Occurrence and importance
Soft scald is sporadic in its occurrence even on highly susceptible varieties, but it sometimes causes heavy losses. Soft scald occurs most frequently on Jonathan, McIntosh, Rome Beauty, and Wealthy, but it is occasionally found on Stayman, Winesap, Grimes Golden, Delicious, and other varieties.
The disease is characterized by peculiar patches and ribbon-like areas of brown tissue on the surface of the apple and by a sharp line of demarcation between the diseased and the healthy tissues (top photo). Sometimes only the skin of the apple is affected, but more often the browning extends into the flesh for 1/8 inch or more (bottom photo). Black spots on areas injured by soft scald are usually caused by a secondary infection, Cladosporium herbarum or Alternaria tenuis.
Soft scald, a nonparasitic disease, is believed to be caused by abnormal respiratory conditions in the fruits, induced by the reaction of mature or late-picked fruits to temperatures bordering on the freezing point of apples. Severity of the disease is increased by a delay between harvest and cooling.
Fruits of optimum maturity can be cooled rapidly to 31°F. However, fruits of advanced maturity from grower lots that are susceptible to soft scald should be held for several weeks at 34° before they are cooled to 31°.
Soft scald, external view
Soft scald, cross-section view