Overcoming dormancy in fruit trees is essential for ensuring a successful growing season and a bountiful harvest. Dormancy is a natural protective mechanism that prevents fruit trees from budding and flowering during the winter months when the conditions are not suitable for growth.

Methods of Overcoming Dormancy on Fruit Trees

The cultivation of temperate fruit trees in tropical and subtropical zones has to tackle several problems, mainly related to the endo-dormancy of the buds. One way is by utilizing suitable cultivars with low winter-chilling requirements. In some cases there is a tendency in existing early-ripening cultivars to have low chilling requirements, but their quality is poor.
During the last few years, breeders have combined the low chilling requirement, early-flowering and earliness with an improvement in quality. In some species the selection of suitable rootstocks has reduced the chilling requirement of the scion by decreasing its vigour. However, in many tropical and subtropical zones the application of some cultural practices is necessary to improve the production and the quality. The practices aim to induce leaf-fall, early dormancy and reduce the vigour of the shoots.
Cultural practices such as withholding irrigation water, defoliation, use of certain methods of pruning and growing the trees with training forms have shown good results. The application of cultural practices can be a valuable strategy both to improve the production of the temperate fruits in tropical and subtropical zones and to reduce possible damage to the environment by spraying chemicals in order to break dormancy.

Defoliation (leaf stripping)

Defoliation (leaf stripping) will create an artificial resting period, which is later broken with a chemical spray. The proper timing of leaf stripping, which is easily done by hand, can be determined from climatic conditions, leaf orientation and appearance. For Kenya it is recommended that this is done approximately during the second half of August for plums and peaches/nectarines, and in the first week of September for apples and pears.
Lately a new approach has been developed where a harvesting cycle of 6 to 7 months for selected apple cultivars is feasible, which means that almost two crops can be obtained per year. Normally, stripping of leaves is performed one month after harvest but may be varied to alter the sequence of ripening in the same region.
Twenty to 30 days after defoliation, pruning, and training (bending) of shoots, mature trees will start flowering, and after a further 5 months the fruits will start maturing.
Depending on the size of an orchard, hand defoliation may not always be feasible and chemical defoliation may be appropriate. Many of the preparations below have been applied with varying success depending on local conditions, but should be tested on site to assess their effectiveness:
  1. combination of 5 % urea + 5 % zinc sulphate
  2. 3 – 4 % urea
  3. 1 % Copper chelate
  4. 0.75 % Magnesium chlorate
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As a rough guide the following activities are advisable:

  1. 1st week: hand or chemical defoliation;
  2. 2nd week: pruning of trees;
  3. 3rd week: application of spray to overcome prolonged dormancy.

Desiccation

It is possible to counteract a lack of winter chilling to some extent by using cultural techniques. The practice of withholding irrigation water in the dry season following harvest to induce early dormancy is fairly widespread.

Chemicals

The problem of prolonged dormancy can be minimized significantly by using chemicals like dinitro-orthocresol (DNOC), thiourea, potassium nitrate (KNO3) and, more recently, hydrogen cynamide (Dormex™).

Spraying of trees causes flowers and leaf buds to open more evenly and at the right time. The use of tar oil or DNOC does not permit the growing of cultivars entirely out of their climatic range, but its effect on borderline cases is quite remarkable; certain cultivars that would not have grown properly unless sprayed do in fact bear good crops after treatment. In trials, pome fruit trees responded better to chemical treatment than stone fruit trees. Furthermore, it seems that thiourea influences mainly the opening of leaf buds, whereas potassium nitrate affects primarily flower buds.

In general, beneficial effects leading to increased yield have been detected by using combinations of various chemicals, especially thiourea (2%) plus oil, and KNO3 (10%) plus oil. The timing of application is critical: trees will not respond if sprayed too early in the dormant season, whilst late spraying may damage any buds that have already broken or are at the stage of bud break.

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When spraying, it is important to wet the whole tree thoroughly. The most popular treatment used commercially to break dormancy of temperate fruit trees following warm winters is a 0.25 % DNOC–6 % mineral oil emulsion. Results obtained in practice are, however, often unsatisfactory due to incorrect timing and phytotoxicity.

In order to overcome these and other shortcomings extensive research with hydrogen cyanamide (Dormex), a plant growth regulator, has been undertaken. Because of the exceptional results obtained Dormex has been used commercially since 1984 in tropical and subtropical countries the world over.

Dormex can overcome the lack of winter chilling, induce uniform bud break and increase yield. Its effectiveness seems less sensitive to the time of application and the interval between treatment and bud break than DNOC–oil. The following Dormex application dosages for selected fruits are recommended:

  1. Grapes / kiwi 5 %
  2. Apple / pear 2 – 4 %
  3. Peach / plum 1 – 3 %
  4. Cherry / nectarine 1 – 3 %

A spreader/sticker should be added to increase effectiveness.
Because of the widely variable agro-climatic zones in which temperate fruit are being grown in regions close to the equator, the recommendations concerning the use and the appropriate timing of any methods for inducing bud break must not be generalized.

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