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Storage Scald in Apples and Pears
Storage scald, also called common scald and superficial scald, affects mainly the skin of fruit and can result in fruit losses. The disorder is believed to be caused by oxidation of a naturally occurring compound in the skin of fruit. Under certain conditions the compound becomes toxic to the cells of the skin.
Scald, evident only after several months of storage, first appears on skin on the green side of fruit as an irregular burned or scalded area. Fruit removed from storage after December may not show symptoms at first, but when allowed to stand at room temperature for 12 to 24 hours may begin to exhibit scald. The flesh immediately below the affected area may become soft and discolored if scald is severe. If this happens, the fruit cannot be peeled easily when processed.
Scald severity is influenced by variety, season, cultural practices, and postharvest treatments. Susceptible varieties include Ben Davis, Cortland, Delicious, Granny Smith, Rhode Island Greening, Rome Beauty, Stayman, Winesap, and York Imperial. Less susceptible are Braeburn, Fuji, Gala, Golden Delicious, Jonathan, McIntosh, Northern Spy, Pink Lady, and Spartan. Warm temperatures during the last 6 weeks before harvest may increase the likelihood of scald. Hot, dry weather increases susceptibility, while cool, damp weather decreases it. Studies in New Jersey with Stayman showed that when fruit experienced 190 hours or more of temperatures below 50°F on the tree during the weeks just prior to harvest, scald did not develop.
Immature fruit is more likely to scald, and there is some indication that fruit high in nitrogen or low in calcium may have a greater tendency to scald. Delays in storage sometimes can reduce scald severity, but they adversely affect the length of storage life. The problem also appears to be worse on fruit stored at high relative humidity.
The best means of controlling scald is to avoid storing immature or overmature fruit. The fruit should be cooled as rapidly as possible, and if placed in CA storage, the desired oxygen level should be achieved quickly.
Treating the fruit with DPA (diphenylamine) or ethoxyquin (labeled for pears only) reduces scald incidence when proper concentrations have been used (see Table 6-2). DPA may retard the loss of green color in the fruit skin. DPA may also cause injury to the fruit if drench solutions accumulate too much dirt. It is recommended that DPA solutions be used at the rate of 30 bins per 100 gallons. Injury from ethoxyquin has occurred when tanks are not properly drained and the material becomes concentrated as the water in the dump tanks evaporates. Ethoxyquin is also very sensitive to sunlight and breaks down rapidly, losing effectiveness. The recommendation is not to exceed 100 bins per 100 gallons of diluted material before changing the solution. Water remaining in the dump tank must not be emptied into residential sewer systems or natural watersheds.
Fruit is normally drenched for 30 seconds in the solution. Longer treatments may result in unacceptably high residue levels. Fruit treated with ethoxyquin may be washed and waxed as soon as it is dry.
While it is being prepared, agitate the solution to obtain a uniform mixture. Since the active ingredients of DPA and ethoxyquin vary with the manufacturer, it is important to read the label when preparing the solution.
Ethoxyquin coverage can be detected under ultraviolet light, enabling you to assess your effectiveness in covering the fruit. Allow fruits to dry, then view them in a dark room. Do not look directly into the ultraviolet light because it can damage your eyes.
Drenching for Scald Control and Storage Requirements by Cultivar (See Tables 6-1 and 6-2)