The main issue with the bagging technique is that it's not fully effective, as some grapes become rotten inside the bags. This problem typically occurs due to several key reasons:
First, the timing of bagging is often incorrect. Ideally, grapes should be bagged during the fruit expansion stage, not too early or too late. If bagging is done too late, some grapes may already be infected by pathogens, making it difficult for sprays to completely eliminate the infection. As a result, these infected grapes continue to grow inside the bags. According to an experiment, the bagging rate was only 2.5% at the time of fruit swelling, but increased to 17.8% after 20 days.
Second, the bagging method itself may be flawed. It's commonly believed that bagging should be completed within six days after spraying, but this isn't always accurate. In practice, it’s best to wait until the pesticide has dried before bagging. Ideally, the process should be done on the same day if there’s no rain or dew overnight. If conditions are favorable, bagging can still be done within two days. For large planting areas, it's practical to divide the fields into sections and plan the daily bagging based on labor availability and speed.
It's also important not to spray pesticides immediately after applying them if the liquid hasn’t dried. Doing so can lead to fruit rot. When bagging, avoid touching the fruit directly with your hands. Make sure to tightly seal the top of the bag to prevent rainwater from seeping in.
Third, improper pesticide application can also contribute to the problem. Spraying should not be done when there's dew, or during midday when the sun is strong and wind is blowing. The best time to spray is between 7 am and 10 am, when dew has cleared and sunlight is less intense. This allows the pesticide to work more effectively. Spraying should be even and thorough—don’t over-spray, but don’t miss any areas either. For example, the sides of trellises and the clusters should both be sprayed. Use high-quality products such as mancozeb, thiophanate-methyl, or DuPont Fuxing. Avoid counterfeit or low-quality pesticides. Also, use a fine nozzle on the sprayer to ensure even and uniform coverage.
Lastly, the quality of the paper bags plays a significant role. Properly used grape bags offer many benefits, including protection against diseases, pollution, birds, and pests, which helps maintain yield and quality. The market price of bagged grapes is usually 1 to 1.5 times higher than non-bagged ones, making it a popular choice. Always purchase standard, qualified wood pulp paper bags, even if they cost more—this ensures safety and effectiveness. Some low-quality bags made from inferior materials, such as recycled newspapers or previously used paper, can lead to mold and rot inside the bags. Therefore, investing in good-quality bags is essential for long-term success.
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