S16 - Session O1 - Keynote: The effect of a mild winter climate (low chill accumulation) on apple production and how this can be mitigated

Authors: Karen I. Theron *, Esmé Louw, Wiehann Steyn

The South African apple industry has developed and is increasing in size even though most production regions have mild winter climates and receive less winter chill than most regular cultivars require. The result of this lack of winter chill is delayed foliation, which manifests in low and protracted bud break and bloom in spring. The result of delayed foliation is a change in tree architecture with the development of more basal dominance, bare shoots with fewer spurs developing and thus a reduction in bearing positions, resulting in a more fruit left per infructescence and subsequently a reduction in fruit size, as well as fruit of mixed maturities at harvest. The application of rest breaking chemicals has long been a standard procedure in apple orchards to combat delayed foliation. Recently, various changes in how these chemicals are used have been introduced. This involves the addition of plant growth regulators or applying a primer application prior to the main restbreaking application. Young tree training has also seen innovative changes as the basal dominance of the trees needs to be reduced and leader growth encouraged. This is achieved by regular application of gibberellin A4 to the terminal 10-15 cm of the tree. In addition, lateral shoots are often removed during early spring after the first winter after planting. Strong heading cuts are used, leaving only ca. 10 cm of the basal section of the lateral shoot. As this delays yield, benzyl adenine is used to stimulate shoots in the same season on the new laterals that develop. Also, a modified Solaxe-system was introduced to reduce the strength of lower branches in the tree. To address variation in fruit maturity, planting systems and rootstocks are adjusted to reduce canopy volume. This, together with the adjusted use of chemical restbreaking agents reduce the length of the bloom period thus reducing the period over which fruit set and start their development. In addition to smaller tree canopies, aminoethoxyvinylglycine is applied to delay maturity in early fruit, thus reducing variation. A very important aspect though is that the growers are supported by highly skilled technicians and a well organised industry that addresses issues through research as they occur.