Orchard Management (1+1)

CROP REGULATION IN FRUITS

Introduction:

• Crop regulation is the basis for the regular and quality crop. A range of methods are used to increase production with enhanced fruit quality by crop regulation. It can be achieved through manual thinning, chemical thinning, selective harvesting, training, summer and winter pruning, prevention of pre harvest fruit drop etc.
• It is well known that competition between fruits is one of the principal factors affecting fruit growth, ripening and fruit quality. Fruit competes with each others for assimilates according to their relative sink strength and the availability of assimilates.
• When crop density is higher as under high fruit set conditions, competition for assimilates is intense, which result not only in decreased fruit size and quality but also promotes alternate bearing. Crop density, a measure of fruit crop, influences fruit growth and ultimately fruit size at harvest. It is largely dependent upon flowering and fruit set.
• Under ideal conditions, most fruits often set heavier crop that cannot be adequately sized to meet market requirement for higher price. This necessitates retention of well spaced fruits (Plate 17.1a & 17.1b) after selective removal of some of the fruits on the trees. Leaf to fruit ratio fairly guide about the extent of thinning to be carried out in a particular orchard. Usually 30-40 leaves per fruit (Plate 17.2 a) are sufficient to produce fruit of good quality (Plate 17.2 b). Though, thinning may reduce the total yield, but net return increases as larger fruits fetch much higher price in the market (Plate 17.3 a & b).
• The regulation of crop load begins at pruning.
• The management of irrigation is also vital, especially during the critical of fruit development, where the most rapid fruit growth occurs.
• Tree nutrition also affects the eventual fruit size.
• However, thinning has a much greater influence on fruit size by adjusting optimum crop load on the tree during the heavy cropping year. It can be done manually by selective removal of young fruit lets at pea size stage.
• In Redhaven peach cultivar, retention of 4-5 fruits per shoot at the time when fruits are of peas (14-15 mm in diameter) size assure optimum leaf to fruit ratio and higher production of large size fruits.
• However, manual thinning is labour intensive, expensive and only small number of trees in an orchard can be thinned at the optimum time.
• Use of chemicals including plant growth regulators (PGRs) as chemical thinning agents is gradually replacing hand and mechanical thinning in stone fruits.
• Various considerations like tree age and vigour, irrigation, pruning and fertilizer practices followed, chemical used, its concentration and time of application and local climatic conditions should be taken into account while adopting chemical thinning.
• Fruit setting in stone fruits is often limited by frost injury to flowers in spring frost affected areas.
• Evaporative cooling of flowering buds in spring has a delaying effect on bloom in stone fruits. In this, trees are sprayed with water during day time at frequent interval from bud swell stage until bloom. Frequency of tree misting depends upon the evaporation rate, which is done as and when water film on the aerial parts of the tree dries up. Evaporation of water from the shoots cools the buds and therefore delays their break.
• Fall (mid-October) application of growth regulators like GA₃ at 200 ppm and ethrel at 200 ppm delay flowering in spring by 3-7 days depending upon species and thus markedly increase fruit set.

Plate 17.1 a) Heavy cropping (left); b) retention of 6 fruit-lets per fruiting shoot after manual after hand thinning in Kiwifruit (right).


Plate 17.2 a. Fruiting branch with lower leaf/fruit ratio; Plate 17.2 b. Fruiting branch with higher leaf/fruit ratio

Uses of PGRs in crop regulation in temperate fruits

1. Delay bloom:

• Autumn application of GA₃ and Ethrel cause delaying of bloom in spring in stone fruit, which can be a useful practice to increase fruit-set in frost affected areas.
• Another advantage of PGRs treatment is that it facilitates better cross-pollination among varieties having different (non-synchronous) bloom period. For example, blooming of Red Beaut plum can be delayed by several days with fall application of GA₃ and Ethrel to help in better synchronization flowering period with cultivar Santa Rosa, inter-planted for cross pollination.
• Concentration of PGRs and time of application depend upon fruit crop and environment.

2. Thinning:

• Use of PGRs as crop thinning agents have been discussed above under crop thinning. In blossom thinning, chemicals are applied at bloom, whereas in fruit-lets thinning, chemicals are sprayed a few days after petal fall i.e. after fruit set.
• Blossom thinning is adopted after assessing crop potential and is always risky, while fruit-let thinning is done after observing actual crop load.
• In apple application of NAA at 10 ppm, 7-15 days after petal fall during the heavy cropping year (On-Year) causes satisfactory thinning and increases return bloom. However, thinning with chemicals is less satisfactory to achieve in stone in comparison to apples. Results are variable depending upon climatic conditions of the area and temperature at the time of application.
• Fruit crops grown in different climatic zones- tropical, subtropical and temperate are required to be trained and pruned accordingly, depending on the type of plant and specific objectives of fruit growers.
• Suitable chemical may be useful for this purpose in one condition but may fail to thin satisfactorily in another area.
• Follow local recommendations for the type of chemical to be used, dose and time of application for better results.

Plate 17.3 - a. Large fruits after thinning in kiwifruit; Plate 17.3 b. NAA at 350 ppm 40 days after flowering in Kinnow

Mango:

Fruit drop:

• In mango, fruit drop is exceptionally high as only approximately 0.1% of the perfect flowers develop fruits to maturity. There are several causes such as lack of pollination, low stigmatic receptivity, defective perfect flowers, competition between fruit lets, and low soil moisture regimes.
• Extent of fruit drop in mango can be controlled by regular irrigation during fruit development period only after fruit set.
• Post setting drop of Alphonso mangoes can be controlled by foliar application of 25 ppm of NAA or 2,4-D (Gokhale and Kanitkar, 1953).
• In Neelum 2,4-D at 30 ppm proved effective control of fruit drop without having any adverse effect on fruit size (rao and Subba Rao, 1963).
• In other cultivars, use of lower (10-15 ppm) of 2,4-D about 6 weeks after fruit set i.e. end of April decreased drop (Gill, 1966). In Chausa mangoes, NAA at 50 ppm or 2,4-D at 10 ppm increased fruit retention (Prakash and Ram, 1985).
• Effective doses however, varies with cultivars and PGRs are effective only in "off'' years only (Chadha, 1963).

Biennial bearing:

• Problem of biennial is a major problem in mango associated with climatic factors, age and size of shoots, C: N ratio and hormonal imbalances in the trees. A number of remedial measures have been suggested from time to time, which include

1. Proper upkeep and maintenance of orchard
2. De-blossoming
3. Smudging and crop regulation through chemicals, pruning etc.

De-blossoming:

• In this, some flowers are removed during ‘On Year’ so that a good crop can be obtained even during ‘Off Year’. When shoots are de-blossomed in ‘On Year’ during full development stage of panicle, it forces panicles to emerge during the ‘Off Year’.

Smudging and chemical regulation: Smudging means building up slow fires, emitting smoke. It has been successful in Philippines but has not shown desirable response in India. Many chemical has been tested including, Ethrel (2-chloroethyl phosphonic acid), auxins, paclobutrazol and KNO₃ with inconsistent and often doubtful results.

Pruning:

• Opening up of the centre of the trees by topping off or thinning of branches have been reported to decrease biennial bearing in Mulgoa, Neelum and Bangalora cultivars.
• This improves light penetration into the interior of tree canopy. However, these practices have been unsuccessful in those cultivars which have inherent problem to bear irregularly.

Citrus

Fruit set:

• In citrus, fruit yield is often limited due to poor fruit set. Experiments with the use of PGRs have given some encouraging but, inconsistent results.
• Fruit set in Washington Navel Orange can be increased with the foliar application of 2, 4-D at 8 ppm.
• In India, a foliar application of 2, 4-D or 2, 4,5-T has been reported to be beneficial in improving fruit set and quality in mandarin.
• Sharma and Chopra (1978) observed increased fruit set in Pineapple and Valencia late orange following the application of 2,4,5-T at 10 to 15 ppm.
• In some studies in California, spray of GA₃ though has increased fruit set in sweet oranges and mandarin, but some undesirable effects like rough skin surface, thickening of skin, less juice contents in fruits often occurred.

Fruit drop:

• Excessive fruit dropping of citrus fruits is a major problem in India.
• If lack of soil moisture is the cause, use of organic mulching material like leaves or black polythene mulch can reduce the extent of fruit drop.
• Auxin, particularly 2, 4-D at varying concentrations is very effective in controlling pre-mature/pre-harvest fruit drop in citrus. Sprays of 2, 4-D at 8 ppm at 1.2cm diameter stage in Valencia orange, and at 15 ppm in Pineapple and 2, 4, 5-T at 30 ppm in Jaffa and Mosambi are useful when applied in October. 2, 4- D have also been found to be useful in reducing fruit drop in sweet lime (Randhawa et al. 1959) and Darjeeling mandarin (Kar et al. 1985).
• In Himachal Pradesh, application of 2,4- D at 10 in May and September controlled fruit drop in Kinnow mandarin.

Fruit thinning:

• In Himachal Pradesh, application of NAA at 350 ppm (Plate 3 b) or Ethrel at 200 ppm in Kinnow mandarin during the ‘On Year’, 40 days after full bloom effectively control fruit drop and reduce the tendency of alternative bearing.

Guava:

Regulation of flowering and fruiting:

• In northern India, guava flower twice a year, once in February and subsequently in June. The February or spring flowering is called Ambe-bahar; its crop mature in June and the July to September or monsoon flowering is called Mrig-bahar; its crop mature from November to January. In western and southern India, a third flowering occurs in October.
• Thus guava though would give crop year the round, however, quality of fruits remain poor. The trees are therefore given some special resting and cultural treatments to skip a particular crop, so as to obtain high quality crop from the remaining cropping seasons.

Bahar treatments:

• In northern India, fruits produced in the rainy season are inferior in quality and this crop is escaped by removing all the flowers, while winter crop is desirable with respect to size and quality and is therefore taken.
• In Deccan region, where the climate is mild, only two desirable crops a year are taken and the third one is escaped.
• The practice, which is popularly called a 'bahar' treatment, is achieved by root exposure and root pruning or exposure to hot sun before the onset of monsoon.
• Root pruning has however, adverse effect on tree longevity.

Withholding of irrigation:

• In Maharashtra, stopping of irrigation and removal of soil around the upper roots by June 10, and later again covering it with soil and manure mixture is a common practice for regulation of cropping in guava. In the pre-monsoon period, two light irrigation before a normally heavy one is recommended. This will allow escaping crop from rainy season.

Plant growth regulators:

• Auxin compounds have been very useful in thinning a crop in guava.
• Post bloom application of NAA at 80- 100 ppm has been useful in reducing fruit set. This treatment can reduce more than 80 % of rainy season crop and increase flowering of the following winter crop. NAD at 50 ppm and 2,4-D at 30 ppm are also effective for de-blossoming of summer flowers.

Pomegranate:

• Irrigation is withheld at least two months prior to the main flowering season. Manuring and fertilization followed by light irrigation is then followed two months later after flowering. Then three to four days later, normal irrigation are given at recommended intervals. This will result in producing new growth and bloom and thus ensure good crop.
• In Deccan, there are three flowering seasons viz,, June-July (mrig-bahar), February-March (ambe bahar) and September- October (hasta bahar). It is however desirable to take just one crop a year depending upon market requirement for better price and availability of water. In the Deccan, mrig bahar is taken due to scarcity of water in hot summer months. For getting crop from mrig bahar, treatment with withholding of irrigation from December to March-May results suppression of growth during this period. Trees will shed their leaves in March and will remain dormant till May. Shallow ploughing of land up to 10 cm depth is practiced in April. In mid May, the manure and fertilizers are applied followed by one or two light irrigation prior to the onset of monsoon rains. Trees will put forth new growth followed by bloom in June and will bear a good crop.
• In irrigated areas, Ambe bahar is also taken in the Deccan. Crop from this will mature in June-July. Irrigation is stopped once rain commence during monsoons. Following monsoon period, when trees shed their leaves in October-November, shallow ploughing is done. Manures and fertilizers are applied in the months of December-January, subsequently in January, first light irrigation is given and flowers will appear a month after irrigation. Quality of fruits and yield are better in Maharashtra from ambe bahar, whereas in Bangalore, fruit from mrig bahar maturing in October-November are superior in quality than ambe bahar. Regulation of hasta bahar is practically not feasible due to rainy season.

Preparation of solution:

• Solutions of plant growth regulators are made by dissolving their measured quantity (Table 17.1) in small amount of solvent and then diluting in water, to make an appropriate strength/ concentration. The types of solvents for different PGRs are as follow:

PGR : Solvent
Auxin : 95% alcohol
Gibberellin : 90% acetone
Cytokinin : Diluted hydrochloric acid (0.1N HCl)

• Commercial formulations of PGRs are also available in liquid form and solutions of desired strength of these are prepared by diluting their required quantities in water.
• Before applying through spray, 0.1% of surfactant (e.g. Tween 20) is added to the solution, for quick absorption by the leaves. Spray operation should be carried out in the morning hours on a clear and calm day to run off.
• For making solutions of higher concentration of auxins as in quick dip method (rooting), the quantity of alcohol to be added to the water should be up to 50 % of the total quantity of solution.

Table 17. 1: Guide for preparing solutions of different strength

% solution	mg/liter	g/liter	ppm
100 10 1 0.1 0.01 0.001 0.0001	1000000 100000 10000 1000 100 10 1	1000 100 10 1 0.1 0.01 0.001	1000000 100000 10000 1000 100 10 1

Percentage (%) = ppm/ 10000
ppm = % x 10000