Flower and Fruit Thinning 235 



Auxins are ineffective for thinning of peach flowers when there 

 is no foliage. Approximately two or three weeks after bloom when the 

 foliage has developed on peach trees, auxin sprays can effectively be 

 used to thin fruits (Hibbard and Murneek, 1950) as shown in figure 

 101. 



The situation with apples is not as clear as with olives. y\uxin 

 applied to the leaves is considerably more effective than applications 

 to the fruits, but overall sprays are more effective still (Batjer and 

 Thompson, 1948). There is some evidence that the initial effect of an 

 overall auxin spray on apple trees is to retard abscission, even of fruit 

 pedicels from which the fruits have been removed (Struckmeyer and 

 Roberts, 1950; Murneek and Teubner, 1953). 



Of course the auxin produced in the flower or fruit has a strong 

 influence on the effectiveness of flower and fruit thinning sprays. If 

 parthenocarpic fruits are set by the spray, as in the case of tomato 

 or Bartlett pear, the auxin-producing mechanism described in the 

 preceding chapter will be released within the ovary and a relatively 

 high auxin content on the side away from, or distal to, the abscission 

 layer will result. This auxin will prevent abscission and hold the 

 flower on the plant. Apple and peach flowers do not readily set fruit 

 parthenocarpically and hence any retarding of abscission by auxin 

 sprays will be short-lived at best. 



Struckmeyer and Roberts (1950) have proposed that auxins thin 

 flowers by causing an excessive number of flowers to set fruit, result- 

 ing in such a competition for nutrients for growth between the young 

 fruits that large numbers of them are forced to abscise for the lack of 

 such nutrients. However, Batjer and Uota (1951) have been unable 

 to confirm their observation that the auxin spray resulted in an initial 

 increase in apples set. 



METHODS OF TREATMENT 



The sodium salt of naphthaleneacetic acid has been most gen- 

 erally used in auxin fruit-thinning work. The reason is not clear, but 

 this auxin is certainly very effective and the sodium salt is convenient 

 in that its solubility is much greater than that of the free acid. While 

 most treatments are made by means of an aqueous spray, dust prep- 

 arations have been used with good success (Southwick and Weeks, 

 1952). 



Naphthaleneacetamide has also come into considerable commer- 

 cial use for thinning apple flowers. This compound is considerably less 

 effective than the salt of the acid, and hence concentrations of approxi- 

 mately twice the strength of acid sprays are used. The amide has the 



