APPLE SCALD, A COMPLEX PROBLEM 



William J. Bramlage 



Department of Plant and Soil Sciences, University of Massachusetts 



Symptoms . "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: 



a. "Rugose scald": skin initially develops a faint 

 bronze color, but later these areas turn Ught 

 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. 



b. "Browning scald": the lenticels do not remain 

 green, the injury progressively invades deeper 

 into the flesh, and areas often slough off 

 because they remain moist. 



c. "Lenlicel spot scald": the injury is 

 predominantly around the lenticels, so that it 

 appears as a spotting rather than a blotchy 

 disorder. 



d. "Stem-end browning": 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, with specific 

 cultivars being more prone to one form or 

 another. However, there are many other forms of 

 fruit injury that also may cause skin damage that 

 is similar to one of these forms of scald. For 

 example, we have noted frequently a large amount 

 of lenticel spotting 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". With very ripe fruit, 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: 1. 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; 2. The next 5 to 8 weeks when changes 

 continue so that scald can no longer be 

 prevented although it still has not appeared; 3. 

 The remainder of storage, when scald may slowly 

 appear; 4. Post-storage, when scald rapidly 

 develops. Thus, the first 6 to 8 weeks after 

 harvest are crucial for applying scald control 

 measures, and post-storage conditions can 

 determine how extensively the scald symptoms 

 will appear. For example, we have noted much 

 more scald under humid post-harvest conditions 

 than under dry ones. 



An outstanding series of research papers in 

 the late 1960's and early 1970's, 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, drying out, and collapse. 

 Since most of the alpha-farnesene is found in 

 the fruit peel, most of the conjugated trienes 



