Pre-Harvest Fruit Drop 251 



factor which undoubtedly contributes to this stimulation of growth 

 rate is the reduced set of Howers which open after the spray is applied. 

 An additional benefit obtained is the prevention of water spot break- 

 down of the fruits after they are picked and placed in storage (Stewart, 

 1949). 



Oil sprays are used in most large orchards in the insect control 

 program and these sprays commonly cause the abscission of some 

 leaves and some fruit stems. The auxin sprays sometimes can neu- 

 tralize these effects (Stewart et al, 1951). 



Some of the problems which have arisen from the use of auxin 

 sprays in oranges include the development of rudimentary seeds, of 

 navels in Valencia oranges or excessively large navels in the Washing- 

 ton navel orange, and the development of abnormally large oil glands 

 on the surface of the fruit (Stewart et al, 1951, 1952). It has been 

 found that these abnormalities are held to a minimum if concentra- 

 tions of 10 mg./l. or less of the auxin is used. As in the case of apples, 

 defective fruits are not held on the tree by auxin sprays (Erickson, 

 1951) and hence the spray does not increase the percentage of defective 

 fruits harvested. 



In using 2,4-D to prevent fruit drop of Washington navel oranges, 

 experimenters have found that increases in fruit size are obtained 

 when the spray is applied six weeks before bloom. In this curious in- 

 stance, auxin applied before the flowers have opened stimulates the 

 subsequent fruit growth rate as well as prevents pre-harvest drop. Thus 

 the auxin effect is exerted over a period of seven months (Stewart 

 et al, 1951). 



The 2,4-D sprays on Washington navel oranges were found to 

 have another beneficial side effect by reducing the amount of fruit 

 stem dieback which normally occurs in this variety. This appears to 

 be a case of actual abscission of stems which occurs naturally, and such 

 abscission is effectively prevented by auxin sprays. 



Lemons 



The pre-harvest drop problem in lemons is slightly different 

 from that in oranges, for the lemon continues to flower over a long 

 period of time, and the fruits on a tree are of diverse ages. Stewart 

 and Hield (1950) have found that 2,4,5-T sprays can effectively hold 

 the fruit on the tree even though the fruit is mature. In this way the 

 sprays make it possible to hold fruit on the tree until the desirable 

 summer market for lemons is at hand. Concentrations of from 5 to 8 

 mg./l. of 2,4,5-T are apparently optimal. The diethanolamine salt is 



