Skin appearance is an important quality characteristic in almost all fruit crops including apples. Compromised skin appearance usually results in reduced market value. Russeting is a common and familiar example of a surface disorder of some apple cultivars. Skin spot is less well known but an economically important apple disorder in ‘Elstar’ and occasionally in ‘Golden Delicious’ (Grimm et al., 2012). The skin spot disorder is usually thought to appear after removal from CA storage (Hennecke et al., 2008; Veltman et al., 2003). Anecdotal reports suggest that fruit are particularly susceptible to skin spot if they are poorly colored, from dense canopies, grown in wet years, or harvested late (Köpcke et al., 2002).
Recent investigations have established that skin spot comprises patches of small, brownish spots arranged in a matrix and caused by the death of individual epidermal and hypodermal cells underlying microcracks in the cuticle (Grimm et al., 2012). The protoplasts appear brownish and coagulated. An individual spot is “sealed off” from the surrounding living tissue by lignin impregnation in the cell walls. Based on these observations, we hypothesize that the skin spot disorder is preceded by the formation of microcracks in the cuticle that result in the desiccation and death of the underlying cells. After a cell’s death, the loss of membrane integrity results in the oxidation of phenols and consequent browning, thus creating a small, brown “dot.” A grouping of these dots creates a skin spot. According to our hypothesis, cuticular microcracking is a prerequisite for the skin spot disorder. Interestingly, surface moisture induces microcracks in the cuticles of many fruit crop species, including apples (Knoche et al., 2011; Knoche and Grimm, 2008).
The objectives here were to identify the causal–chain relationship among fruit surface water applied by overhead sprinklers, the formation of microcracks in the cuticle, and the severity of skin spot in ‘Elstar’ apples grown under rain shelters. In addition, we analyzed weather records for the 2005–13 growing seasons to look for potential correlative relationships between the occurrence of rain and the severity of the skin spot disorder.
Curry, E.A. 2008 Effects of 1-MCP applied postharvest on epicuticular wax of apples (Malus domestica Borkh.) during storage J. Sci. Food Agr. 88 996 1006
Curry, E.A. 2009 Growth-induced microcracking and repair mechanism of fruit cuticles. Proc. SEM Annu. Conf., Soc. Expt. Mechanics, Albuquerque, NM. 23 Dec. 2013. <http://sem-proceedings.com/09s/sem.org-SEM-2009-Ann-Conf-s078p04-Growth-induced-Microcracking-Repair-Mechanisms-Fruit-Cuticles.pdf>
Grimm, E., Khanal, B.P., Winkler, A., Knoche, M. & Köpcke, D. 2012 Structural and physiological changes associated with the skin spot disorder in apple Postharvest Biol. Technol. 64 111 118
Knoche, M., Khanal, B.P. & Stopar, M. 2011 Russeting and microcracking of ‘Golden Delicious’ apple fruit concomitantly decline due to gibberellin A4+7 application J. Amer. Soc. Hort. Sci. 136 159 164
Maguire, K.M. 1998 Factors affecting mass loss of apples. PhD thesis, Massey University, Palmerston North, New Zealand
Maguire, K.M., Lang, A., Banks, N.H., Hall, A., Hopcroft, D. & Bennett, R. 1999 Relationship between water vapour permeance of apples and micro-cracking of the cuticle Postharvest Biol. Technol. 17 89 96
Quast, P. 2004 Beispiele zum Wirkungspotenzial von 1-MCP bei der Reifehemmung verschiedener Apfelsorten unter verschiedenen Lagerbedingungen Mitteilungen des Obstbauversuchsringes des Alten Landes 59 329 335
Roy, S., Conway, W.S., Watada, A.E., Sams, C.E., Erbe, E.F. & Wergin, W.P. 1999 Changes in the ultrastructure of the epicuticular wax and postharvest calcium uptake in apples HortScience 34 121 124
Veltman, R.H., Verschoor, J.A. & van Dugteren, J.H.R. 2003 Dynamic control system (DCS) for apples (Malus domestica Borkh. cv. ‘Elstar’): Optimal quality through storage based on product response Postharvest Biol. Technol. 27 79 86