies, help reduce humidity and improve air circulation. Third, chemical control is necessary when the disease appears. In protected vineyards, a Bordeaux mixture (1:0.7:200) is applied before uncovering the film and repeated every 10 days for three times. Early-stage infections can be managed with fungicides such as 20% metalaxyl, 40% methotrexate, 40% fosetyl-aluminum, or 50% wettable methoxyphenoxy. These products should be used according to recommended dilution rates. Finally, planting disease-resistant varieties in endemic areas is a long-term strategy to reduce the impact of downy mildew. Educating farmers on disease identification and management practices is also vital for effective control.Smc Standard Cylinder,Standard Air Cylinders,Airtac Standard Cylinder,Standard Pneumatic Air Cylinder
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