Quantifying the impact of insufficient winter chill on apple fruit quality

Insufficient winter chill is a reality in most of the apple producing areas of South Africa but little scientific data is available to quantify the effect that this has on the quality of the fruit. This study aims to determine the variability in fruit quality in two climatically contrasting areas. Secondly it aims to determine if the variability can be mitigated by rootstock choice and investigate the rootstock effect on dormancy progression.

For the first objective ‘Golden Delicious’ orchards (Elgin and Koue Bokkeveld) were harvested by selecting six trees and removing the fruit from two branches into six categories. Destructive and non-destructive quality parameters were measured before and after storage and statistical analysis was performed to determine the variability. Similar methods were applied on a trial site with seven rootstocks grafted with ‘Reinder’s Golden’. Dormancy progression of the rootstock/scion trial was determined and compared.

High levels of starch and firmness variability are reported for orchards that were harvested at the determined release date. The relative bloom date of a flower has an effect on the ultimate quality of the fruit. Rootstocks influence the dormancy progression and fruit quality of the scion, but can not mitigate the variability found in an orchard.

A high amount of variability was found within the fruit appearance, maturity and quality parameters of ‘Golden Delicious’. Surprisingly, orchard and tree differences were found to contribute more to the variability compared to regional (climatic) differences. Although a protracted flower period (associated with lack of winter chill) occurred, it was a surprisingly low contributor to the total variance found in fruit quality across sites. Rootstocks do not seem to mitigate much of fruit quality variability. Malling rootstocks, especially M9, has a tendency to have a lower maximum dormancy level compared to Geneva rootstocks.

Variability in fruit appearance, maturity and quality is high and the major known contributors are orchard and tree differences. Rootstocks do have an effect on the quality parameters, but so not mitigate the variability as fruit position and light exposure are overriding factors.