GROWING DECIDUOUS FRUIT TREES IN THE TROPICS - A NEW APPROACH TO AN OLD PROBLEM

Deciduous fruit trees such as apples, pears, peaches and plums, originating from the temperate zones, have been grown in the subtropics and tropics for several centuries.

Until a fairly short time ago, the cultivation of these crops in countries subject to high temperatures was seriously hampered by certain developmental disorders. The disorders occurring in the subtropical areas in spring have been described under the synonymous expressions: prolonged rest, prolonged dormancy and delayed foliation.

They occur because the winters in these areas, when the trees should be in their winter dormancy, are relatively mild. The period of adequately low temperatures is too short to properly satisfy the natural cold or chilling requirements of these crops which is needed for breaking winter dormancy.

The harmful effects of prolonged rest in the subtropics can be largely prevented by treating the trees with a number of chemical compounds. Spraying with oil emulsions to which dinitro - ortho - cresol in concentrations of 0. 06% or less is added, or the spraying of thiourea in a 2% concentration in water alone, or combined with the standard oil/DNOC spray, has been found to be very effective.

In the high-altitude fruit growing areas of countries such as Guatemala, in latitude 14^oN, climatic conditions represent a transition to those in the subtropical areas. In Guatemala, there are considerable seasonal fluctuations in temperature. This is a clearly defined cold season of some 4 months, in which temperatures at 2 000–2 400 m above sea-level are sufficiently low to satisfy the crops' chilling requirements. Following this cold season, spring and summer are sufficiently warm to promote reasonable growth and fruit production. Prolonged rest problems, if any, in these areas can be overcome by treatment with the earlier mentioned chemical compounds.

In the tropical countries near the equator, such as Indonesia and Kenya, however, the situation is different from that existing in the subtropics.

Here there are no definite seasons, only slight seasonal changes in temperature. In the medium-altitude areas, temperatures are so high and even throughout the year, that branches can continue growing and remain leafy, at least at their apex. Below the leafy apex, however, the branches are bare, because after shedding of the senescent leaves has occurred no sprouting of the axillary buds and development of lateral branches occurs. These buds remain dormant due to apical dominance from the growing leafy branch tip. Consequently, apple trees in such areas, if left without attention, develop long whippy branches which, apart from the terminal cluster of leaves, are almost devoid of lateral