Managing Disorders in 'Honeycrisp' Apples
There has been considerable advancement in understanding the finicky and complex 'Honeycrisp' apple. Postharvest practices to obtain good fruit quality, reduce physiological disorders and extend the storage life of 'Honeycrisp' are reviewed below.
Harvesting at optimum fruit maturity is extremely important for maximizing storage life and maintaining good apple quality. However, determining the optimum harvest maturity for 'Honeycrisp' is difficult. Standard maturity indices, such as internal ethylene concentration, starch index, soluble solids concentration and fruit firmness are not always consistent. Current recommendations suggest that harvest should occur when the ground color begins to change from green to yellow and the starch index is close to 6 (on the Cornell chart). There has been no consistent relationship of internal ethylene to harvest date and differences in maturity do not always exist among 'Honeycrisp' apples with varying levels of red color. For example, brilliant red fruit can exhibit very similar internal ethylene concentrations, starch content, and firmness values as those having poor red coloration.
Figure 1. Left to right and top to bottom: soft scald, soggy breakdown, senescent browning, internal CO2 injury (with and without flesh cavitation), bitter pit, and lenticel blotch pit in 'Honeycrisp' apples.
Harvesting at optimum maturity is the best way to achieve the characteristic flavor of 'Honeycrisp'. Immature 'Honeycrisp' harvested too early will not ripen properly and those fruit do not develop good flavor and quality characteristics. Conversely, late harvested or over-mature 'Honeycrisp' can develop fermentation products, such as ethanol and acetaldehyde, which cause undesirable flavors and poor fruit quality. 'Honeycrisp' harvested at advanced maturity are also more prone to several major disorders.
'Honeycrisp' is extremely susceptible to physiological disorders (Figure 1) and there can be substantial variation among orchards and trees.
Soft scald is a major chilling-related disorder that can develop in 'Honeycrisp'. It is characterized by sharply defined, irregularly shaped, smooth, brown lesions of the skin. Peel tissue is initially affected and then hypodermal tissue is damaged as the disorder continues to develop. Skin lesions are often then invaded by secondary pathogens, such as Alternaria or Cladosporium.
Soggy breakdown is another major chilling-related disorder that can be found in 'Honeycrisp'. It is distinguished by moist, soft, brown, spongy flesh tissue, which can form complete rings in severe cases. Both soft scald and soggy breakdown develop more often in apples harvested at advanced maturity.
Senescent browning or diffuse flesh browning becomes more prevalent with extended storage durations and warmer temperatures. The fruit remains firm and there are usually no external symptoms. Advanced fruit maturity at harvest time also promotes this disorder.
Internal CO2 injury can become a major problem in 'Honeycrisp' when storing in controlled atmosphere (CA). Injury is characterized by flesh browning with or without flesh cavities, and it can sometimes be confused with soggy breakdown. 'Honeycrisp' tends to be extremely sensitive to CO2 and therefore injury can also be found in air-stored fruit.
Lenticel breakdown develops as darkened or black lenticels, or superficial small brown spots surrounding the lenticels. The lesions may become sunken over time and allow for the invasion of pathogens. Fruit with advanced fruit maturity are more susceptibility, as well as those in long-term storage. The disorder can be aggravated by various chemicals and coatings.
Bitter pit may appear prior to harvest or during storage, and usually develops in the calyx end of the fruit. Pits are dark, sunken lesions at or beneath the fruit surface. The cause for bitter pit is a mineral imbalance in the apple flesh, associated with low levels of calcium.
Lenticel blotch pit is similar to both lenticel breakdown and bitter pit. Irregular sunken patches form around the lenticels, which can resemble coalesced pits. Blotches usually form near the calyx end.
Prior to cold storage, conditioning at 10°C for 1 week is advised to reduce the incidence of soft scald and soggy breakdown. Conditioning at warmer temperatures has been shown to substantially reduce acidity, which has also been noted within sensory evaluations. Bitter pit can develop more rapidly at warmer temperatures, so conditioning at 10°C is a compromise between bitter pit and soft scald development. After conditioning at 10°C for 1 week, 'Honeycrisp' is best stored in ambient air at 3°C.
Limited success has been observed using CA storage consisting of 3% O2 and 1.5% CO2 at 3°C. CA storage tends to substantially reduce greasiness, as well as maintain acidity. 'Honeycrisp' tends to be very sensitive to CO2, so related disorders can develop easily (i.e. internal CO2 injury with or without flesh cavities). However, delaying the establishment of CA for 4 or 8 weeks has been shown to reduce these disorders in Ontario trials.
Ethylene production, respiration, and greasiness can be reduced by SmartFreshTM (1-MCP) on 'Honeycrisp'. 1-MCP tends to be slightly more effective when applied at the onset of the conditioning period at 10°C, compared to after that 1 week period. However, always be aware of any CO2 accumulation during the 1-MCP treatment, as this has potential to cause CO2 injury. There is little loss of firmness in 'Honeycrisp' during storage, so any improved firmness retention caused by 1-MCP treatment may be difficult to discern.
Diphenylamine (DPA) can reduce CA-related disorders in apples. DPA has been shown to reduce internal CO2 injury (with or without flesh cavities) in 'Honeycrisp' stored in CA.
Acknowledgements: Thanks to the Ontario Apple Growers, Apple Marketers' Association of Ontario, Les producteurs de pommes du Québec, AgroFresh Inc., and Storage Control Systems Inc. for their support throughout the duration of this research; as well as to Norfolk Fruit Growers' Association and Pommes Philip Cassidy Inc. for their direct collaboration; and Lorie Walker and Geoff Lum for technical assistance. Recent work pertaining to 'Honeycrisp' storage has been funded in part through Growing Forward 2, as part of the Canadian Agri-Science Cluster for Horticulture 2 and the Agri-Innovation program in partnership with Agriculture and Agri-Food Canada and the Canadian Horticultural Council.
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