Apples

Watercore

Symptoms are a glassy, water-soaked appearance of the flesh resulting from accumulation of liquid, predominantly sorbitol, in the intercellular spaces (Marlow and Loescher, 1984). Watercored tissues are usually associated with vascular bundles of the core line, although other tissues may be affected. These include the flesh near the surface, which may develop watercore as a result of heat stress and, in severe cases, can be observed through the skin. Watercore develops only on the tree and can lead to crinkle, a disorder characterized by breakdown of the flesh and shallow depressions on the skin. During storage, the disorder can dissipate, but fruit with severe watercore can develop tissue breakdown.

Watercore occurs in most commercial cultivars, but some are more susceptible than others. In 'Fuji', its presence may be desirable, while in other cultivars such as 'Delicious', risk of watercore breakdown during storage has resulted in development of strict grade standards that prevent packing of affected fruit for markets. Development of watercore is associated with harvest of overmature fruit and/or low night temperatures. Watercore development may be due to changes in membrane integrity, rather than inability to metabolize sorbitol. Timing of harvest is the primary method to avoid or obtain (if desired) fruit with watercore.

Sunscald

Sunscalded areas, often golden-brown patches, occur on the exposed cheeks of fruit. The damaged areas may darken in storage. Sunscald development is a nonenzymatic and nonoxidative process. All cultivars can be damaged by sunscald, and because its development cannot be prevented by postharvest treatments such as diphenyl-amine (DPA), sunscalded fruit should be removed during grading.

Bitter Pit

Symptoms are discrete necrotic lesions on the skin and/or in the flesh, often occurring at the calyx end of the fruit first. Bitter pit is predominantly a storage-related disorder, although it is sometimes discernable on the tree. Another bitter pit-related disorder is lenticel blotch. Bitter pit is cultivar specific, with 'Cox's Orange Pippin' and 'Cortland' being particularly susceptible. Susceptibility to bitter pit is associated with early harvest and preharvest factors that result in low fruit calcium, such as low crop load and large fruit.

Both pre- and postharvest methods may be used to control risk of bitter pit development (Ferguson and Watkins, 1989). Preharvest methods include management practices to improve calcium availability in the soil, such as lime application, pruning and thinning practices that reduce competition between fruit and leaves, and application of calcium salt sprays during the growing season. Postharvest methods include harvesting more mature fruit, drenching fruit with calcium salts, rapid cooling of fruit, and application of storage conditions that delay fruit ripening, e.g., controlled atmosphere (CA). Prediction techniques for bitter pit risk, based on calcium and other minerals, have been developed in some growing regions.

Jonathan Spot and Lenticel Spot

In these disorders, brown to black spots develop on the skin, particularly on the blushed side of the fruit. Jonathan spot is characterized by haloes surrounding the lesions that occur randomly over the fruit surface, while lenticel spots may be slightly depressed areas around the lenticels. The disorders are reduced by avoiding excess nitrogen, harvest at optimum maturity, rapid cooling, and keeping fruit at the optimum storage temperature.

Senescent Breakdown

Senescent breakdown occurs widely in fruit stored at higher than optimal temperatures or for too long. Flesh softening is followed by development of mealiness and browning. The skin and flesh may split in advanced cases. Other senescent breakdown-like disorders that are recognized in the literature are Mcintosh breakdown, Jonathan breakdown, and Spartan breakdown.

Breakdown incidence can be reduced by pre- and postharvest calcium applications to fruit, harvest at the optimum stage of maturity, prompt cooling, and storage at optimum temperatures and humidities. CA storage generally reduces senescent breakdown incidence.

Superficial Scald (Storage Scald)

Development of superficial scald is associated with long-term, low-temperature storage of apples and is probably a chilling injury. Susceptibility to scald is affected by cultivar, growing region, and harvest date. 'Delicious' and 'Granny Smith' are highly susceptible, while 'Gala', 'Empire', and 'Braeburn' are scald resistant. Cooler growing regions have a lower scald risk, apparently related to the cooler nights that are experienced by the fruit before harvest. Typically, more mature fruit have lower scald risk than those harvested earlier.

Superficial scald is controlled by postharvest drenches of DPA. Product labels regulate the maximum DPA concentrations that should be used on specific cultivars to ensure control with a minimal risk of chemical damage. Risk of chemical injury increases if DPA is not discarded when soil accumulates in the solution, or if DPA is used with chlorine. DPA residues are not allowed on fruit by some import ing countries, and there is concern about possible consumer issues with postharvest chemical use. Therefore, nonchemical methods of control for superficial scald, such as low-oxygen CA storage, are used in some growing regions. A fungicide to reduce decay incidence, and calcium salts to reduce bitter pit or senescent breakdown incidence, are often applied with DPA.

Low-Temperature Breakdown

Symptoms include a diffuse browning of the outer cortex that develops into breakdown. Low-temperature breakdown can be distinguished from senescent breakdown by the occurrence of a band of unaffected tissue under the skin, dark vascular strands, and moistness of the tissue. However, at advanced stages it can be difficult to separate the two disorders.

Susceptible cultivars include 'Bramley's Seedling', 'Cox's Orange Pippin', 'Empire', 'Mcintosh', and 'Jonathan', when stored at temperatures less than 2 to 3°C. Disorder incidence increases with prolonged storage. Preharvest factors include low crop loads, large fruit, and cool weather during the latter part of the growing season. Low calcium and phosphorus concentrations in the fruit may be associated with higher susceptibility. High humidity and elevated carbon dioxide concentrations in the storage can increase fruit susceptibility to the disorder.

The primary control method is to maintain higher storage temperatures for susceptible cultivars. Because fruit maintain better firmness at lower temperatures, susceptible cultivars are sometimes kept at potentially injurious temperatures for short periods, the length of which varies by cultivar and growing region. Stepwise lowering of temperatures during the storage period has been utilized, but may detrimentally affect fruit quality (Little and Holmes, 2000). Fruit are typically stored at slightly higher storage temperatures under low-oxygen CA storage.

Soft Scald (Deep Scald)

Symptoms are discrete brown lesions that are smooth and slightly sunken where the underlying tissue has become affected. The flesh tissue is initially pale brown, soft, spongy, and moist and is sharply demarcated from the unaffected tissue. A similar disorder, known as ribbon scald, occurs as a low-oxygen injury.

Soft scald is a low-temperature injury of certain cultivars kept at less than 3°C. Susceptible cultivars include 'Jonathan' and 'Honey-crisp'. Susceptibility to soft scald is greater in fruit harvested later than earlier, which may be related to higher fruit respiration rates when cooled. In general, delays between harvest and storage increase injury development. Orchard factors that increase fruit susceptibility to the disorder are dull, cool, wet summers; light crops; large fruit; and vigorous trees on heavy soils. Control methods rely mainly on harvesting fruit at a less mature stage and use of storage temperatures above 2 to 3°C.

Brown Core (Coreflush)

This disorder is known as brown core in North America and coreflush elsewhere. Its symptoms include a pinkish or brownish discoloration of the core tissue, either as a diffuse circular area or as individual angular areas between the seed cavities. The discolored flesh tends to be firm and moist. Susceptible cultivars include 'Granny Smith' and 'McIntosh'.

Brown core incidence may be aggravated by late harvest and cool growing seasons and is more common in cooler than warmer growing regions. It is essentially a low-temperature disorder, but incidence has also been related to high carbon dioxide in CA storage and senescence. Control procedures include harvest at optimum maturity, avoiding low storage temperatures, and low-oxygen and low-carbon dioxide CA storage.

Low-Oxygen Injury

Symptoms consist of brownish areas with definite margins on the skin, which can range from small patches to most of the fruit. Internally, brownish corky sections occur, with occasional cavities that may be contiguous with external injury. Additional symptoms are alcoholic off flavors, brownish flesh discoloration caused by alcohol injury, bleaching or scalding of the skin, and purpling of the blushed areas of the skin. Skin purpling may be the first visual symptom of low-oxygen injury but is not always evident. Early stages of off-

flavor development and skin darkening can be reversed by removal of fruit to air.

Other forms of low-oxygen injury are epidermal cracking and ribbon scald. In epidermal cracking, the flesh tissue is usually dry and mealy and yields readily to pressure but is distinct from mealy breakdown. It may be aggravated by high humidity in the storage atmosphere. Ribbon scald appears as smooth, brown, irregular-shaped, well-defined lesions of the skin.

Cultivars vary in susceptibility to low-oxygen injury and in expression of injury symptoms, probably as a function of sensitivity of tissues to low oxygen concentrations and to physical features such as skin characteristics that affect gas diffusion into the fruit. Factors that increase risk of low-oxygen injury include late harvest, delays between harvest and application of CA storage, slow fruit cooling, and low storage temperatures. Low-oxygen injury is prevented by maintaining oxygen concentrations above the minimum for the cultivar, and by avoiding the factors described earlier. Other considerations include the length of exposure of fruit to low oxygen, carbon dioxide concentrations, and the storage temperature. Usually, storage temperatures for low-oxygen CA storage are higher than for standard CA storage.

Brown Heart (Core or Flesh Browning)

Affected fruit have patches of brown flesh, which may be distributed randomly or as a zone between the core and the flesh, depending on the cultivar. Usually fruit appear externally normal. However, the disorder can be observed on the fruit surface in severe cases. The brown tissue is initially firm and moist but may become dry with cavity formation. Development of brown heart usually ceases when causal conditions are removed.

The primary cause of brown heart is elevated carbon dioxide in the storage atmosphere, damage being related to concentration and length of exposure to the gas. Injury is usually associated with incorrect CA storage but can occur in air storage conditions where ventilation is poor, e.g., in cartons and in ship holds. Apple fruit are less sensitive to elevated carbon dioxide than pear fruit. However, in both fruit types, cultivar effects are important, perhaps reflecting anatomical differences such as size of intercellular spaces and rates of gas diffusion in the tissues.

Other factors affecting susceptibility to brown heart include growing region, orchard, and harvest date. Disorder risk is increased with more mature fruit, large fruit size, delayed cooling, low storage temperature, and low oxygen. The importance of each factor can vary greatly by cultivar. Delays between harvest and exposure to elevated carbon dioxide can reduce susceptibility of fruit to brown heart. In some cultivars, maintenance of low carbon dioxide during the first four to six weeks of CA storage is recommended to minimize risk. DPA used to control superficial scald also controls carbon dioxide injury, and fruit losses have occurred when DPA use has been discontinued.

External Carbon Dioxide Injury

Symptoms of external carbon dioxide injury are irregularly shaped, colorless, brown, or black lesions on the skin, often partly sunken with sharply defined edges. Appearance of injury under elevated carbon dioxide concentrations can occur within a few weeks. Factors that affect fruit sensitivity to the disorder are similar to those described for brown heart, except that early rather than late harvest increases injury risk of susceptible cultivars. Carbon dioxide concentration and duration of exposure, rapid establishment of high carbon dioxide before fruit are cooled, and presence of free moisture on the fruit surface can affect disorder incidence. DPA treatments control external carbon dioxide injury.

Pears

Bitter Pit

This disorder is also known as corky spot on 'Packham's Triumph' (South Africa), and cork spot or Anjou pit in 'D'Anjou' pears (United States). Symptoms may appear both before and after harvest. Bitter pit in pears is similar to that described for apples, and similar control methods apply.

Breakdown

External symptoms include skin yellowing during storage, while internally flesh softening, breakdown, and browning develop. A number of related disorders that develop in pears with breakdown, in cluding senescent scald, vascular breakdown, internal breakdown, and core breakdown, are described by Snowdon (1990).

As with senescent breakdown of apples, senescent breakdown in pears is a disorder of overmature fruit that are cooled slowly and stored at temperatures above the optimum, or for extended periods. The disorder, therefore, can be controlled by attention to these factors.

Superficial Scald (Storage Scald)

Development of superficial scald on pears is associated with long-term storage and has most of the characteristics described for apples. Scald is controlled by using postharvest ethoxyquin drenches, as DPA is not registered for pears.

Low-Oxygen and High-Carbon Dioxide Injuries

Brown core of 'D'Anjou' pears, pithy brown core of 'Bosc', and flesh browning or cavitation of 'Bartlett' occur with certain oxygen and carbon dioxide combinations (Lidster, Blanpied, and Prange, 1999). In brown core and pithy brown core, the core tissue surrounding the carpel turns brown, followed by cavity formation, while in flesh browning the cortex tissues next to the vascular region and outside the core line turn slightly brown and show many little cavities. Dark brown skin discoloration of 'D'Anjou' pears occurs under prolonged CA storage with low oxygen, high carbon dioxide, or a combination of both. These injuries can be avoided by maintaining appropriate storage atmospheres.

Peaches and Nectarines

Woolliness

Symptoms are the development of mealy texture, a lack of flavor and juiciness, and a failure of fruit to ripen when removed from cold storage. Susceptibility to woolliness development is associated with late-maturing cultivars, cool growing seasons, and harvest of less-mature fruit. Disorder incidence can be reduced by cultivar and harvest management and postharvest techniques such as modified atmo sphere storage, delayed storage, ethylene treatments, and intermittent warming of fruit during storage.

Internal Breakdown

Symptoms are diffuse, internal discoloration of the flesh and dry, soft flesh. The disorder mostly appears after transfer from low temperatures to ripening temperatures.

Low-Oxygen Injury

Low oxygen results in development of intense skin browning and grayish brown breakdown of the flesh near the skin or surrounding the stones. Low-oxygen injury can be distinguished from internal breakdown by presence of both external and internal symptoms, well-defined areas of injury, browning, and flesh injury that is not necessarily dry (Lidster, Blanpied, and Prange, 1999). Symptoms can appear at any time during storage. Injury can be avoided by maintaining appropriate storage atmospheres.

Plums

Internal Breakdown

Symptoms are internal browning near the stone followed by breakdown of the affected tissue into a gelatinous mass. Development of internal breakdown usually occurs after harvest, but it can be observed before harvest. Orchard and climactic factors, such as hot weather, predispose fruit to development of breakdown.

Cherries

Surface Pitting

Symptoms are irregular depressions that occur on the shoulders or sides of fruit (Looney, Webster, and Kupferman, 1996). Pitting results from impact damage, where cells beneath the skin dehydrate when injured. The majority of pitting occurs during packing operations. Warmer cherries are more resistant to damage and develop fewer pits than colder cherries when subjected to the same forces.

Storage at temperatures near 0°C or the transfer of fruit from cold storage to room temperature worsens pitting.

Control measures include minimizing damage events during harvesting and handling. Low-oxygen, high-carbon dioxide, and high-humidity atmospheres do not affect surface pitting incidence, but preharvest sprays or postharvest dips of calcium salts can decrease its incidence on 'Van' sweet cherries.

Low-Oxygen and High-Carbon Dioxide Injuries

Cherry stems are more sensitive than flesh to high-carbon dioxide (development of red-brown color) and low-oxygen (development of black-brown color) atmospheres (Lidster, Blanpied, and Prange, 1999). Fruit darkening associated with cell membrane rupture and leakage can occur after removal from inappropriate CA conditions. High carbon dioxide can also cause droplets of exudates to form on the fruit, followed by surface browning. These injuries can be avoided by maintaining appropriate storage atmospheres.

Many physiological disorders are known in temperate tree fruit. Most industries have the knowledge about cultivar susceptibility, orchard management, and storage techniques to minimize risk, although the impact of preharvest factors, especially climate, can markedly affect susceptibility and thus potential losses due to these disorders. Even though many postharvest injuries are caused by inappropriate storage regimens, there is major variation among cultivars of fruit in response to these treatments. A closer linkage between plant breeders, who often select material solely based on appearance and productivity, and postharvest scientists to ensure that selection decisions also incorporate susceptibility of fruit to physiological disorders should be encouraged.

Related Topics: FRUIT MATURITY; HARVEST; PACKING; POST-HARVEST FRUIT PHYSIOLOGY; PROCESSING; STORING AND HANDLING FRUIT

SELECTED BIBLIOGRAPHY

Ferguson, I. B. and C. B. Watkins (1989). Bitter pit in apple fruit. Hort. Rev. 11:289355.

Fernandez-Trujllo, J. P., J. F. Nock, and C. B. Watkins (2001). Superficial scald, carbon dioxide injury, and changes of fermentation products and organic acids in 'Cortland' and 'Law Rome' apple fruit after high carbon dioxide stress treatment. J. Amer. Soc. Hort. Sci. 126: 235-241.

Hansen, E. and W. M. Mellenthin (1979). Commercial handling and storage practices for winter pears, Agric. exper. station special report 550. Corvallis, OR: Oregon State Univ.

Kays, S. J. (1997). Postharvest physiology of perishable plant products. Athens, GA: Exon Press.

Lidster, P. D., G. D. Blanpied, and R. K. Prange, eds. (1999). Controlled-atmosphere disorders of commercial fruits and vegetables, Pub. 1847/E. Ottawa, Ontario: Agric. Agri-Food Canada.

Little, C. R. and R. J. Holmes (2000). Storage technology for apples and pears. Knoxfield, Victoria, Australia: Dept. of Nat. Resources and Envir.

Looney, N. E., A. D. Webster, and E. M. Kupferman (1996). Harvesting and handling sweet cherries for the fresh market. In Webster, A. D. and N. E. Looney (eds.), Cherries: Crop physiology, production and uses (pp. 411-441). Oxon, UK: CAB International.

Marlow, G. C. and W. H. Loescher (1984). Watercore. Hort. Rev. 6:189-251.

Snowdon, A. L. (1990). A color atlas ofpost harvest diseases and disorders of fruits and vegetables, Volume 1. Boca Raton, FL: CRC Press.

Wang, C. Y. (1990). Chilling injury of horticultural crops. Boca Raton, FL: CRC Press.

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