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‘Honeycrisp’ apples are susceptible to bitter pit, a physiological disorder that impacts peel and adjacent cortex tissue. ‘Honeycrisp’ is also susceptible to chilling injury (CI) that can be prevented by holding fruit at 10 to 20 8C after harvest for up to 7 days. This temperature conditioning period reduces CI risk but can enhance bitter pit development. Previous research demonstrated a controlled atmosphere (CA) established during conditioning can reduce ‘Honeycrisp’ bitter pit development without inducing other physiological disorders. The objective of this research was to evaluate the duration of CA needed to reduce bitter pit development. Experiments were conducted in 2014, 2016, and 2017 with fruit obtained from commercial orchards in Washington State and, in 2017 only, Ontario, Canada. Half the fruit were treated with 42 mmol·L 1-methycyclopropene (1-MCP) for 24 hours at 10 8C immediately following harvest. The untreated fruit were held at the same temperature (10 8C) in a different cold room. Following 1-MCP treatment, all fruit were conditioned at 10 8C for an additional 6 days, then fruit was cooled to 2.8 8C. During conditioning, fruit were held in air or CA (2.5 kPa O2, 0.5 kPa CO2) established 1 day after harvest, for 1 to 8 weeks, then in air. All fruit were removed from cold storage after 4 months and then held 7 days at 20 8C. Fruit from most orchards/years stored in CA developed less bitter pit compared with fruit stored continuously in air. CA during conditioning also reduced poststorage peel greasiness but CA for 2 weeks or longer enhanced cortex cavity development in some orchard lots. Treatment with 1-MCP did not reduce bitter pit but enhanced development of peel leather blotch and core browning for some orchards/years. 1-MCP–treated fruit slowed the loss of soluble solids content, titratable acidity, and reduced internal ethylene concentration.Results suggest the potential for postharvest management of bitter pit development in ‘Honeycrisp’ apples by CA established during conditioning with minimal development of other postharvest disorders. ‘Honeycrisp’ apples (Malus ·domestica) are a high-value cultivar with a desired crisp texture and a distinct flavor profile. These characteristics make this cultivar popular among consumers (Luby and Bedford, 1992; Mann et al., 2005; Yue and Tong, 2011). Increased ‘Honeycrisp’ production (U.S. Apple Association, 2018) has resulted in a need for strategies to improve the storage performance to maintain quality and reduce fruit losses. One challenge is controlling bitter pit, a physiological disorder that is associated with fruit calcium content (Miqueloto et al., 2014; Rosenberger et al., 2004). Economic losses resulting from bitter pit can be considerable, and incidence is often unpredictable annually, between orchards in the same region, or even tree-to-tree. Bitter pit symptoms are characterized as depressed brown lesions in and just beneath the peel within the first 5 mm, especially in the distal portion of the fruit (Amarante et al., 2006, 2013; Cobb, 1895; Ferguson and Watkins, 1983, 1989; Freitas et al., 2010; Garman and Mathis, 1956; Jaeger, 1869). Bitter pit may be visible at harvest, but typically develops during the initial storage period. Factors increasing susceptibility of ‘Honeycrisp’ to bitter pit are hot, dry growing conditions (Watkins, 2009), young trees (less than 5 years old) (Rosenberger et al., 2004), excessive vegetative vigor (W€unsche and Ferguson, 2005), large fruit size (Telias et al., 2006), fruit nutrient content (Rosenberger et al., 2004; Telias et al., 2006; Torres et al., 2017), fruit maturity at harvest (Prange et al., 2011), and crop load (Robinson and Watkins, 2009; Serra et al., 2016). Bitter pit can be more severe in fruit picked immature compared with fruit harvested more mature (Le Grange et al., 1998; Perring and Pearson, 1986; Prange et al., 2011; Volz et al., 1993). Practices that can reduce ‘Honeycrisp’ bitter pit development include field-applied calcium (Biggs and Peck, 2015; Peryea et al., 2007; Rosenberger et al., 2004), optimal crop load (Delong et al., 2006; Robinson and Lopez, 2012) and harvest at optimal maturity (Prange et al., 2011). After harvest, bitter pit development can be reduced by calcium dips (Reid and Padfield, 1975) as well as CA storage (Hewett, 1984; Sharples, 1982; Webster and Forsyth, 1979). A short period of CA that creates low oxygen stress established during temperature conditioning also can reduce bitter pit development (Pesis et al., 2010) as can a nonstress CA that is continued throughout cold storage (Mattheis et al., 2017). The ethylene action inhibitor (1-MCP) prevents ethylene-mediated ripening processes in apple fruit (Fan et al., 1999a, 1999b). This treatment also impacts development of fruit physiological disorders including senescent breakdown, CO2 injury, CI, and superficial scald (Blankenship and Dole, 2003; Contreras et al., 2014; DeEll, 2010; DeEll et al., 2015; Fan et al., 1999b; Watkins, 2008; Watkins and Nock 2012). Reduction in bitter pit following 1-MCP treatment and/or CA established during ‘Honeycrisp’ conditioning has been reported (Mattheis et al., 2017; Mirzaee et al., 2014). However, other studies reported inconsistent effects of postharvest 1-MCP treatments on apple bitter pit development (Gago et al., 2015; Mirzaee et al., 2015). Postharvest 1MCP application to ‘Honeycrisp’ apples can affect fruit quality by limiting soluble solids content (SSC) and titratable acidity (TA) loss during air storage (DeEll, 2010). Postharvest applications of 1-MCP also can increase CArelated internal disorders such as CO2 injury, cavities, and internal browning (DeEll, 2010; Watkins and Nock, 2012). ‘Honeycrisp’ is a chilling sensitive cultivar and can develop soft scald and soggy breakdown when cooled immediately after harvest (Watkins and Rosenberger, 2000). However, the use of a temperature conditioning period at a relatively warm storage Received for publication 30 Apr. 2019. Accepted for publication 30 May 2019. Financial support for this research was received from the Washington Tree Fruit Research Commission. Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture. We thank Janie Countryman, Brenda Steady, and Katie Mullin for excellent technical assistance. Corresponding author. E-mail: james.mattheis@ ars.usda.gov. 1532 HORTSCIENCE VOL. 54(9) SEPTEMBER 2019 temperature (10 to 20 C) after harvest for up to 7 d followed by storage at 3 C can reduce the development of CI during cold storage (Contreras et al., 2014; Delong et al., 2004, 2006, 2009; Watkins et al., 2004). Impacts of delaying cooling can be cultivar specific and are known to reduce (Argenta et al., 2000; de Castro et al., 2007) or enhance disorder development (DeEll et al., 2016; Neven et al., 2000; Watkins et al., 2004). Storage temperature and CA gas composition affect apple physiological disorder development (Watkins and Liu, 2010). Critical O2 and CO2 concentrations that can lead to injury vary with cultivar (Gran and Beaudry, 1993) and temperature can influence fruit response to CA as respiration rate increases with temperature. ‘Honeycrisp’ apples can be sensitive to injury during CA (Contreras et al., 2014; DeEll et al., 2015; Watkins and Nock, 2012), with delayed establishment of CA known to reduce injury development risk (DeEll et al., 2016). Commercially, conditioning is a typical ‘Honeycrisp’ management practice regardless of how long fruit will be stored. As some fruit are marketed soon after conditioning is completed, the duration of CA established during conditioning that is necessary to reduce bitter pit development is a relevant commercial issue. The objective of this study was to determine if a short period of CA initiated during conditioning with or without previous 1-MCP treatment affects the development of bitter pit and other physiological disorders as well as fruit quality of ‘Honeycrisp’ apples. Materials and Methods Plant material and postharvest treatments. ‘Honeycrisp’ apples were obtained in Washington State (WA) at commercial harvest from two orchards in 2014 (lots A and B), two in 2016 and 2017 (lots C and D), and an additional lot in 2017 (lot E) in Ontario, Canada (ON) (Table 1). Apples without external disorders were selected the day of harvest and placed onto pressed fiber trays (WA) or into plastic containers (ON). All fruit were held at 10 C and some were exposed to 42 mmol·L 1-MCP (AgroFresh, Inc., Spring House, PA) for 24 h in an 800-L gas-tight metal cabinet (WA) or an air-tight treatment tent (ON; DeEll and Lum, 2017). Control fruit were not held in the same areas where 1-MCP application was performed. After 24 h, fruit was removed from the treatment chamber or tent and moved to the same cold room as controls. All fruit were held at 10 C for 6 additional days, then the storage temperature was reduced to 2.8 C. During conditioning, fruit were stored in air or in CA (2.5 kPa O2, 0.5 kPa CO2) established the day after harvest. WA fruit were stored in 0.14-m CA chambers for up to 8 (2014) or 2 weeks (2016, 2017). The CA system was operated as described previously (Mattheis et al., 2017). ON fruit were stored in boxes in air or CA as described in DeEll and Lum (2017). After CA, all fruit were held in air for 4 months followed by 7 d at 20 C after removal from cold storage. Harvest maturity and fruit quality assessment. Fruit maturity and quality were evaluated from a random sample of 10 (ON) or 16 (WA) fruit on the day of harvest. Nondestructive (weight, fruit size, peel background color) and destructive (firmness, starch score, SSC, TA, internal disorders) assessments were as previously described (DeEll and Lum, 2017; Mattheis et al., 2017). Internal ethylene concentration (IEC) of ON fruit was determined by withdra

Responses of ‘Honeycrisp’ Apples to Short-term Controlled Atmosphere Storage Established During Temperature Conditioning