Pre-harvest determination of bitter pit potential in apples

[摘要] Bitter pit fruit in commercial consignments of apples still poses an economic threat toexporters from South Africa. Bitter pit develops pre-harvest, but gets progressively worseduring storage and is only traceable once the lesions appear after storage. Accurate, earlyindications of bitter pit incidence will allow for remedial pre-harvest measures in the field,e.g. Ca foliar applications, to reduce the potential losses. Similarly, the automatic detection ofa bitter pit fruit during packing will reduce financial losses by identifying unacceptable fruitbefore shipping.Fluorescence imaging is a fast, non-destructive technique, able to evaluate numerous fruitsindividually. Results of pre-harvest imaging on apples to identify fruit susceptible to bitter pitshowed that pitted fruit were correctly classified, but misclassification of non-pitted fruit withfluorescence imaging was still too high.NIR-spectroscopy point meter readings could distinguish visible bitter pit lesions fromhealthy tissue. Important wavelengths associated with visible bitter pit were identified. Thistechnique could also identify immature apples, more prone to bitter pit development. It couldhowever not distinguish between bitter pit and non-pitted fruit when applied randomly on thecalyx end of apples at harvest.Pre-harvest foliar applications to increase fruit Ca content and reduce bitter pit incidence, is astandard practice world wide. External Ca uptake by fruit was monitored to determine theefficacy of applications during different stages of fruit development. Two periods of efficientuptake of external Ca were identified, viz., cell division and the last few weeks before harvest.Foliar Ca applications from 40 days after full bloom were more effective in increasing fruit Ca content and reducing bitter pit incidence than at 80 days after full bloom, which wasrecommended previously.Mineral analysis of fruit has been used with variable success to predict bitter pit prior toharvest. The possibility of increasing the accuracy of existing predictive models by usinganalysis of individual fruit rather than pooled samples, was investigated. By improving thenormality of different mineral distributions and decreasing the overlap between pitted andnon-pitted fruit classes, it was attempted to improve the reliability of predictions based onvariable threshold values. The Ca distribution showed a variation between pitted and nonpittedclasses, but still a significant overlap between classes reduced the accuracy of thepredictive capacity of this distribution. Even though our results produced a correctclassification of 85% for non-pitted fruit, which can be useful, this was still below therequired tolerance, of less that 2%, expected on the market.The effect of pruning and fruit bearing position on two-year-old wood on dry mass and Caallocation of fruit was determined. 'Golden Delicious' fruit set was the lowest at the basalbearing position compared to the other positions evaluated and was contrary to expectations.Fruit in a terminal bearing position was superior to the basal position regarding total dryweight and fruit size. Distal wood possibly inhibited growth and set on the basal position viaauxin distribution. Ca allocation differed between seasons and cultivars and could either beinfluenced by bearing position or presence or absence of re-growth.

[发布机构] Stellenbosch University