MAJOR PROBLEMS IN FRUITING OF HORTICULTURAL FRUIT CROPS
Author: P. L. Deshmukh
Ph.D. Scholar, Department of Horticulture,
Dr. Panjabrao Deshmukh Krishi Vidyapeeth, Akola, Maharashtra.
It is common that the fruit grower is often faced with difficult situation of poor fruiting in his orchard. The tree may be in good health and may not show any apparent cause of the malady. The growth and fruitfulness of a plant is greatly influenced by various external as well as internal factors.
I. External Factors
A plant is exposed to different phases of environment during the different stages of its life cycle such as germination, flowering, fruiting, harvesting, etc. These various phases of environment influence the blossoming and fruit setting. This environmental impact may render the plant fruitful or unfruitful. Let us see what are the main external factors influencing the fruitfulness of the plant.
The most important aspects of environment are temperature, rainfall, wind, light, etc. These factors can prevent or favour the pollination and fruiting.
Temperature is a variable factor which is controlled by time, season, latitude, altitude, slope, plant cover, soil type, etc. Temperature profoundly influences reproduction, embryonic development and growth of a plant. Air temperature 25 to 30 degree C is the most favourable range for tropical plant. The optimum temperature for fruiting ranges from 22 to 28 degree C and it varies according to the fruit trees.
Light supply is also important in determining the setting of fruits in deciduous trees (pertaining to the falling of leaves, fruits and petals of flowers). The development of stamens (male organ in the flower) and petals (each of the division of a corolla, the coloured part of a flower) in strawberry flowers is reported to take place only when the plants are exposed to long day light conditions.
Rainfall at blossoming is recognised as one of the most important limiting factors of fruit setting. Heavy rains drop down the newly formed flowers from the tree or may severely damage the vital organs of the flowers such as stamens, pistils, etc. Consequently, there is very poor fruit setting or no fruit setting at all.
Wind is one of the most important agents in the transfer of pollen from stamens to stigma (the portion of ovary surface which receives pollen). Many plants such as walnut, aonla, etc. are wind- pollinated. A reasonable amount of wind at blossoming is necessary in securing good fruit set, but strong and desiccating wind will adversely affect the fruit setting.
B. Insect pests and diseases
Insects also play an important role in pollination. Bees and other pollen carrying insects work more effectively in a still atmosphere.
The flowers of many fruit trees are subject to the attack of various insect pests and diseases. The result is serious reduction in the fruit set. Mango hopper can attack mango tree at blossom and damage flowers to a great extent. It can even destroy whole orchard. Malformation of flower panicles in mango, and anthracnose, die- back and fruit-rot in citrus plants cause heavy losses through poor fruit set and blossom drop. The control of these pests pest and diseases and adopting suitable preventive measures before attack, will greatly : increase the fruit set.
II. Internal Factors
A. Sex distribution
For proper setting of fruits presence of sufficient number of male and female flowers in a plant is necessary. But some plants (of many species) do not fruit because their flowers are having only one sex that is either male or female. So in such a case, the orchard must have a proportionate number of plants bearing male and female flowers. Otherwise fruiting will not take place. Ego papaya, mango, strawberry, fig, etc.
This denotes the defective floral structure such as short styles and long filaments in some flowers and long styles and short filaments in other flowers. Due to this abnormal structure of flowers, self pollination will be prevented and this will lead to poor fruit set
C. Physiological factors
These include premature or delayed pollination. When mature pollen grains are applied to immature pistils, they penetrate the styles and enter the ovules and if the ovules are not ready for fertilization, the flowers soon fall off. The health of the plant is affected due to this. Pollens from healthy trees germinate better than those from weak trees.
In some trees male and female sex organs develop and mature at different periods. This will inhibit the self-pollination of the perfect flowered plants (the perfect flowers are those which have both male and female sex organs). This is known as dichogamy. It is incomplete where there is overlapping of the seasons of maturity of sex organs, otherwise it is complete. Eg. avocado.
D. Genetic incompatibility
This is another cause of self-unfruitfulness. Genetic incompatibility is a condition in which the pollens of a variety are incapable of fertilizing the flowers of another variety, or of the same variety. Hybrids of distantly related plants produce self-sterility. These trees may blossom and bear flowers but they may be without pistils and petals. Numerous stamens also may be present but are malformed. Sometimes the ovules and the pollens of these flowers are fertile in themselves but fail to conjugate. Eg. mango, sweet cherry, plum, pear and almond.
E. Nutritional status of plants
The nutritional status as affected by soil, water supply, manuring, soil culture and pruning also exert a profound influence on fruit set The growth and fruitfulness of a plant is greatly influenced by the relative proportions of carbohydrate and nitrogen. For the better development and satisfactory growth, a balance should be maintained between carbohydrate and nitrogen. This balance of carbohydrates and nitrogen in the plant is called C/N ratio.
The building up of a surplus food material (carbohydrate) i.e. over and above the requirement of plant's physiological activities and new tissue formation is a pre-requisite to fruitful condition of a tree. This accumulation may occur due to the rapid manufacture of carbohydrates or to their less utilization. The factors affecting this rapid manufacture are wider spacing in the orchard, light, water, nutrients and more favourable temperature. This surplus accumulation and vigorous growth of the plant can be brought down by the reduction in supply of water or nutrients (especially nitrogen) and also by lowering the prevalent temperature.
III. Methods for Inducing Fruiting
A. Mechanical measures
It is very difficult to tackle related physiological, genetical and climatic factors responsible for unfruitfulness. However, certain cultural practices such as root exposure, bending, pruning, notching, girdling, thinning and smudging can influence the accumulation of food materials and help in fruiting. These practices are discussed below.
1. Root exposure
This is widely practised in India for inducing flowering and fruiting in oranges. About two months prior to blooming, the soil around the trees is removed near the main roots from an area of 60 cm radius. The main roots are exposed and fibrous roots are removed. The orchard soil is ploughed and the trees are allowed to go dry until the leaves wither and even some of them fall. It normally takes three to four weeks. After some leaves have fallen the exposed roots are covered with a mixture of soil and manure. The trees are then irrigated immediately.
During this period of root exposure, the tree is forced to take rest as upward movement of water and downward movement of food material is affected adversely. Within a period of about three weeks of covering the roots and watering, the tree will burst into a heavier bloom. The root exposure is only practicable in dry weather when the soil moisture and nitrogen can be reduced to the minimum.
3. Ringing or girdling
This is one of the well known methods for inducing fruit bud formation. The operation consists of removing a strip of bark from branch or trunk of the tree. This will interrupt the downward movement of the carbohydrates and thus force them to accumulate above the ring. The branches which are 15 to 30 cm thick are ringed by removing a strip of bark about two cm wide all round the base of the branch, a little above the point where it joins another branch. Ringing is sometimes resorted to induce flowering in over-vigorous mango trees. The heating of the ring and complete restoration of new bark on the ringed portion is highly imperative after fruits have set. Weak and exhausted trees should not be ringed.
This practice is well accepted in guava plant especially in the erect growing varieties. In guava the flowers are borne on new shoots. For mere fruits naturally there should be more branches. In erect growing varieties, the apical buds produce new shoots while the lower buds remain dormant. By bending the branches, these dormant buds sprout and give rise to more shoots. The large branches are bent towards and tied arch fashioned with similarly bent branches of the neighboring tree. This practice is not necessary in those guava varieties which naturally spread obliquely and droop giving rise to side branches.
Notching is also a sort of ringing of a branch above a dormant lateral bud. A small narrow strip of bark just above a dormant bud is removed. As a result, the bud is forced to grow into either a vegetative shoot or a mixed shoot containing both foliage and flowers as would be consistent with plant's natural habit of growth. It can be successfully practiced in fig trees. The effect of notching above the bud is three fold.
Smudging is a practice of smoking the trees. The operation consists in burning brushwood on the ground and allowing smoke to pass through the centre of the crown of the tree. The tree is smoked heavily and continuously for a week. Thereafter, light fires are made in the morning and evening for about a month or until the trees come into bloom. The smoking is discontinued as soon as the terminal buds begin to swell. The smudging is applied in mango to produce off-season crop.
7. Thinning of fruits
This practice means the removal of a few young fruits from heavily bearing clusters or fruiting branches, so that the remaining fruits may have greater advantage of space, light, water and food. Due to reduced competition they can be expected to be larger in size, better in quality and brighter in colour.
Thinning should be done soon after the natural drop of young fruits has started i.e. a little before they have grown one fourth of their normal size. Thinning may invariably reduce the total yield of the tree. It is generally practised in apple, pear, plum, papaya, etc.
B. Chemical measures
Of late, different chemicals or plant hormones have been used for inducing flowering and fruit setting in different fruit crops. It has been found that different concentrations of gibberellic acid (GA3), naphthelene acetic acid (NAA), 2,4-D, etc are very effective in inducing flowering and fruiting. NAA (5 ppm) or GA3, (10 ppm) can be used for inducing flowering in mango. Weak concentration of NAA can also be used for inducing flowering and fruiting in pineapple. Figs, citrus, apple and pear all have been found to bear fruits by the use of auxins or GA3, or both in different concentration.
Similarly, chemicals especially pesticides can be used to check crops which sometimes are the main causes of unfruitfulness or poor fruit setting in these crops.
About Author / Additional Info:
Ph.D. Scholar, Department of Horticulture, Dr. Panjabrao Deshmukh Krishi Vidyapeeth, Akola, Maharashtra. (India)