24 CIRCULAR 867, U. S. DEPARTMENT OF AGRICULTURE 



in orchard 1 (22 and table 6) where, in addition to good drainage, 

 the 1946 bloom weather was exceptionally favorable for cross-pollina- 

 tion. 



It has been pointed out that naphtha leneacetic acid reduces fruit 

 set to a greater extent on weak wood lacking food reserves than on 

 vigorous wood containing adequate amounts of stored food. Trees 

 exhibiting nitrogen deficiency seldom set so heavily as those well sup- 

 plied with this element. For this reason the treatment is likely to 

 overthin when the nitrogen level is insufficient for good vigorous 

 growth. Any condition that would reduce the efficiency of the foliage 

 during the previous season, such as spray injury or insect damage, 

 would likely interfere with the accumulation of reserve food and 

 predispose the trees to a light set. All such factors should be con- 

 sidered before using a hormone spray for thinning. 



The question of combining the naphthaleneacetic acid for thinning 

 with one of the normal postbloom fungicide and insecticide sprays is 

 frequently raised. Many of the fungicides and insecticides contain 

 wetting agents, as do the naphthaleneacetic acid preparations, and 

 when combinations are made excessive runoff may take place. It is 

 also possible that some of the numerous materials now employed for 

 pest control may interfere with the solubility of naphthaleneacetic 

 acid or its absorption by the foliage, thus modifying the results. 

 Sprays for the control of insects and diseases are usually ajDplied to 

 all trees in the orchard while a thinning spray may be used only on 

 certain varieties, and the appropriate concentration for these may 

 vary. It, therefore, seems advisable to apply the thinning spray as 

 a separate operation. 



OTHER CHEMICALS USED FOR THINNING APPLES 



Most of the investigational work on chemical thinning as well as 

 the commercial application of the practice has centered around the 

 use of dinitro bloom sprays or postbloom applications of naphtha- 

 leneacetic acid. There are, however, several other types of materials 

 that have been tested for reducing the set of apples which should 

 receive mention. Following the preliminary work of Auchter and 

 Roberts (£), Shepard (36) reported 6 years' results on the spraying 

 of apples for the prevention of fruit set. The main purpose of this 

 work was to defruit biennial-bearing varieties so as to change the 

 year in which the trees bore their crop. Ajnong a number of materials 

 used, cresylic acid and a tar oil distillate were the most effective in 

 preventing fruit set. 



Magness, Batjer, and Harley (31) used sprays of tar oil distillate 

 I TOD) and dinitro-ortho-cyclohexyphenol in oil (DXO) with two 

 objects in view : (1) To defruit the trees as a possible means of lower- 

 ing the codling moth population; and (2) to determine the possibility 

 of reducing the set of biennial-bearing trees sufficiently during the 

 heavy -bloom year to cause fruit -bud formation for the year follow- 

 ing. The reduction in set due to treatment was found to vary mainly 

 with the variety, the stage of development at which the spray was 

 applied, and the type of spray or dosage. The data presented from 

 experiments conducted in the States of Washington and West Vir- 



