Physiological Disorders

T. R. Wright.

Diseases of fruits and vegetables caused by adverse environmental conditions during growth in fields or orchards or during harvest, storage, and marketing are called physiological diseases.

One important cause of a number of physiological disorders is suboxidation, or anaerobic respiration. The first part of this chapter is devoted to symptoms of suboxidation on a number of commodities; the latter section is devoted to the description of miscellaneous disorders.

Fruits and vegetables are living organisms. If their supply of oxygen is greatly reduced or withheld during postharvest handling they are likely to smother with progressive death of various parts of their tissues. Off-odors and off-flavors accompany the smothering of tissues, and the tissues become susceptible to decoy because of weakening and death of cells.

Potato black heart may occur in the field during excessively hot weather in waterlogged soils because the tubers cannot get enough oxygen to supply the respiratory demands at high temperatures under those conditions. It may occur in potatoes in transit when the car temperatures are allowed to go over 90 F., or in storage houses where bins are so large that the middle of piles receive insufficient ventilation, Flues through the piles and the false floors and walls with which newer storage houses are being equipped provide enough aeration to alleviate this trouble in most cases.

Excessive temperatures may induce black heart in storage by increasing the rate of respiration of tubers to such an extent that the oxygen within the tissues is used up more rapidly than it can be replenished. Black heart is externally visible only in the most serious cases. It appears then as moist areas, purplish at first and later changing to deep brown or black. Tissues of the central part of the tuber become dark gray, purplish, or black. The affected tissues, clearly delineated from healthy areas, are firm or leathery. They may dry out and separate to form cavities.

Snap beans are subject to surface blemishes of a chestnut-brown color, called russeting. Both green pod and wax pod varieties are affected. The lesions are of various sizes and shapes, but most often occur as parallel diagonal streaks. They resemble those of sunscald but occur on both sides of the pod. Although the cause of this defect has not been definitely established, similar symptoms may be produced by holding the beans in a moist, warm environment without ventilation. Russeting may develop during long periods in transit.

Lemons are affected sometimes by a disorder called albedo browning in which the albedo, the white spongy inner part of the rind, becomes discolored. It may be intense enough to show through the skin as a diffused browning that varies from a barely discernible blemish to quite extensive browning. The disorder is aggravated by storage at 32 and poor ventilation. Cranberries are subject to smothering when carbon dioxide accumulates and oxygen is depleted in overcrowded Storage rooms or in tight containers. The disorder usually affects berries near the center of containers and is associated with large amounts of decay. Affected berries lose their crispness and bright color, become dull red, and have a leathery texture.

Apple brown heart is a brown discoloration of the flesh of apples held for long periods in atmospheres containing high concentrations of carbon dioxide gas. The disorder has been particularly troublesome in ships exporting Australian apples to England. It may occur also when apples are held in sealed containers or are heavily coated with waxes that interfere with normal air exchange.

Packaging of consumer units of fresh fruits and vegetables in sealed transparent films has caused some suboxidation problems. The films differ in permeability to oxygen, carbon dioxide, and water vapor, and the different kinds of fruits and vegetables vary in their tolerance to atmospheres low in oxygen and high in carbon dioxide. In general it has been found advisable to use either unsealed or perforated packages in order to avoid suboxidation troubles.

Other functional disorders besides those caused by suboxidation develop in storage or transit, often without apparent cause.

Apple scald is a serious physiological disease of some apple varieties. It is a superficial browning that may become visible in storage or, more likely, after storage while the fruits are being marketed. Usually only the skin tissue is affected, the flesh remaining edible, but in serious cases the trouble may extend into the flesh. Then decay fungi readily invade the disrupted tissue.

Apple scald is usually associated with immaturity at harvest. Low storage temperatures do not reduce scald but retard its development. In fact, the disorder usually does not appear on susceptible fruits stored at 32 until after their removal to higher temperatures. Delay in storing the fruit is not conducive to scald development if enough ventilation is provided. The actual cause of scald, while associated with fruit volatiles, is still unknown.

Humidity, low oxygen levels, and excess carbon dioxide do not affect the incidence of scald. Early investigators tried a large number of absorbents for removing the gases given off by apples, but only those known to have an affinity for esters offered any degree of scald control. The best results were had with fats and oils, such as creamery butter, tallow, mineral oil, and neat's-foot oil. The most practical control was obtained by wrapping the fruit with paper wraps impregnated with mineral oil. Nearly as effective control of scald was obtained by scattering shredded oil-impregnated paper throughout the apple pack.

The development of oiled wraps made possible further investigation of scald. The most critical period in the postharvest life of apples was found to begin with picking and to extend 6 to 8 weeks. The scald-producing processes were most active and the control measures effective only when applied during that period.

Oiled wraps are being used throughout the world to control scald. The wraps are impregnated with about 15 percent by weight of tasteless, odorless, mineral oil. When shredded oiled paper is used a minimum of one-half pound for each bushel of apples is recommended for satisfactory results.

Oiled paper, however, will not entirely prevent scald. Extremely susceptible varieties, such as Arkansas (Black Twig), develop the disorder despite oiled paper but to a lesser extent. Susceptible varieties picked immature or held in poorly ventilated common or cold storages through the critical 6 to 8 weeks after harvest are so susceptible that oiled paper is of little benefit.

Charcoal air filters are used in some storage rooms to remove volatile products of respiration of the fruit. The air in the room is passed through the filters approximately three times an hour. Although scald is reduced by this air purification, it is generally controlled more effectively and economically with oiled wraps.

Apple bitter pit is an example of a physiological disorder that starts in the orchard but becomes important in storage. It causes nearly round, sunken spots one-eighth to one-fourth inch in diameter on the surface of the fruits and necrotic, corky areas in the flesh. Affected tissue may be bitter. The spots are usually brownish but may vary from gray green to reddish brown on the blush side of fruits. Bitter pit is associated with irregularities in moisture and nutritional requirements, particularly just before harvest. Australian investigators have suggested that the pits arise as the result of uneven conversion of starch to sugar bringing about unbalanced osmotic relationships between cells. The cells that remain starch-filled are subject to desiccation, collapse, and discoloration. Picking fruit when mature and prompt cold storage help keep damage at a minimum.

Jonathan spot of apples, which is a skin disease, appears during storage almost entirely on apples of the Jonathan variety. Its cause is unknown.. The round spots are blackish on the blush side and greenish brown on the pale side of the fruit and have sharply defined borders. The lesions range from pin-point size to three-eighths inch in diameter. During storage the spots may increase in size and merge. Secondary decay fungi often are found in the spots. Jonathan spot develops as the acidity of the fruits decreases with the approach of maturity. Storing the fruit in atmospheres containing carbon dioxide prevents it. The spotting may. be avoided largely by picking at an early stage of maturity and storing promptly at 32 F. Delays after harvest and common storage at temperatures above 40' allow ripening to progress and favor spot development.