Occurrence and Control of Grape Downy Mildew in Southern China
Grape downy mildew is one of the most significant diseases affecting vineyards in southern China. In recent years, with the increasing use of greenhouse cultivation and rain-shelter systems, the conditions have become more favorable for the disease to develop rapidly after periods of exposure. The infection weakens the plant’s vitality, affects fruit quality, causes premature leaf drop, and hinders flower bud formation, ultimately leading to lower yields the following year. This makes it a critical issue that grape growers cannot ignore. The symptoms of the disease begin as translucent, oily spots on the leaves, which may range from pale yellow to reddish-brown. Within 4 to 5 days, a dense layer of white mildew appears on the underside of the affected areas. As the disease progresses, the leaves become dehydrated, scorched, and eventually fall off. The tips of the shoots may become swollen and curved, and the white mildew turns brown before the tissue dies. Similarly, young fruits may show discoloration and a grayish coating on their surface. The pathogen causing downy mildew belongs to the genus *Peronospora*, which is part of the class Oomycetes. It is an obligate parasite that develops within the host plant's cells. The primary source of inoculum is oospores found in infected tissues, mainly leaves, and these can survive for up to 1–2 years. In the following spring, oospores germinate within 1–2 months, producing zoosporangia. These release motile zoospores that are splashed onto nearby leaves by rain. Once they reach the host, the zoospores germinate and penetrate through stomata and lenticels. The hyphae then grow between the cells, forming appressoria that draw nutrients from the host. Eventually, sporangia form on the surface and are released into the air, spreading to new tissues and continuing the cycle. Infected leaves and other tissues combine to form oospores, which overwinter and start the infection cycle again. Environmental conditions play a crucial role in disease development. Cool and humid climates are highly favorable for the spread of the disease. The optimal temperature range for oospore germination is 13–25°C, with sufficient moisture required. Sporangium germination occurs at temperatures between 5–10–15–27°C, also requiring free water. Mycelial growth thrives between 7–29°C, while sporangium formation requires 95–100% relative humidity. The pathogen needs 70–80% humidity to infect plants. Under ideal conditions, the incubation period lasts 4–7 days, but this varies depending on the variety’s resistance. Some resistant varieties can remain asymptomatic for up to 20 days. Several factors influence the occurrence of the disease. First, weather conditions: downy mildew thrives in cool, wet, and cloudy environments, especially during spring and autumn when there is little wind and frequent rainfall. Rainfall is the main driver of disease outbreaks. Second, orchard management: poor ventilation, dense planting, low trellises, and overcrowded branches reduce air circulation and light penetration, creating a favorable environment for the pathogen. Third, variety selection: European and American grape varieties generally exhibit stronger resistance compared to Eurasian types. Resistant varieties often have fewer and smaller stomata, along with white deposits around them, which help prevent infection. Prevention and control strategies should focus on integrated approaches combining cultural practices and chemical treatments. First, sanitation is essential. During winter, remove and burn fallen leaves and diseased branches to eliminate overwintering inoculum. Deep plowing the soil helps bury pathogens. A lime sulfur solution (Baume 3–5 degrees) mixed with sodium pentachlorophenol (0.5%) can be applied during pruning to reduce initial infections. Reapplication during the bloom stage further enhances protection. Second, improve the microclimate by adjusting vine spacing, orienting trellises with the wind direction, and avoiding overly low structures. Reduce nitrogen fertilizer and increase phosphorus, potassium, and lime to strengthen plant resistance. Keep foliage from becoming too dense and raise the lower wires of the support system. Ensure proper irrigation and drainage to maintain optimal moisture levels. Third, chemical control is necessary in high-risk areas. In protected vineyards, a Bordeaux mixture (1:0.7:200) should be sprayed one day before removing the cover, followed by applications every 10 days for three times. For early-stage infections, fungicides such as 20% metalaxyl, 40% methotrexate (400–600 times), 40% bisphosphonate (200–300 times), or 50% wettable methoprene (400–500 times) can be used effectively. Finally, promoting the use of disease-resistant varieties in endemic regions is a long-term and sustainable strategy. Educating farmers about the importance of resistant cultivars can significantly reduce the impact of downy mildew on grape production.Pressure Control Switch For Air Compressor,Advanced Pressure Control Valve,Air Compressor Pressure Control,Air Filter Regulator For Air Compressor
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