Why does rotten fruit appear on grapes in summer?
Many fellow growers have reported that their grapes start developing rotten fruit as soon as summer arrives. What exactly is going on? Let's analyze this below.
1. Pathogen infection
The main diseases that generally cause grape rotten fruit include gray mold, anthracnose, white rot, dead-arm disease, and sour rot; in some special years, black rot also occurs.
2. Dense branches and leaves, poor ventilation and light penetration
When the foliage on the grape trellis is too dense and too many clusters are retained, ventilation and light penetration are poor, making pests and diseases more likely and causing rotten fruit.
3. Nutritional imbalance
When the vine is weak, nitrogen fertilizer is applied excessively, and trace nutrients such as potassium, calcium, and boron are lacking, or when too much fruit is set and the load is too heavy, the root system is placed under greater strain, the plant grows weakly, and disease resistance is reduced, causing water-pot disease, sunscald, fruit cracking, and other rotten-fruit situations to occur on a large scale. Reasonable application of fully decomposed farmyard manure and organic activated nutrient package fertilizers such as Jiamei Hongli, Yinglilai, and Neijiade Nao Baijin can enhance the plant's resistance to many pests and diseases and worsen the nutrition of piercing-sucking pests, whereas insufficient base fertilizer, excessive nitrogen fertilizer, insufficient phosphorus, potassium, calcium, and micronutrients, and waterlogged soil will all promote serious pest and disease occurrence and cause large-scale rotten fruit.
4. Problems during spraying
First, the spraying time is wrong. Do not spray when there is dew, when strong sunlight shines directly at noon, or when a strong wind is blowing. It is best to spray between 7 and 10 a.m., which avoids dew and strong sunlight and helps finish bagging the same day.
Second, spraying must be even; do not spray too much, and do not miss any spots. For trellised grapes, both sides must be sprayed, and for overhead trellises, both sides of the clusters must also be sprayed.
Third, use good pesticides, such as mancozeb, thiophanate-methyl, DuPont Fuxing, etc., and be very careful not to spray counterfeit products. When spraying, choose a fine swirl disc for the sprayer nozzle, which helps produce a fine and even spray.
5. Wrong bagging time and method
Relatively speaking, bagging grapes earlier is better, but too early is also not good; generally bag during the swelling stage. Grapes bagged late have already been infected by pathogens, and spraying cannot completely eliminate them, so they continue to multiply inside the bag. According to experiments, when bagged during the swelling stage, the grape rotten-fruit rate is only 2.5%, while when bagged 20 d (days) late, the rotten-fruit rate is 17.8%.
Some people believe that after spraying bagged grapes, bagging should be completed within 6 d (days). However, based on the author's many years of practical experience, after spraying grapes, once the spray liquid has dried, bagging should be done promptly, preferably completed the same day. If there is no rain and no dew at night, completing it within 2 d (days) is also acceptable. If the grape area is large, it can be divided into blocks; based on labor, bagging speed, etc., calculate the number of bags to be applied in 1 d (day), and spray only as much as can be bagged. Never bag immediately after spraying before the liquid has dried, otherwise rotten fruit is likely. Also note that when bagging, try to avoid touching the berries with your hands, and be sure to tie the upper opening tightly to prevent rainwater leakage.