Scald, mild

Market Diseases of Apples, Pears, and Quinces
Ordinary Scald

Occurrence and importance
Ordinary scald may occur on susceptible varieties of apples wherever they are grown. Golden Delicious appears to be the only popular variety presently grown commercially that is immune (see Senescent Scald). There is a wide variation in susceptibility of other varieties. Among the varieties most seriously affected are York Imperial, Rome Beauty, Stayman, Delicious, Winesap, Cortland, Yellow Newtown, Baldwin, Grimes Golden, and Arkansas (Mammoth Black Twig). The Arkansas, although not widely grown at present, is the most seriously affected of any variety.

Immature fruits are affected more than mature ones, and the greener side is particularly susceptible. Bright red areas are generally not seriously affected. Ordinary scald can be a serious problem, however, on immature fruits of red sports of some varieties in spite of uniformly red color.

Over the years ordinary scald has unquestionably caused more loss during marketing than any other disease of apples. Present scald-prevention chemicals provide the most effective control that has been found. Various contributing factors, however, may interfere with effective control so that the hazard of ordinary scald development remains very important in the storage and marketing of apples.

Symptoms
The symptoms of ordinary scald are expressed in the death and browning of affected tissues. In mild scald only the skin is affected, but in some varieties some of the underlying tissues are also affected.

Usually the symptoms of ordinary scald are not evident on fruits during the first three months of storage at 31° to 32 °F, but may develop rapidly after that if the fruits are transferred to warm air. Symptoms become evident on the fruits after 4 to 5 months at cold-storage temperatures, and they intensify after the fruits are moved to warm air.

The appearance and severity of ordinary scald and the degree of browning of affected skin varies among apple varieties and also within any susceptible variety. In mild cases such as are often seen on Grimes Golden, the affected tissues are very superficial and are often intermingled with healthy areas of skin (top photo). If more seriously affected, however, Grimes Golden, Rhode Island Greening, Delicious, and many other susceptible varieties often show large areas of uniformly dead, brown skin (second photo). Severe ordinary scald on York Imperial and sometimes on Rome Beauty appears brown and roughened, apparently because of moisture loss from the killed tissues (third photo). Arkansas is the most seriously affected of all susceptible varieties. Large severely affected, brown areas of skin are typical. Underlying tissues are sometimes killed to a depth of 1/4 inch. After several days at 70 °F, the killed layers break down and slough off readily from underlying healthy tissues during handling. The breakdown is not due to decay, however, because rots usually spread down into the flesh in round or conical shapes, whereas ordinary scald is diffuse.

Causal factors
Ordinary scald, a physiological disorder of apples, is sufficiently complex that its true cause has never been definitely established. A number of factors affecting individual fruits, together with weather conditions during their growth, have a direct bearing on susceptibility to ordinary scald. The disease is usually worse in years when the weather is hot and dry during the last few weeks before harvest. Susceptibility varies greatly from season to season. Ordinary scald is primarily a disease of immature fruits. It is worse on apples from heavily irrigated trees than on those from trees receiving moderate irrigation. Disease development is favored by storage temperatures above tight packages. Stacking containers to allow good air circulation in the storage room reduces, but does not control, scald.

Delaying storage may greatly increase the development of scald, especially if the fruits receive little ventilation during the delay. If immature fruits are adequately ventilated, however, a delay in storing sometimes reduces the development of scald.

Ordinary scald is associated with the accumulation within apple skin tissues of toxic levels of volatile substances produced by metabolic activities of the fruits. Much experimental work seems to support this belief, although not all workers have accepted it.

Control measures
For many years the control of ordinary scald consisted of harvesting apples at proper maturity, storing and cooling them promptly, and handling them in a manner to reduce accumulation of volatiles. This was done by providing ventilation and good air circulation in the storage and by packing the apples in paper wraps or shredded paper that was impregnated with mineral oil.

This practice continued, with varying degrees of success, until the discovery of the scald-control properties of antioxidants. These scald-inhibiting chemicals apparently act to prevent oxidizing enzymes from killing and browning the apple skin.

Two scald-inhibiting chemicals, diphenylamine (DPA) and ethoxyquin (6-ethoxy-1,2-dihydro-2,2,4-trimethylquinoline), have been found to control ordinary scald. For maximum effectiveness, the materials should be applied to the fruits within a week after harvest. For most varieties, scald control is reduced with each additional week of delay in applying the inhibitor. The materials can be applied as postharvest sprays, dips, or impregnated paper wraps. Thorough coverage is essential to good control. With some varieties preharvest sprays with scald inhibitors are effective. If preharvest sprays are used, then postharvest applications should be omitted because of the danger of accumulating a chemical residue in excess of the legal tolerance.

Varietal differences have been found in the effectiveness of chemicals for control of ordinary scald. For example, Winesap apples respond well to both ethoxyquin and diphenylamine, whereas the Delicious variety responds poorly to ethoxyquin but diphenylamine is effective in the control of ordinary scald.

In addition to the use of scald inhibitor chemicals, air purification units may also be used to absorb volatiles. Such units are standard equipment in controlled-atmosphere storage.

Even with improved methods of ordinary scald control, good horticultural practices such as harvesting at proper maturity, prompt storage, and rapid cooling are essential.

It is also a good plan to take a sample of fruits from storage frequently and hold them at room temperature to determine whether scald will develop, thus checking on the effectiveness of the treatment. The higher temperature does not cause ordinary scald, it merely brings out what is already latent in the fruits.

Because of differences in varietal response, the recommendations of the State extension service or experiment station should be followed in the selection of the proper material, dosage, and method of application.