Physiological disorder of Apple

Scald

This storage disorder is of serious concern for apple growers. Susceptibility to this storage  disorder varies with variety with the variety of apple,environment and cultural practices.granny smith , Rome Beauty, Delicious, Winesap and Yellow Newton varieties are very susceptible whereas Gala and Fuji are moderately susceptible. The Immature fruits are most susceptible to scald which is aggravated by warmer temperatures in storage. Light scald fruits may be severely affected during marketing.

Symptoms

Light mottling on greener surface of fruit are initial symptoms of scald. Irregular brown patches of dead skin develop within 3 to 7 days due warming of the fruit after removal from the cold storage. Darkening becomes more severe with elapsed time and ultimately extends to red surface also. Scald usually affects the skin only but in severe cases it may extend to fruit flesh. Symptoms may be visible  in cold storage when injury is severe. The warm temperatures do not cause the scald but allow symptoms to develop from previous injury, which occurred during cold storage.

Causes

• Hot and dry weather before harvest
• Immature fruit at harvest
• High nitrogen and low calcium in the fruit
• Inadequate ventilation in storage rooms or in packaging boxes

Control

• The most common method used to control scald is application of an antioxidant immediately after harvest
• Commonly used antioxidant is Diphenylamine.
• Spray of CaCl₂   two weeeks before harvest is very effective for controlling scald effectively
• Harvesting at proper maturity and ventilation in cold storage helps in reducing the scald incidence.

Bitter pit

Bitter pit is considered at the most serious disorder of apple which reduce the fresh market  quality of fruit. This physiological disorder is influenced by climate and orchard cultural practices.young tress that  are just coming into bearing are the most susceptible. The fruit of northern spy,Golden Delicious ,Yellow Newton and Gravenstein are most  susceptible although it affects almost all apple varieties.

Symptoms

Small brown lesions of 2-10 mm in diameters develop in the flesh of the fruit. Bitter pit is characterized by small sunken spots on the fruit surface which are more prevalent near the blossom end. the tissue below the skin becomes dark and corky. At the initial stage small water soaked areas appears which after loss of water shrink and turn brown and ultimately becomes brown and corky due to the dead tissue. Corky tissues are never bitter in taste. The spots generally turn darker, become more sunken than  the surrounding skin and get fully developed after one or two months in storage.

Causes

• Heavy doses of nitrogenous fertilizers as it result in lowering the soil pH or inducing  excessive vigour.
• Early  and over thinning  of fruits increases bitter pit
• Irregular water supply.

Control

• Avoiding excessive doses of nitrogeneous fertilizers.
• Thinning of fruits should be done judiciously.
• Maintaining moderates tree vigour and smaller fruit size.
• Harvesting  mature fruit

Internal browning

Importance

Because internal browning is not detectable externally, except in very severe cases, affected fruit can be discovered by buyers or consumers thereby affecting future confidence in the product. Although fruit susceptibility varies season to season and orchard to orchard, considerable losses of controlled atmosphere-stored fruit can result from this disorder.

Symptoms

Brown discoloration in the flesh, firm but moist, usually originating in or near the core. Brown areas have well defined margins and may include dry cavities resulting from desiccation. Symptoms can range from a small spot of brown flesh to nearly the entire flesh being affected in severe cases. When the entire apple is affected, a margin of healthy, white flesh usually remains just below the skin. Symptoms develop early in storage and may increase in severity with extended storage time.

Physiology

Evidence to date indicates that internal browning occurs as a result of CO2 injury to the apple. Injury incidence and severity increase with increasing concentrations of CO₂ in the storage atmosphere. Variability in susceptibility of apple varieties and in apples of different maturities may be due to anatomical differences (cell size, size of intercellular spaces) rather than biochemical differences. Later harvested fruit generally have greater susceptibility to internal browning and it has been shown in many fruit that diffusivity of CO₂ decreases as fruit mature and ripen. A reduction in the ability of CO₂ to diffuse out of the apple fruit would result in higher internal CO₂ concentrations and thus more chance of injury. Internal browning can be more severe in fruit when watercore is also present. The accumulation of sorbitol in the intercellular spaces would result in reduced gas diffusion in the affected tissue. Some reports suggest that internal browning injury is more severe at 0°C (32°F) than at 2.2°C (36°F). CO₂ solubility is greater at lower temperatures; however, the influence of temperature requires additional study before warmer storage temperatures could be safely recommended. Apple fruit can accumulate acetaldehyde and ethanol as well as various organic acids during exposure to high CO₂ concentrations; however, it is not known if these are causal agents of fruit injury.

Control

Avoid harvesting overmature fruit. Harvest at the optimum maturity, especially for controlled atmosphere storage. Maintain CO₂ concentrations below 1% in controlled atmosphere storage and air storage. CO₂ concentrations of less than 0.5% are recommended for Fuji apples in California. Assure good air circulation in storage rooms to prevent pockets with higher CO₂ concentrations. CO₂ can develop to damaging levels in air storage or marine containers if fruit temperature is 3°C or warmer and ventilation is not good. Proper temperature management and good ventilation will prevent buildup of CO₂. Avoid heavy wax coatings and thoroughly and rapidly cool fruit after waxing and packaging.

Water core 

All apple growing regions but especially arid or semi-arid climates. Susceptible varieties include Jonathan, Delicious, Stayman, Winesap, Granny Smith and Fuji. Water core is associated with high maturity fruit. Low night temperatures in fall hasten maturity and promote watercore development. Watercore is promoted by large fruit, high leaf to fruit ratio, high fruit nitrogen and boron, low fruit calcium, excessive thinning, high light exposure, Ethrel sprays, and girdling of trunks and limbs. Another type of watercore which is unrelated to maturity may occur during unusually hot weather. This watercore is found more on the exposed side of the apple and may be associated with sunscald.

Importance

Can be difficult to detect externally unless symptoms are very severe. If moderate to severe, results in reduced storage life of the apple as internal breakdown develops in affected areas. Affected fruit must be marketed rapidly. Incidence varies season to season but incidence can be very high.

Symptoms

A preharvest disorder resulting in water soaked regions in the flesh, hard and glassy in appearance, only visible externally when very severe. Water soaked areas are found near the core and around the primary vascular bundles but may occur in any part of the apple or involve the entire apple. Symptoms often increase rapidly as fruit become overmature but does not increase postharvest. If symptoms are mild to moderate, they may disappear completely in storage. However, when watercore is severe, internal breakdown can develop.

Physiology

The water soaked appearance of watercore affected fruit results from the accumulation of sorbitol-rich solutions in the intercellular spaces. Sorbitol is the carbohydrate source translocated into the fruit from the tree. Sorbitol must be converted to fructose by the apple fruit. The reason for accumulation in the intercellular spaces is not known. It is speculated that sorbitol may be translocated to the fruit faster than it can be assimilated perhaps due to an inability of the apple tissue to convert sorbitol to fructose. Comparative studies have shown differences in vascular tissues between susceptible and resistant varieties. The browning and breakdown that results from severe watercore is likely due to reduced gas diffusion in the affected tissue and may involve an accumulation of ethanol and acetaldehyde.

Control

The most effective way to reduce the incidence of water core is to avoid delayed harvests. As fruit approach harvest maturity, samples of fruit from the southwest quadrant of the tree should be cut to look for water core. Fruit should be harvested before water core develops extensively. A subsample of fruit at harvest should be cut prior to storage. Lots with moderate to severe water core should not be placed in CA storage but should be marketed quickly.