What to Do About Grape Flower Drop and Fruit Drop? Causes and Pesticide Control Methods
The shedding of flower buds one week before grape flowering and of the ovary after flowering is called flower drop and fruit drop; when the rate of flower drop and fruit drop exceeds 80%, it is called flower drop and fruit drop disease. So how do you prevent grape flower drop and fruit drop? What are the causes? What pesticide to spray? What pesticide to spray?
How to prevent grape flower drop and fruit drop?
1. Choose superior varieties. Choose disease-resistant grape varieties with relatively well-developed floral organs; for female-flower varieties, pollinator trees must be reasonably arranged.
2. Supplement tree nutrition. Most of the grape's nutrient supply is completed in the early growth stage. The best time to supplement nutrients is from after harvest to before winter protection. Generally, early and mid-season grapes are fertilized after harvest, and late-season grapes before harvest, at the latest in early October. Human manure and urine, livestock and poultry manure, compost, and barnyard manure are the main materials, applied together as base fertilizer with 2/3 of the total nitrogen fertilizer amount and half of the total phosphorus and potassium fertilizer amounts.
3. Apply nitrogen fertilizer reasonably. The internal condition of the tree that causes flower drop and fruit drop is a high water-soluble nitrogen content and low carbohydrate content in the new shoots at flowering, which induces vigorous growth of new shoots. Therefore, nitrogen fertilizer should be applied reasonably; before flowering, do not apply nitrogen fertilizer (as much as possible) and apply base fertilizer early. Grapes like fertilizer and water and require more organic fertilizer to strengthen tree vigor, but some varieties such as Kyoho are very sensitive to nitrogen. Once there is too much nitrogen fertilizer, especially a large amount of nitrogen topdressing before flowering, it easily leads to excessive growth of shoots and vines, causing flower drop and fruit drop. Therefore, do not apply nitrogen fertilizer before flowering; wait until the young fruit has set and after autumn harvest before applying nitrogen fertilizer. Spring topdressing should be applied early before germination. Therefore, topdressing should grasp 3 key periods: before bud break, at the end of physiological fruit drop, and during the berry coloring stage. The first two applications should focus on nitrogen, phosphorus, and potassium, and the third on phosphorus and potassium fertilizers. To prevent excessive growth of fruiting branches after germination, germination fertilizer can be omitted under conditions of sufficient base fertilizer.
From 3-5 days before flowering to the flowering period, control irrigation. During the flowering period, avoid drought or heavy irrigation, otherwise it easily causes flower drop and fruit drop. After flowering, irrigation should be even more reasonable.
4. Spray boron on the leaves. Boron can promote pollen germination and pollen tube elongation, help complete pollination and fertilization, and enable the corolla to fall off normally, thereby increasing the fruit-setting rate. Spray 0.3% boric acid solution once each at the bud stage, early flowering stage, and full flowering stage. If 0.3% potassium dihydrogen phosphate is also sprayed, the effect is better. However, to prevent phytotoxicity, an equal weight of lime can be added to the boric acid. For severe boron deficiency, before grape germination in early spring, mix 1 kg of borax with 25 kg of fine soil and apply it around the plants.
5. Scientific pruning. An unreasonable pruning amount will cause shoots to grow excessively or weakly, which is also one of the causes of flower drop and fruit drop. Scientifically thin flowers and fruits, reasonably retain branches and fruits, reasonably load the vine, and maintain stable tree vigor. Only by following the principles of light pruning for strong branches, heavy pruning for weak branches, moderating strong vigor, supporting weak vigor, and maintaining moderate vigor can flower drop and fruit drop be effectively prevented.
(1) Bud rubbing. The principle is to rub off heavily for weak tree vigor and lightly for strong tree vigor. Do it in two rounds. The first should be as early as possible, and can be done after the buds swell; the second is done at leaf expansion, removing double buds, small buds, deformed buds, and diseased buds, and retaining round, plump buds.
(2) Shoot thinning. The principle is to remove small ones and keep large ones, remove weak ones and keep strong ones. Do it in two rounds. The first is done when new shoots grow to about 10 cm, and the second when new shoots grow to about 20 cm. Shoots should also be thinned according to the space size of the trellis surface and the growth vigor of the main and side canes and fruiting mother branches. For hedge trellis cultivation, leave 1 new shoot about every 10 cm; for pergola cultivation, leave about 20 new shoots per sq m. Vigorously growing main and side branches and fruiting branch rings may appropriately retain more fruiting branches; thick and strong fruiting mother branches may retain 2-3 fruiting branches each, moderate ones may retain 1-2 fruiting branches, and weak ones retain only 1 fruiting branch or have the inflorescence pinched off and changed to a vegetative branch.
(3) Thin and adjust inflorescences. According to the load and new shoot growth, promptly thin out excess inflorescences. Strong branches may retain two inflorescences, ordinary fruiting branches retain one inflorescence, and weak branches retain none. From 7-10 days before flowering, cut off 1/4 or 1/5 of the tip of the retained inflorescence, and also cut off the secondary clusters and the spikelets on the shoulder of the cluster, keeping 10-12 spikelets in the middle of the inflorescence.
(4) Pinching. Pinching should be moderate. For thick and strong fruiting branches, strong pinching gives a high fruit-setting rate and good fruit quality; for medium fruiting branches, weak pinching is best. From 7-10 days before flowering, pinch the tips of overly vigorous new shoots; for weak development branches, pinching can be appropriately delayed. From 5-6 days before flowering, pinch the fruiting branches, and it is suitable to retain 5-7 functional leaves above the inflorescence. While pinching, tendrils should be removed and secondary shoots rubbed off. All secondary shoots below the inflorescence should be removed; for those above the inflorescence, except that the top secondary shoot is pinched leaving 5-7 leaves, other secondary shoots are repeatedly pinched leaving 1-2 leaves. Pinching should be done at leaves about 1/3 the size of mature leaves. For fruiting branches with strong growth vigor, twisting the shoot above the inflorescence before flowering can increase the fruit-setting rate by about 20%.
(5) Cane girdling. The girdling time is generally best 5-6 days before flowering. The girdling site should preferably be selected 3 cm in front of where the fruiting branch bears the fruit cluster or at the previous internode. Either a double-blade girdling knife or a budding knife can be used. The width of the cut should be 2-3 mm, reaching the xylem. After removing the bark, immediately bind it tightly with clean plastic film strips. Girdling is only suitable for vigorously growing plants and branches, and is limited to use on fruiting branches; it should not be done on the main cane.
6. Spray regulators. This point is mainly adopted for tetraploid varieties such as Kyoho that are prone to flower drop and fruit drop; diploid varieties are not suitable. Spraying 300-fold B9 or 250-300-fold PBO solution on the leaves once 10 days before flowering can significantly increase the fruit-setting rate.
7. Prevent physiological fruit drop. Physiological fruit drop is natural fruit thinning through competition for nutrients and can improve grape fruit quality. However, to prevent excessive shedding, the excessive growth of grape new shoots should be controlled in time. At this time, water less, avoid nitrogen fertilizer, and apply phosphorus and potassium fertilizers.
What are the causes of grape flower drop and fruit drop?
1. Variety characteristics. For example, the Kyoho variety has a genetically incomplete ovule development characteristic, with an abnormal ovule rate as high as 48%, and its filaments curl backward, which is unfavorable for pollination. Some varieties have defective female flower structures, some have degenerated stamens, and unreasonable pollinator tree arrangement for female-flower varieties can all cause flower drop and fruit drop.
2. Insufficient stored nutrition in the tree. Insufficient stored nutrition in the tree during the early growth stage of grapes increases incomplete flowering, causes poor ovule development and low pollen germination rate, resulting in fruit drop. Because grapes have a large flower volume, their consumption of water and nutrients is also very large. When trace elements, especially potassium, phosphorus, boron, and other elements, are lacking during the grape flowering period, the fertilization ability of the flowers declines, leading to flower drop and fruit drop.
3. Improper cultivation and management technical measures. An unreasonable fertilization plan, excessive nitrogen fertilizer before flowering, excessive irrigation during flowering, and improper implementation of techniques such as shaping and pruning, new shoot tying, pinching, and cluster thinning intensify the contradiction between vegetative growth and reproductive growth during flowering, leading to flower drop and fruit drop. Spraying pesticides during flowering can burn the stigma, affect fertilization, and cause fruit drop.
4. Meteorological factors. (1) Temperature during flowering. The optimum temperature during grape flowering is 20-25°C. If the temperature before flowering is below 10°C, it affects normal flower bud differentiation; if the temperature during flowering is below 14°C, the floral organs develop poorly and fall off; if the temperature during flowering exceeds 35°C, it easily causes wilting and necrosis of the floral organs. (2) Rainfall during flowering. Continuous rainy weather during flowering makes it difficult for the corolla to fall off, and the anthers cannot shed pollen normally. (3) Insufficient sunshine. Continuous low light before flowering reduces new shoot assimilation, and nutrient supply to the fruit cluster during flowering becomes tight.
5. Physiological fruit drop. When grape berries grow to 3-4 mm in size, some berries stop developing and fall off due to malnutrition, which is called grape physiological fruit drop.