Crop Load Affects Mineral Concentrations and Incidence of Bitter Pit in `Cox's Orange Pippin' Apple Fruit

Apple fruit (Malus domestics Borkh. CV. Cox’s Orange Pippin) were harvested in four orchards from trees growing under the same conditions but differing in crop load. Regardless of fruit size, apples from light-cropping trees had lower Ca and higher K concentrations and more bitter pit than did fruit from trees with heavy crop loads. The inverse relationship between Ca concentration in the fruit and the incidence of bitter pit also varied according to crop load and could affect the ability to predict incidence of bitter pit from Ca measurements. Differences in fruit maturity that would influence bitter pit incidence were not associated with crop load. The enhanced susceptibility to storage disorders, such as bitter pit, in fruit of all sizes from light-cropping trees suggests the need to handle fruit from such trees differently for postharvest storage. Light-cropping apple trees tend to bear fruit that are more susceptible to storage disorders such as bitter pit than are fruit from mediumto heavy-cropping trees (Ferguson and Watkins, 1989). The reason for this has been attributed largely to the increased size of fruit growing on light-cropping trees; large fruit are more susceptible to bitter pit since they tend to have lower Ca concentrations (Ferguson and Triggs, 1990; Perring and Jackson, 1975). Most research on the effect of crop load has involved manipulation of the crop by techniques such as fruit thinning and pruning. Thinning has resulted in ‘Cox’s Orange Pippin’ fruit with higher bitter pit incidence (Sharples, 1964, 1968), and while Ca concentrations were the same in thinned and control fruit in these experiments, K, P, and Mg were all higher in apples from thinned trees. Little effect of fruit thinning on bitter pit incidence was found in another study with the same cultivar (Van der Boon, 1980b). Summer pruning can also result in production of fruit with less better pit, although the effect is not consistently related to changes in Ca concentration (Marini and Barden, 1982; Perring and Preston, 1974; Van der Boon, 1980b). A better understanding of the effect of crop load on fruit quality may come from studies on natural variation, rather than from artificial manipulation of the crop. Palmer (1943) found higher bitter pit incidence in fruit from trees with light crops, and Van der Boon (1980a) found that high fruit : leaf ratios (heavy crop) correlated with lower bitter pit incidence. Because fruit size influences mineral concentrations and incidence of storage disorders, studies assessing the influence of crop load should compare fruit of similar sizes. Sharples (1968) found higher bitter pit incidence in fruit from thinned trees within four size classes. The relationship between Ca, fruit size, and bitter pit maybe affected by crop load (Ferguson and Triggs, 1990; Ferguson and Watkins, 1989). Consequently, the objective of the present study was to assess the effect of crop load on mineral concentration and quality of fruit after storage, particularly bitter pit for publication 5 July 1991. Accepted for publication 2 Jan. 1992. C. Batt. J.H. Bowen. R.L. Cotter. and A. Gunson for their assistance New Zealand Apple’ and Pear Marketing Board for financial supcost of publishing this paper was defrayed in part by the payment of rges. Under postal regulations, this paper therefore must be hereby dvertisement solely to indicate this fact. r. Soc. Hort. Sci. 117(3):373-376. 1992. incidence. Results on several apple cultivars suggest that fruit from light-cropping trees tend to mature earlier (Hulme, 1954; Palmer, 1943; Raphael and O’Loughlin, 1964; Sharples, 1964). Since bitter pit incidence is usually higher in early harvested fruit (Raphael and O’Loughlin, 1964; Watkins et al., 1989b), maturity has to be considered in an assessment of the effects of crop load on bitter pit incidence. We have consequently measured, over a range of fruit sizes, the effects of crop load on mineral content of the fruit, bitter pit incidence and storage quality, and on maturity of fruit at harvest. Materials and Methods Four orchards of ‘Cox’s Orange Pippin’ trees were chosen in the Nelson region, South Island, New Zealand. Within each orchard, a block was chosen that had trees growing under the same soil and cultural conditions. In each block, five uniform trees with a heavy crop load (120 to 130 kg/tree) and five with a light crop load (80 to 90 kg/tree) were chosen. From each tree, fruit covering a range of fruit sizes was picked at the time of commercial harvest (12 Feb. 1990). Fruit were weight-graded by orchard and crop load category into average fruit weights of 190, 168, 152, 138, 127, and 117 g, corresponding to commercial size classes of 100, 113, 125, 138, 150, and 163 (19 kg box), respectively. Two samples of 10 fruit per size class were taken for mineral analysis using two cortical plugs per fruit from each of 10 fruit (Turner et al., 1977). At the same harvest time, four fruit averaging 138 g from each tree were used to assess maturity. From each fruit, 1 ml of internal gas was drawn into a syringe through a hypodermic needle inserted into the core cavity and analyzed for ethylene by gas chromatography (Philips PU4500 fitted with a flame ionization detector; Philips, England). Background color was assessed using a Minolta Chroma Meter II color reflectance meter (Minolta, Tokyo) under CIE illuminant D65 conditions. Two readings were taken from each fruit and the a* : b* ratios were used to describe the changes in color (Hirst et al., 1990). The percentage of surface with blush was assessed visually by a single assessor. Flesh firmness was measured on opposite sides of pared fruit using an Effegi pressure tester (Effegi, Italy) fitted with an 11.1-mm-diameter head. Soluble solids concentration was measured with a refractometer using juice expressed during flesh firmness measurements. The starch pattern index