THE SOUTH AFRICAN HIGH DENSITY SYSTEM

In South Africa, tree planting densities for apple appear to have stabilised at around 1,500 trees/ha. The predominant system currently used is a tree with a single central leader with four weak scaffold branches and additional fruiting branches higher up on the leader. The tree can be described as a Spindle or a Vertical Axis. The system is unique in the sense that its evolution is the result of an improved understanding of the reaction of the apple tree to a lack of winter chilling, hot growing conditions and poor soils. Whips are the predominant nursery tree available since feathered trees with sufficient correctly positioned side shoots of optimal dimension are difficult to grow. The whips are not headed at planting and are supported by a simple trellis. The next major obstacle is managing bud break with suboptimal chilling (and resultant delayed foliation problems) to obtain the correct number of well positioned shoots to create the complexity required for early production. In this respect there are two solutions. Nursery trees can be chilled in the cold room (4°C) before planting or (as is required in subsequent years) branching is managed firstly with restbreaking agents and then by scoring and notching and applying Promalin (6-Benzyl adenine and Gibberellins A4+A7). The requirement of scoring and notching appears to be due to increased paradormant inhibition of the distal shoot tissues associated with insufficient chilling. An additional problem of insufficient chilling, which is exaggerated with progressively less chilling is the increase in basal dominance of shoot autonomy, even on non headed whips. Shoot autonomy opposes the development of a well defined hierarchy required for the formation of a pyramidal shaped tree with optimal light distribution. In this respect the uneven vigour of side shoots is aggressively managed by bud flicking and bending. In the growing season it is often necessary to bend these autonomous shoots ("bulls") to avoid subsequent removal via pruning. The shift to high density has occurred using the same rootstock as more extensive orchards, i.e. the vigorous Merton 793. Only recently with improved soil preparation, better irrigation and more knowledge of minimising transplant shock, are more dwarfing stocks becoming a viable option. By managing this complexity, via less pruning and more manipulation (notching, scoring, bending and bud flicking), vigour is controlled to a large extent by early fruiting. Vigour is further controlled by girdling and occasionally by the use of growth regulators. Light distribution within the system is primarily controlled by the complete removal of excessively vigorous fruiting branches.