CHAPTER XIII 



Control of Pre-Harvest Fruit Drop 



A serious problem confronting growers of tree fruits is the loss of 

 mature fruits by abscission from the tree before harvest. Perhaps the 

 crop most seriously affected in this way is the apple. As the apple crop 

 nears maturity, a spell of windy or hot weather can result in the 

 abscission and loss of almost the entire crop. Essentially the same prob- 

 lem confronts the growers of stone fruits and citrus fruits. The fact 

 that auxin could prevent abscission of leaves was demonstrated in 

 1936 by LaRue, and that this same control could be applied to mature 

 apples was first shown in 1939 by Gardner et al. Their findings have 

 led to the development of one of the most valuable new tools of the 

 pomologist. 



PHYSIOLOGICAL BASIS FOR PRE-HARVEST DROP CONTROL 



With respect to abscission in general, it has gradually become 

 apparent that plant organs have a general tendency to abscise, and 

 that the auxin production within the organs suppresses the tendency. 

 The retention of leaves, flowers or fruits on a plant are therefore 

 dependent upon the auxin supply in the organ, and abscission occurs 

 at times when the auxin production in the organ becomes low. In the 

 case of the apple, auxin is produced in three waves: (1) upon fertiliza- 

 tion, the ovary produces a flush of auxin, (2) in the fourth week of 

 growth, the endosperm produces auxin, and (3) from the 8th to 12th 

 week, the embryo in the seed produces a large amount of auxin. These 

 waves of auxin production have been worked out in detail by Luckwill 

 (1948, 1953), and he has shown that spells of fruit abscission occur 

 between each of these production waves, and after the last one. A large 

 drop in auxin production as maturity approaches is readily seen in 

 figure 103. Each of the low ebbs in auxin production by the fruit 

 would be expected to result in a weakened auxin gradient, and conse- 

 quently in fruit abscission. Because the seeds are the apparent sources 

 of the last two auxin waves, it is not surprising that as maturity ap- 

 proaches the fruits with the fewest seeds are the first to abscise (Hei- 

 nicke, 1917). 

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