What causes low fruit set in apple trees? How can it be solved?

Apples are one of the fresh fruits people commonly eat, because they contain a wide variety of minerals and vitamins, and their “moisture” is easily digested and absorbed by the body, with the practical effect of whitening and moisturising the skin, which is why they are so popular with everyone. In recent years, along with the continuous increase in apple planting area, the apple fruit set rate has dropped sharply, and many growers are very troubled by this. So why is the apple fruit set rate low? How should it be solved? Let us take a look together below.

I. Reasons for low fruit set in apple trees

1. Factors of the variety itself

Different apple varieties differ greatly in their own fruit-bearing capacity, and the reason is mainly related to flower production and pollination. Compared with the Red Fuji apple variety, which has the largest planting area, varieties of the Delicious line and Qinguan produce flowers very easily; differences in mother branches also cause great differences in fruit set rate. For example, varieties such as Golden Delicious mainly bear on long and medium mother branches, varieties such as Xinhua and Red Fuji mainly bear on short mother branches, and varieties such as Liaofu easily produce axillary flower buds. Research points out that varieties that mainly rely on short mother branches and have the habit of bearing fruit on axillary flower buds usually have a higher fruit set rate.

2. External climate and environmental factors

First, low temperatures in spring cause varying degrees of freezing damage to the apple receptacle, ovary and so on, to the point that the pistil stigma of opened flowers freezes to death and cannot complete pollination and fertilisation, such as the low-temperature damage in early and mid-April 2018; second, sandy and windy weather in some planting areas reduces the mucus during the apple flowering period, leading to poor pollination and fertilisation, such as in the Jingning apple planting area in Gansu; finally, there is drought weather. Although people appropriately drain water during the spring shoot growth period of apples to increase yield, if it is not controlled properly and the drought becomes too severe, the spring shoots will not grow abundantly, the growth amount will be insufficient, they will set (fruit) early, and flower bud differentiation will be poor.

3. Insufficient stored nutrients in the tree body

The nutrients needed for apple flower bud differentiation come from the nutrients stored in the tree body, and these nutrients stored in the tree body are usually the portion that the plant absorbs and stores when growers apply base fertiliser in autumn. Therefore, if growers do not apply base fertiliser in time, the quality of flower buds produced by the apple trees the following year will be poor, and the fruit set rate in the later stage will drop greatly.

4. Insufficient provision of pollinator trees

The vast majority of apple varieties are self-unfruitful or have a relatively low fruit set rate. When establishing an orchard, no less than 20% pollinator trees should be provided to help them better complete pollination and fertilisation. If too few pollinator trees are provided in the orchard, or if no pollinator trees are retained after top grafting and variety replacement, and artificial supplementary pollination is not used during the flowering period to assist pollination, then in the later stage a low fruit set rate will appear due to poor pollination and fertilisation.

II. Solutions for low fruit set in apple trees

1. Reasonable arrangement of pollinator trees

Providing sufficient pollinator trees when establishing an orchard is an important measure to increase the apple fruit set rate. Generally, the ratio of main planted varieties to pollinator trees is 3:1-5:1, and the plum blossom stake style and row-by-row style are relatively appropriate. It must be noted that the spacing between pollinator trees and main planted varieties should be less than 50 m. For orchards undergoing top grafting and variety replacement, 2 branches can be kept on each tree as pollinator tree branches. In addition, when providing pollinator trees, it must be noted that the flowering period of the pollinator tree variety must overlap with that of the main planted variety, the pollen amount must be large, and the flowering period must be long.

2. Artificial supplementary pollination

At present, artificial supplementary pollination mainly consists of hand pollination and placing bees during the flowering period. Among these, the hand pollination process is relatively complicated. Workers must, after the morning dew has dried, pick bell-shaped flowers about to open or flowers that have just opened, then extract pollen indoors, and afterwards apply the pollen to the stigma of the flower on the second day after the apple flower has bloomed, in order to increase the fruit set rate. As for placing bees during the flowering period, this mainly relies on bees carrying pollen to complete pollination and fertilisation, and this is currently the most widely used method of artificial supplementary pollination in orchards. At present it is mainly divided into honeybee pollination and mason bee pollination. Honeybee pollination usually requires placing one beehive every 5-10 mu, because the range of activity of honeybees is mainly concentrated within about 50 m; compared with honeybees, mason bees not only have the advantage of being easy to raise, but also have a much larger workload. Research points out that their workload is 80 times that of honeybees. Placing 50-80 mason bees per mu of apple orchard can help complete pollination and fertilisation, and the effect is also relatively good.

3. Resistance to adverse climate and weather

For resistance to low-temperature weather in spring, you should get into the habit of listening to weather forecasts and make preparations before the low temperature arrives.

① Through measures such as irrigating the ground and sprinkling water on the tree body, regulate the temperature near the ground, ease the drop in tree body temperature, and delay the flowering period to prevent cold damage;

② By applying brassinolide and other plant auxins, increase the nutrients of the tree body, ease the damage of low temperature to the flowers, and thereby increase the fruit set rate;

③ In the orchard, use straw, weeds, broken leaves and so on to light fires and produce smoke to drive away the cold air in the orchard, but environmental protection is now managed quite strictly, so it is not recommended that growers use this, in order to reduce unnecessary trouble.

For adverse weather such as sandy wind, shelterbelt strips can be planted around the orchard for prevention; and for drought, appropriate supplementary moisture is enough, but be careful not to overdo it, otherwise it easily causes excessive growth of spring shoots.

4. Balanced fertilisation and soil improvement

When applying base fertiliser to apple trees in autumn, the types should be complete: high-efficiency organic fertiliser, high-efficiency compound fertiliser, medium-element fertiliser and so on must all be applied. For apple trees in full production period, apply 3500-5005 kg of organic fertiliser per mu, using radial trench application; when topdressing, spring is mainly based on nitrogen fertiliser, summer is mainly based on phosphorus and potassium fertiliser, and it is suitable for rotary tillage and shallow application; as for nutrient elements, either apply them together with base fertiliser, or directly spray them on the leaves during the apple tree growth period according to the growth condition of the tree body. In addition, techniques such as growing grass in the orchard can also be used to increase the organic matter content in the soil and improve the soil structure, so as to cultivate robust tree form on the basis of increasing soil organic matter.

5. Coping measures in other aspects

Through scientific pruning methods, build a tree shape with good ventilation and light transmission, and cultivate robust mother branches; through prevention and control of early defoliation disease, branch ring rot, aphids, planthoppers and other diseases and pests that greatly harm flower bud differentiation, cultivate a strong tree body; through scientific flower and fruit thinning, reduce ineffective nutrient consumption, concentrate nutrients to produce sufficient and plump flower buds, and increase the fruit set rate.

The above are the reasons for low fruit set in apple trees and the solutions. There are many reasons for low apple fruit set. Growers must innovate their management, analyse the actual reasons according to the specific situation, and deal with them promptly. It is hoped that the above methods can be of some help to apple growers.