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Cindy B.S. Tong, Hsueh-Yuan Chang, Jennifer K. Boldt, Yizhou B. Ma, Jennifer R. DeEll, Renae E. Moran, Gaétan Bourgeois, and Dominique Plouffe

Multiple types of flesh browning can occur as storage disorders in ‘Honeycrisp’ apple (Malus ×domestica Borkh.) fruit. Predicting its occurrence is hindered by differing definitions of the types of browning, incomplete understanding of their etiologies, and difficulty in assessing harvest maturity of ‘Honeycrisp’ fruit. In 2013, of ‘Honeycrisp’ fruit grown, harvested over multiple weeks, and stored in Maine, Minnesota, Ontario, and Quebec, only the Quebec fruit developed diffuse flesh browning. A detailed comparison showed that the Quebec fruit differed in size, but not in other quality attributes, from fruit of the other locations. The Quebec fruit experienced lower temperatures during active fruit growth and were increasing in cell size up to harvest. Analyses of climate data from 2009 to 2015 indicated that accumulated growing degree-days (GDD) 50–60 day after full bloom (DAFB) could account for 31% of the variation in diffuse flesh browning, and seasonal GDD <500 are associated with a greater likelihood of injury. Fruit that exhibited diffuse flesh browning had higher magnesium and lower fructose levels than unaffected fruit. As these measurements were made after browning was assessed, the timing of the onset of these characteristics in relation to browning cannot be determined.

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Yin Xu, Yizhou Ma, Nicholas P. Howard, Changbin Chen, Cindy B.S. Tong, Gail Celio, Jennifer R. DeEll, and Renae E. Moran

Soft scald is an apple (Malus ×domestica Borkh.) fruit disorder that appears in response to cold storage after about 2–8 weeks. It appears as a ribbon of dark tissue on the peel of the fruit, with occasional browning into the flesh. Several apple cultivars are susceptible to it, including Honeycrisp. The objectives of this study were to examine the cellular microstructure of fruit exhibiting soft scald and determine if any aspect of the peel microstructure at harvest could be indicative of future soft scald incidence. Light and electron microscopy were used to examine the peel microstructure of ‘Honeycrisp’ fruit that were unaffected or affected by soft scald. Tissue with soft scald had brown pigmented epidermal and hypodermal cells, whereas unaffected fruit peel epidermal cells were unpigmented. Cuticular wax of unaffected peel had upright wax platelets or clumps of wax, but peel surfaces with soft scald exhibited flattened granules and were more fragile than that of unaffected fruit. Epidermal cells of fruit with soft scald were more disorganized than that of unaffected fruit. Light microscopy was used to examine peels of ‘Honeycrisp’ fruit from four growing locations and fruit from a ‘Honeycrisp’ breeding population at harvest. ‘Honeycrisp’ and ‘Honeycrisp’ progeny fruit were also stored at 0 °C for 8 weeks and scored for soft scald incidence. Cross-sections of unaffected peel of stored ‘Honeycrisp’ fruit looked similar to that of freshly harvested fruit. No significant correlations were found between soft scald incidence and measured microstructural attributes of ‘Honeycrisp’ fruit at harvest, suggesting that peel microstructure cannot be used to predict possible soft scald incidence after storage.

Open access

Cindy B.S. Tong, Hsueh-Yuan Chang, James J. Luby, David Bedford, Benham E.L. Lockhart, Roy G. Kiambi, and Dimitre Mollov

MN55 is an apple (Malus ×domestica Borkh.) cultivar recently released by the University of Minnesota apple breeding program, with fruit marketed in the U.S. as Rave®. When stored for 4 months at 0 to 4 °C, MN55 fruit can develop several storage disorders, including skin dimpling. Skin dimpling incidence was greater for fruit harvested 1 week later than those harvested earlier. Dimpling was not alleviated by prestorage treatments of 1-methylcyclopropene or diphenylamine or by holding fruit at room temperature for 1 day before long-term cold storage. However, dimpling incidence was very low when fruit were stored at 6 to 7 °C. Because viruses have been implicated in other fruit dimpling disorders, the presence of viruses in MN55 leaves and fruit was studied. Apple stem pitting virus (ASPV) was detected by microscopy, reverse transcriptase polymerase chain reaction (RT-PCR) methodology, and high throughput sequencing (HTS) in peel of fruit from MN55 trees that exhibited skin dimpling after 4 months of storage at 0 to 1 °C. ASPV was also detected in supermarket-purchased fruit of other cultivars with noticeable skin dimpling. Although ASPV was not conclusively demonstrated to cause skin dimpling in our work, its prevalence indicates that further investigations are warranted to determine the relationship between viruses and skin deformities in stored apples.