Apple Scald, a Complex Problem
Scald is a term loosely applied to a group of skin disorders of apples and pears. It involves brown or gray discoloration of irregularly shaped areas on the surface of the fruit during or following storage. On apples, Wilkinson and Fidler (5) described the following forms of scald:
Skin initially develops a faint bronze color; but later these areas turn light brown to very dark brown. The surface layers of cells are dead and so they dry out and collapse, leaving a brown, sunken appearance: Usually many lenticels remain green, however, standing out prominently from the sunken areas.
The lenticels do not remain green, the injury progressively invades deeper into the flesh, and areas often slough off because they remain moist.
Lenticel spot scald
The injury is predominantly around the lenticels, so that it appears as a spotting rather than a blotchy disorder.
The injury is primarily on the shoulder, radiating from the stem-end cavity, which remains relatively free of the disorder.
It appears that all of these forms are expressions of the same problem; specific cultivars are more prone to one form or another. However, many other forms of fruit injury also may cause skin damage similar to one of these forms of scald. For example, we have noted frequently a large amount of lenticel spoking after storage which is clearly the result of field treatments, presumably pesticides, even though there was no evidence of damage at harvest. This spotting could easily be mistaken for lenticel scald. Especially on McIntosh, we often see black scald, a clearly defined black area almost always occurring on the red side of the fruit. This injury is actually a form of sun scald, even though it usually is not present at harvest, and could be confused with browning scald. When fruit is very ripe, friction damage can cause injury that could be confused with either browning scald or stem-end browning. On pears, scald is often a symptom of over-storage. Thus, there is often much confusion about what is being called scald.
Nature of Scald
True scald is an expression of damage and death within the surface layers of cells in localized regions. It never occurs on the tree, only after relatively long periods of storage. Its development is believed to be divided into four stages.
The first 6 to 8 weeks after harvest, when changes occur in the fruit that create the potential for scald development, although scald does not yet occur.
The next 5 to 8 weeks when changes continue so that scald can no longer be prevented although it still has not appeared.
The remainder of storage, when scald may slowly appear.
Poststorage, when scald rapidly develops. Thus, the first 6 to 8 weeks after harvest are crucial for applying scald control measures, and poststorage conditions can determine how extensively the scald symptoms will appear. For example, we have noted much more scald under humid postharvest conditions than under dry ones.
Causes of Scald
An outstanding series of research papers in the late 1960s and early 1970s, mostly from Australia, established much of what we know about the chemistry of scald development. It was shown that early in storage fruit accumulate a chemical called alpha-farnesene; being a volatile compound, much of it can evaporate from the fruit. As storage time lengthens the alpha-farnesene is oxidized to a group of compounds called conjugated trienes, which do not evaporate and continue to accumulate as long as the fruit are kept in storage. These conjugated trienes apparently are toxic to the cells, damaging them and eventually causing their death, which is accompanied by their brown or black discoloration, by drying out, and collapse. Since most of the alpha-farnesene is found in the fruit peel, most of the conjugated trienes are made in the peel, and therefore, these are the cells that are killed.
Factors Affecting Scald
Different cultivars vary greatly in scald susceptibility. For example, Cortland is extremely susceptible and nearly was abandoned until effective scald-control methods were developed. On the other hand, Golden Delicious has very low susceptibility. The huge increase in production of Granny Smith worldwide has intensified concern about scald, since this cultivar is extremely scald susceptible.
Susceptibility of a given cultivar is not constant, however. It is widely recognized that immature fruit tend to be more susceptible than overmature fruit. Although this relationship is not invariably true, it is strong enough that growers should be much more concerned about scald on early-picked than on late picked fruit.
Color is another important factor. Scald is more likely to occur on a green area than on a well-colored area of the fruit. This relationship is probably indirect; good exposure to sun is probably what reduces scald susceptibility, rather than red pigments. Thus, the production of red strains of susceptible cultivars largely obscures the fact that shaded areas and shaded fruit are more susceptible than exposed areas and exposed fruit. Excessive tree vigor and inadequate pruning (hence, fruit shading) probably increase scald susceptibility, while summer pruning probably decreases it.
Scald susceptibility varies considerably from year to year for a given cultivar. To a large extent this variability is the result of the influence of weather on scald susceptibility. Studies in England (2) showed that weather conditions from late July to the beginning of September were very important. Hot, dry weather increased scald susceptibility; cool, damp weather decreased it. Indications were that water stress may have been more important than temperature in this relationship. Studies in New Jersey (3) showed that hot weather shortly before harvest increased scald susceptibility. When Stayman Winesap apples had experienced 190 or more hours of temperatures below 50°F they did not develop scald, but as this total dropped, scald susceptibility increased. Thus, a cool moist August and a cool harvest season should greatly reduce scald susceptibility; whereas, a hot, dry August and a hot September should increase it. How these two periods interact is not clear. For example, 1987 had a hot dry August but a cool September. Is one of these situations more important than the other?
Numerous approaches to controlling scald have been developed, since losses to the disorder can be devastating. When researchers recognized that scald was caused by a volatile compound, they aimed early approaches at maximizing evaporation of the compound from the fruit during storage. These techniques included use of air purifiers in the storage, storage ventilation, and paper wraps that were impregnated with mineral oil. These techniques reduced the amounts of scald that developed, but did not control it.
Controlled atmosphere (CA) storage greatly reduces scald. Both low oxygen and high carbon dioxide can be effective. However, since carbon dioxide is most effective at concentrations above 5% and most cultivars are susceptible to carbon dioxide injury above 5%, for most cultivars the greatest benefit from CA is from the low oxygen. The low oxygen impeded oxidation of alpha farnesene to conjugate trienes, the toxic materials. This effect is much greater at 1 to 2% oxygen than at 3% oxygen, and many researchers have shown that scald can be nearly completely controlled at 1 to 1.5% oxygen. However, in the Northeast we have generally been unable to store fruit at less than 3% oxygen, so we are unable to take full advantage of the scald control from CA. At our recommended CA conditions, scald is still a potential problem.
An important factor in scald control through CA is the rapidity with which CA conditions are established. Delayed sealing or slow generation of an atmosphere can greatly increase the risk of scald development after storage. Rapid CA is an excellent scald control measure, especially where oxygen cannot be reduced below 3%.
Ethylene-scrubbing during CA storage can also control scald. In England, scald virtually was eliminated from fruit taken from a commercial ethylene-scrubbed storage (1). However, the feasibility of ethylene scrubbing in commercial storage for most cultivars is doubtful, so this method seems to have limited application.
The most reliable scald-control measure is probably the use of the antioxidant chemicals diphenylamine (DPA) and ethoxzyquin. In the mid-1950s, Smock (4) found that these materials provided excellent control of scald. Following their approval by the Food and Drug Administration, they became standard commercial treatments as postharvest dips for fruit destined for long-term storage. These materials interfere with the oxidation of alpha-farnesene to conjugated trienes, as does low oxygen in a CA atmosphere.
Use of antioxidants is not without its problems. The materials must be used with care, since excessive dosage can cause severe fruit injury. Even use at recommended dosage often leads to injury due to entrapment of solution in cavities, between fruit, or in wooden containers. As this trapped solution evaporates, the antioxidant concentrates to injurious levels. There is also concern about the risks to consumers from residual antioxidants; since these materials are volatile, little or no residue should persist at the end of storage if the material is used properly. However, these materials have not been approved in some countries, so treated fruit cannot be exported to such countries.
During the past 3 years we have been conducting extensive studies on scald. Our goal is to reduce dependence on the antioxidant chemicals for control. Current recommendations are generally based on a worst-case scenario, since growers simply cannot risk scald development. However, as is described above, scald susceptibility is extremely variable and maximum treatment is often unnecessary. If we can better quantify the factors affecting scald, we should be able to quantify the potential for scald and adjust the recommended treatment to the actual need. One approach to this is through careful collection of climatological data in relation to scald development. A cooperative study involving a number of fruit researchers (Ed. note: Washington has joined this study) and directed by Dr. David Blanpied at Cornell University is in progress. We are also attempting a different approach: a search for a chemical index of scald susceptibility in the fruit that might signal the need (or lack thereof) for chemical treatments at the time of harvest.
Scald was probably the single most important postharvest problem for apples until antioxidant chemicals were approved. For 20 years after approval little further attention was given to this problem. Now interest is renewed, largely due to the need to reduce the use of chemicals wherever possible. Growers can expect to hear much more about scald control measures in coming years.
Dover, C.J. 1985. Commercial scale catalytic oxidation of ethylene as applied to fruit stores. Pages 373?383 in J.A. Roberts and G. A. Tucker (eds.). Ethylene and Plant Development. Butterworths, London.
Fidler, J.C. 1956. Scald and weather. Food Sci. Abstracts 28:545-554.
Merritt, R.H., W.C. Stiles, A.V. Havens, and L.A. Mitterling. 1961. Effects of preharvest air temperatures on storage scald of Stayman apples. Proc. Amer. Soc. Hort. Sci. 78:24-34.
Smock, R.M. 1957. A comparison of treatments for control of the apple scald disease. Proc. Amer. Soc. Hort. Sci. 69:91-100.
Wilkinson, B.G., and J.C. Fidler. 1973. Physiological disorders. Pages 67-131 in Fidler, J.C., B.G. Wilkinson, K.L. Edney, and R.O. Sharples (eds.). The Biology of Apple and Pear Storage. Commonwealth Agricultural Bureaux, East Malling, Kent, England.
William J. Bramlage
University of Massachusetts, Department of Plant and Soil Sciences
Post Harvest Pomology Newsletter, 6(2): 11-14