FRUIT THINNING WITH CHEMICAL SPRAYS 41 



dew on the orchard cover and throughout the day from puddles near 

 tilling tanks or in the ruts made by spray machinery. Arsenic con- 

 tamination of such water sources from preblossom sprays has often 

 proved very destructive to bees. The possibility of damage from 

 dinitro residues in any available water should not be overlooked. 

 During dry bloom seasons the considerate grower would fill in all 

 depressions where water contaminated with any poisonous spray 

 material might collect and then furnish the bees with a nearby supply 

 of suitable water for their use. 



SUMMARY AND CONCLUSIONS 



Application of chemical thinning sprays to fruit trees in order to 

 reduce excessive fruit set has received considerable study in recent 

 years. These sprays are applied during or shortly following the 

 bloom period for the purpose of partially or completely overcoming 

 the necessity of hand thinning. While results of numerous investi- 

 gators have varied widely and possible hazards are attendant with 

 their use, nevertheless, chemical thinning has become a commercial 

 practice in several of the major fruit-producing areas. The results of 

 various investigations seem at present to warrant the following 

 conclusions. 



Of a wide variety of materials tested for the purpose of reducing 

 fruit set, the dinitro chemicals and naphthaleneacetic acid ( a growth- 

 regulating substance) have proved generally the most satisfactory. 

 No outstanding differences in effectiveness have resulted in experi- 

 ments involving the use of various forms of dinitro chemicals. At 

 the present time (1950) sodium dinitro cresylate and dinitro-ortho- 

 cyclohexylphenol are used more extensively than the other forms of 

 dinitro chemicals. 



For best results with apples generally, dinitro sprays should be 

 applied as near the full-bloom stage as possible. Under many condi- 

 tions, however, spray applications as late as 1 to 3 days following full 

 bloom have proved as effective, or only slightly less effective, than at 

 the full-bloom stage. For greatest effectiveness with peaches and 

 other stone fruits, dinitro sprays should be applied a day or two 

 ahead of full bloom or when about 75 to 95 percent of the blossoms 

 are open. Sprays applied at the full-bloom stage or later often result 

 in considerably less thinning than if applied earlier. Exact timing 

 is, therefore, more important with stone fruits than with apples. 



Dinitro thinning sprays seem to reduce fruit set on apples by 

 direct action as a pollinicide, and also may prevent many of the 

 fertilized flowers from developing indirectly through temporary 

 alteration of growth processes. The latter type of thinning is more 

 pronounced when the sprays are applied to trees in low vigor and/or 

 under conditions of cool, humid weather. With stone fruits there 

 seems to be much less thinning of the indirect type, which in all 

 probability accounts for the necessity of more exact timing and the 

 frequent failures of obtaining any appreciable thinning if the sprays 

 are delayed until most of the blossoms have been pollinated and 

 fertilized. 



