What measures are there to improve the fruit-setting rate of pear trees?
1. Protecting flowers and protecting fruit They are two technical measures with completely different meanings, but their purpose is the same: to improve the fruit-setting rate. Only when these two measures are skillfully combined on the same tree can it be ensured that the tree bears enough fruit for a good harvest.
Protecting flowers and protecting fruit is by no means an emergency measure taken when the flowers on the tree are about to open. This is because the flower buds of pear trees begin differentiation in mid-June of the previous year, and after a long period the floral organs gradually form, not opening until March of the following year. If pest and disease control is neglected, especially the pear lace bug and pear black spot disease that cause massive leaf drop, the leaves of the pear tree will basically all fall by August. Then, when the temperature is suitable in September-October and there is sufficient rain, new leaves will emerge and a second round of flowers will bloom. The flowers that should have bloomed in the following year will have already bloomed in the previous year, so naturally there will be no flowers left to open the next year. In some pear orchards, many leaves fall in August. Because there are not enough leaves to manufacture nutrients, flower bud differentiation is inadequate, and although there are many flowers the following year, fruit set is not high. It can thus be seen that the decisive measure for protecting flowers and protecting fruit is protecting leaves.
Flower thinning and fruit thinning, and protecting flowers and protecting fruit, are technical measures that complement each other. Most pear varieties form flower buds easily, have many flowers, and have a high fruit-setting rate. If too much fruit sets, first, the fruit is small in that year and economic returns are low; second, it causes alternate bearing, so flower thinning and fruit thinning must be emphasized.
2. Hand pollination Pear trees are fruit trees that cross-pollinate. Without pollination by flowers of a different variety, the fruit-setting rate is very low. If no pollinator variety was planted when the orchard was established, or the number of pollinator varieties is insufficient, or the selected pollinator variety is unsuitable, its flowering period may meet or overlap with that of the main variety for only a short time, or the main variety and the pollinator variety may begin flowering at different tree ages, or the flowering period may be continuously rainy, affecting normal pollination. In these situations, if hand pollination can be carried out, the fruit-setting rate can be greatly increased, and a relatively good yield can be obtained under unfavorable conditions.
Of the pollen grains that land on the stigma, only 10 enter the ovary to fertilize and form seeds. However, the hormones produced by the pollen grains that enter the style but do not reach the ovary can stimulate cell division in the young fruit. Therefore, the more pollen grains that land on the stigma, the greater the possibility that the fruit will grow into a large fruit. Trials indicate that when 300 pollen grains land on the 5 stigmas of a flower, the young fruit has the most cell divisions and is large. As the number of pollen grains decreases, the young fruit accordingly becomes smaller. Poor pollination and fertilization not only affect fruit size, but also often increase the number of deformed fruits. With the development of the market economy, it is necessary to promote hand pollination as a routine technique.
(1) Timing of pollination The period during which opened flowers have the ability to be fertilized is 5-7 days. According to investigations, the earlier pollination is done after flowering, the better. Fruit obtained from pollination after the fourth day of flowering develops poorly and is small, and after 5 days the fruit-setting rate declines markedly. Pear pollen at 15-17°C germinates about 50% in 1 hour, 80%-90% in 2-3 hours, and after 3 hours almost all of it can penetrate the stigma. This shows that under normal temperature conditions, rain 3 hours after pollination has little effect on fruit set. Pollination can be done from 8 a.m. to 6 p.m. At noon, when the temperature is high, pollen germinates quickly and the effect is even better.
(2) Collection and handling of pollen The pollen collected should be pollen from trees of a different variety. In production, mixed pollen from several varieties can be used. It is best to pick flower buds that are about to open; pollen from buds that are too young is of poor quality. After the flowers are brought back, remove the pink anthers from the buds as quickly as possible, spread them out on paper, and dry them so that the yellow pollen inside the anthers is released. If pollen is urgently needed, the pollen can be dried in a constant-temperature cabinet at 20-25°C, or a 15-watt light bulb can be placed in a paper box to provide heat, drying the pollen, speeding up the splitting of the anthers, and causing the pollen to be released. Note that the temperature should not exceed 25°C; if the temperature is too high, the pollen loses its germination ability. If it exceeds 30°C, the pollen germination rate will also decline. Therefore, when drying pollen, a thermometer must be used to constantly monitor temperature changes. After the pollen is dried, put it centrally in a small bottle for later use. The life span of pollen is not long. Under conditions of 50% humidity in a greenhouse, the germination rate of pollen after 10 days is 57.6%, and after 50 days only 33.9%. If the pollen is dry, the germination rate will decrease rapidly. At pollination, the amount of pollen needed per flower is very small. To save pollen, a filler such as pine pollen, milk powder, or lotus root starch can be added to the pear pollen before pollination. The dilution amount is 10-20 times the amount of pollen, and it must be stirred evenly.
(3) Pollination methods The diluted pollen can be put into a gauze bag, then tied to the end of a pole, and the pole lightly tapped with a wooden stick so that the pollen is scattered onto the stigmas of the flowers. With this method, about 750 grams of pollen are needed for 1 hectare of pear orchard in full fruit stage. The pollen can also be mixed into a 15% sucrose solution and then sprayed onto the stigmas of the flowers with a sprayer. With this method, 1200-1500 grams of pollen are needed for 1 hectare of pear orchard. A writing brush or the eraser end of a pencil can also be dipped in pollen to spot-pollinate flowers. When hand-pollinating pear flowers, it is not necessary to pollinate every flower or every inflorescence; it is enough to pollinate 1-2 flowers on inflorescences that are well positioned and blooming normally. When pollinating, simply lightly touch the pollinator to the stigma of the flower being pollinated; pressing too hard will injure the stigma and affect normal pollination and fertilization.
In orchards where the pollinator trees are improperly arranged or insufficient, the thorough solution is top-grafting pollinator varieties. For emergencies, hand pollination must be carried out, though the workload is relatively large. For simplicity, flowering branches of the pollinator variety can be cut off, placed in bottles filled with water, and hung in the upper part of the pear trees that need pollination; a large tree needs 4-5 bottles hung on it. This method is simple, but the effect is not as good as the above methods. In addition to the above methods, bees can also be introduced for pollination. This is suitable for pear orchards where pollinator trees account for more than 20% of the whole orchard and are arranged fairly evenly. Place the beehives in the orchard 2-3 days before flowering; 3 boxes for 2 hectares is more suitable.
3. Flower thinning and fruit thinning The nutrients needed for flowering and young fruit development all come from the nutrients stored in the tree the previous year. If there are many flowers and the fruit-setting rate is high, the stored nutrients are quickly exhausted, it is very difficult for the young fruit to fully swell into marketable fruit, and the tree vigor is weakened. Therefore, flower thinning and fruit thinning should be carried out on adult fruiting trees and weaker-growing trees.
(1) Flower thinning Flower thinning actually begins during winter pruning. During winter pruning, some excessive flower buds can be thinned out. If there still seem to be too many flowers before flowering, the flower buds within the buds can be removed while keeping the leaf buds, in order to increase the amount of foliage. This is very beneficial for restoring tree vigor.
In order to expand the canopy, all the flower buds of axillary flower buds on the extension shoots of the main branches and lateral branches should be thinned out. On 3-4-year-old branches, spur branches are generally spaced about 10 cm apart, so that the fruit is evenly distributed, close to the main branches and trunk, and bears fruit in three dimensions.
(2) Fruit thinning The purpose of fruit thinning is to ensure that the remaining fruit on the tree can obtain sufficient nutrients and grow into marketable fruit. Therefore, the earlier fruit thinning is done, the better. It can begin 2 weeks after petal fall. The second fruit thinning is carried out 25-30 days after flowering. When thinning fruit, first remove fruit damaged by pests and diseases and deformed fruit. For the same variety, keep young fruit that is longer in shape, has a long and thick fruit stalk, and has a glossy fruit surface.
The number of fruit retained varies with variety and fruit size. The number of fruit to retain is determined according to the planned yield and fruit size. For example, for Huanghua pear, a medium-sized variety, with a planned yield of 45,000 kg per hectare and a requirement of more than 250 grams per fruit, 180,000 fruit need to be retained, plus a 10% safety margin, for a total of 198,000 fruit retained. While considering the number of fruit to retain, it is also necessary to consider having enough leaves. Generally, about every 30 leaves supply the nutrients for one fruit, that is, about 1,000 sq cm of leaf area is needed to ensure the needs of one fruit's growth and development. This differs among varieties. Some late-maturing varieties (large-fruited) need more than 40 leaves to satisfy the nutrients required by one fruit.
Fruit thinning methods. Generally, fruit is retained according to the tree. Strong trees and vigorous branches retain more, and the reverse retains less. The top of the canopy retains more, and branches with small branch angles retain more. The general principle is to thin the weak and keep the strong, thin the small and keep the large, thin the dense and keep the sparse, and thin the top and bottom and keep the two sides. Generally, 1-2 fruit are retained every 18-26 cm. For varieties that do not easily form flowers, the method of retaining fruit at alternate buds should be considered to ensure stable production.