The effect of a mild winter climate (low chill accumulation) on apple production and how this can be mitigated

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.
Delayed foliation results in the development of increased basal dominance, bare shoots with fewer spurs and thus fewer bearing positions as well as fruit of mixed maturities at harvest.
Increased basal dominance makes it more difficult to fill the allocated space in the upper part of the canopy while fewer bearing position requires leaving more fruit per infructescence with a subsequent reduction in fruit size.
Mixed maturities can increase the incidence of postharvest defects and negatively affects the appearance and eating quality of batches of fruit.
The application of rest breaking chemicals has long been a standard procedure to combat delayed foliation in apple orchards.
Recently, various changes in how these chemicals are used have been introduced.
This involves 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 sometimes stubbed during early spring after the first winter after planting leaving only ca. 3-10 cm of the basal section.
This delays yield, but it is considered more important to ensure that the canopy fills its allocated space for optimal light interception and yield potential.
Also, a modified Solaxe-system was introduced to reduce the strength of lower branches in the tree.
It has recently become evident that high density plantings with dwarfing rootstocks can preclude the need for stubbing and bending below the horizontal after it was long believed that dwarfing rootstocks are not adapted to South African conditions.
These changes in planting systems and rootstocks to reduce canopy volume is also addressing the variation in fruit quality and maturity variation of lower density plantings.
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 ripening in early fruit, thus reducing variation.
A very important aspect though is that the growers are supported by highly skilled technical advisors and a well-organized industry that addresses issues through research as they occur.

XXXI International Horticultural Congress (IHC2022): International Symposium on Innovative Perennial Crops Management

K.I. Theron, E.D. Louw, W.J. Steyn

apple production, climate change, endodormancy, low winter chill

https://doi.org/10.17660/ActaHortic.2023.1366.1