Grape downy mildew is a widespread disease found in almost all grape-growing regions across the country, with the most severe damage occurring in the coastal areas of Shandong. In other producing regions, the disease typically appears in years with higher rainfall. During epidemic years, infected leaves often scorch and dry prematurely, becoming curled and stunted, which significantly affects the plant's health and overall yield.
The main symptoms of the disease begin to appear between May and June, with the peak incidence occurring from August to September. The pathogen can affect all young aerial parts of the grapevine. When leaves are infected, they initially show small, semi-transparent, oily spots with unclear borders. These spots may merge into larger lesions, usually yellow to brown in color. Under high humidity or wet conditions, a white, powdery layer of sporangia forms on the underside of the lesions—this is the visible sign of the pathogen. As the disease progresses, the affected leaves turn brown, dry out, and fall off early.
New shoots, tendrils, clusters, and petioles also show similar symptoms. They start as translucent, oily spots that develop into irregular, yellow to brown sunken lesions. Wet conditions promote the formation of the white downy mildew layer on these areas as well. Infected tissues may stop growing, become deformed, or even die.
When young fruit is infected, the affected areas become soft, discolored, and sink. A white layer of downy mildew develops, and the fruit eventually falls off. If the infection occurs when the fruit is nearly mature, it turns brown, becomes soft, and quickly shrinks and drops. This not only reduces yield but also lowers the sugar content and overall quality of the grapes.
Although the disease is most common in autumn across most grape-producing regions, it can also occur earlier in the growing season during rainy or wet years. This early infestation can have a significant impact on the crop, making it crucial for growers to monitor weather conditions and implement timely control measures. Understanding the disease cycle and environmental factors that favor its spread is essential for effective management.
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