‘Honeycrisp’ and ‘McIntosh’, two major cultivars in Canada and the United States, are prone to fruit quality loss and storage disorders that are directly influenced by harvest maturity (Smock, 1977; Watkins et al., 2005). Harvest at early stages of maturity can lead to bitter pit, whereas harvest at later stages can lead to chilling injury (CI) during storage in ‘Honeycrisp’ (Watkins et al., 2005). Harvest of ‘McIntosh’ at an early stage can lead to core browning and at advanced maturity to excessive softening (Meheriuk et al., 1994). To minimize these problems, growers depend on accurate measurement of fruit maturity at harvest.
Several maturity indicators are used to schedule harvest, but accuracy in measuring changes in maturity varies with the cultivar. The starch pattern index (SPI) has been used as a fruit maturity indicator in many cultivars (Blanpied and Silsby, 1992), but it is not a consistent indicator of maturity in ‘Honeycrisp’ (Watkins et al., 2005). The starch pattern index is also influenced by crop load and weather in addition to fruit maturity (Serra et al., 2016; Smith et al., 1979). An increase in internal ethylene concentration (IEC) and ethylene production have been useful for determining when to complete harvest of ‘McIntosh’ (Blanpied and Silsby, 1992; Dilley and Dilley, 1985), but the rise in IEC does not consistently match advancing maturity in some varieties, or decreases from an initial spike in concentration (Doerflinger et al., 2016; Watkins et al., 2005). Ground color changes and chlorophyll breakdown occur in the peel during ripening (Knee, 1972), but this can be subjective in the case of ground color and difficult to measure on large numbers of fruit in the case of chlorophyll.
The Delta Absorbance Meter (DA meter) indirectly measures the decrease in chlorophyll a concentration in the peel by measuring the index of absorbance difference (IAD) or the difference in absorbance at wavelengths 670 and 720 nm. Because of its convenience, it has been tested on many cultivars including Honeycrisp, but harvest values may be region specific (DeLong et al., 2014) and cultivar dependent (Farneti et al., 2015; Nyasordzi et al., 2013). For ‘Ambrosia’ and ‘Honeycrisp’, IAD is a better measure of harvest maturity than SPI, which can be sensitive to yearly variation and crop load (Serra et al., 2016; Toivonen, 2015). An indicator of maturity should be consistent from year to year and from site to site within a region. Year-to-year and orchard-to-orchard stability occurred in the use of IAD as an indicator of when to harvest ‘Ambrosia’ (Toivonen, 2015). Consistency among orchards was reported for ‘Honeycrisp’ in the region of Nova Scotia (DeLong et al., 2014). Variation among regions may occur but has not been documented. Standards that are region- and cultivar-specific would be needed for accurate measurement of fruit maturity but can be cumbersome to deploy effectively for the many cultivars in production and for growers who manage several cultivars. Developing one standard for regions with similar growing conditions would simplify the use of IAD as an indicator of harvest maturity.
The ability to predict storage disorders at harvest could prevent loss of fruit if highly susceptible apples could be detected and segregated from apples placed in storage. This has been shown with superficial scald in the less susceptible ‘Cripps Pink’, but not in the highly susceptible ‘Granny Smith’ (Farneti et al., 2015). In ‘Honeycrisp’ grown in Nova Scotia, apples have reduced the risk of bitter pit and soft scald when IAD is in the range of 0.36 to 0.59 (DeLong et al., 2014). However, these studies were based on harvest means of IAD, and they do not consider the variation of maturity that can occur within one harvest date or additional factors involved in disorder development (such as air temperature and precipitation) that also change with time while fruit remain on the tree to further ripen. Segregation of fruit into IAD classes may increase our understanding of how maturity influences susceptibility to soft scald and bitter pit without the interference of other orchard factors that change with calendar date, a necessary step in developing predictive models.
The objective of this research was to evaluate the regional variation and predictive capabilities of harvest IAD in ‘Honeycrisp’ and ‘McIntosh’ apples to determine the universality of IAD as a harvest indicator.
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