12 MISC. PUBLICATION 168, U. S. DEPT. OF AGRICULTURE 



that cause bitter pit. The disease at harvesttime is characterized 

 by small to medium-sized surface depressions, or there may be a 

 general corrugation of the surface of the fruit (pi. 4, C). Corky 

 spots occur in the flesh immediately below the surface depressions 

 and sometimes also at some depth in the flesh (pi. 4, D). The 

 York Imperial is not subject to boron-deficiency cork, a fact 

 which eliminates the need for comparing York spot with cork. 



The presence of high concentrations of certain salts in irriga- 

 tion water is sometimes associated with and seems to be the 

 cause of a condition in apple fruits that can hardly be distin- 

 guished from bitter pit. In the Pacific Northwest instances have 

 been known of spotting following the use of irrigation water 

 containing a high percentage of epsom salt (magnesium sulfate). 



Bitter pit can be distinguished from fruit spot by the char- 

 acteristics mentioned on page 35. When confined to the surface 

 region, bitter pit is hard to distinguish from some forms of 

 stigmonose, except for the facts that the individual corky areas 

 associated with stigmonose are often larger than most bitter pit 

 spots and minute punctures can sometimes be seen in the skin 

 that covers them. 



Bitter pit is a nonparasitic, or physiological, disease, and its 

 occurrence is largely determined by orchard conditions. It is 

 worse on fruit from young trees, especially if the crop is light, 

 than on fruit from older trees, worse on large apples than 

 on small ones, and worse on apples picked when immature than 

 on those picked when mature. In dealing with susceptible varie- 

 ties, growers could probably avoid losses and costly repacking 

 by segregating light-crop fruit and disposing of it to processing 

 plants. 



Bitter pit is greatly increased by heavy irrigation and heavy 

 rainfall, particularly when these occur late in the growing season, 

 and by heavy nitrogen fertilization. Conversely, everything that 

 contributes to the stabilizing of moisture conditions in the soil 

 and to even, normal growth of the fruit throughout the season 

 is of value in preventing the disease. 



Investigators in the United States and Australia have ob- 

 tained evidence that the disease is due to the killing of immature, 

 starch-filled tissues of rapidly growing apples or of fruit in 

 storage, probably resulting from excessive transpiration that 

 induces osmotic action between the starch-filled cells and those 

 in which most, possibly all, of the starch has been changed to 

 sugar. According to this explanation, the starchy areas are killed 

 by excessive desiccation. It should, therefore, be possible to re- 

 duce the amount of bitter pit by speeding up the rate of ripening. 

 Bitter pit develops more rapidly at 50° F. than at 32° and more 

 rapidly at both these temperatures than at 70°. Considerable 

 bitter pit may develop within 7 to 10 days on susceptible varieties 

 at 50°. In some of the susceptible fruit, however, bitter pit will 

 not have developed by the end of that time. Bitter pit often 

 develops in a month or 6 weeks at cold-storage temperature, and 

 develops further, especially on immature fruit, after removal from 

 such storage. 



Bitter pit may begin to appear before the apples are harvested, 

 but the disease is seldom fully manifested at that time. Susceptible 



