Soft scald is a chilling-related, abiotic disorder of apple and pear (Pyrus communis L.) fruit in which the peel develops soft, brown, sunken lesions, but the interior initially looks fine (Brooks and Harley, 1934). The fruit flesh may eventually brown and have a soft and spongy texture and later turn dry. The disorder often appears after about 6–8 weeks of storage at temperatures below 2 °C. Apple cultivars susceptible to soft scald include Delicious, Fuji, Honeycrisp, Jonathan, McIntosh, Monark, and Rome Beauty (Watkins et al., 2004).
The metabolic reactions leading to soft scald remain unclear, and methods to mitigate its occurrence have not always been effective. Wills (1972) found that some hexyl compounds injected into apple cores induce the disorder. Tissue from fruit exhibiting soft scald contains less linoleic acid than unaffected tissue, and low linoleic acid content increases fruit susceptibility to soft scald (Hopkirk and Wills, 1981). Although postharvest application of some antioxidants decrease the incidence of soft scald, ascorbic acid, a water-soluble antioxidant, is not effective. In addition, the antioxidant diphenylamine reduces, but does not eliminate soft scald incidence in ‘Honeycrisp’ fruit (Watkins et al., 2004). Preconditioning fruit by holding them at a warm temperature before cold storage is the most effective method used by the industry, but can be ineffective under some conditions (Moran et al., 2010). Increasing success in predicting the probability of its occurrence would allow growers to better manage susceptible fruit.
Predicting the incidence of soft scald is difficult partly because of fruit-to-fruit, tree-to-tree, orchard-to-orchard, and year-to-year variation (Hopkirk and Wills, 1981; Lachapelle et al., 2013; Moran et al., 2009; Watkins et al., 2005). Because of the large variability in occurrence, identification of these unknown orchard factors may improve prediction of susceptibility. Lack of precipitation 30 d before harvest accounted for 53% of the year-to-year variation in soft scald incidence in Maine and 28% of the variability in Ontario (Moran et al., 2009). Precipitation early in the growing season, from full bloom until fruit reached 10 mm in diameter, was associated with increased soft scald incidence in Canada (Lachapelle et al., 2013). Predictive factors leading to tree-to-tree and fruit-to-fruit variation remain unknown. Not every fruit from a tree will develop soft scald, so soft scald incidence is scored as a percentage of a batch of stored fruit. Labor and storage space could be saved if growers could avoid harvesting susceptible individual fruit or to cull fruit at harvest that might develop soft scald during cold storage.
To determine whether commonalities exist in fruit susceptible to soft scald, the peel microstructure of ‘Honeycrisp’ fruit from different growing locations and of different genotypes from a ‘Honeycrisp’ breeding population with varying susceptibilities to soft scald were compared in this study.
Permanent address: Department of Bioengineering, Jiangsu Vocational College of Agriculture and Forestry, Jurong City, Jiangsu Province 212400, China.
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