FRUIT THINNING WITH CHEMICAL SPRAYS 17 



higher concentrations. Indolebutyric acid at 10 and 50 p. p. m. in 

 the same alcohol-water carrier had no significant effect on set. 



Davidson, Hammer, Keimer, and Dutton (8) included the sodium 

 salt of naphthaleneacetic acid at a concentration of 10 p. p. m. in the 

 regular insecticide and fungicide applications during the 1941 grow- 

 ing season on trees of the following varieties : Stark, Grimes Golden, 

 Mcintosh, Jonathan, and Golden Delicious. Similar plots of these 

 varieties received only the normal insecticide and fungicide applica- 

 tions. Since the addition of the hormone was for purposes other than 

 thinning, no fruit-set records were made. However, counts at harvest- 

 time showed a large reduction in the number of fruits borne per tree 

 where the sodium salt of naphthaleneacetic acid had been included 

 in the spray schedule. The reduction in the number of apples har- 

 vested per tree was as follows: Stark, 90 percent; Grimes Golden, 

 85; Mcintosh, 64; Jonathan, 70; and Golden Delicious, 85. Thus, a 

 very significant thinning effect was obtained from repeated applica- 

 tions of the sodium salt of naphthaleneacetic acid, none of which 

 were applied in the blossoming stage. 



Following these observations Davidson and coworkers (8) con- 

 ducted experiments in 1942, 1943, and 1944 to evaluate the sodium 

 salt of naphthaleneacetic acid as an agent for thinning apples. Con- 

 centrations of 10 and 20 p. p. m. were used, and single applications 

 were made at different stages of development, starting at the pink 

 or open cluster stage and extending up to and including the second 

 cover spray, 4 weeks after bloom. In most cases the hormone was 

 applied alone as a special application. In these tests, which included 

 six varieties, a concentration of 20 p. p. m. was more effective in re- 

 ducing set than 10 p. p. m. The greatest thinning, in some cases too 

 severe, occurred when the application was made at full bloom. Sprays 

 applied just before or just after bloom were less effective than at full 

 bloom but were often adequate. Treatments made 2 and 3 weeks after 

 petal fall were less effective than at petal fall, and when the spray 

 was delayed until 4 weeks after petal fall there was little or no effect. 

 There was considerable variation in the response of different varieties, 

 and the same variety did not respond the same each year. Foliage 

 injury was reported as not being a factor in these experiments. 



Timing of Sprays and Mechanism of Action 



In most of the recent experiments involving the use of hormone 

 sprays for thinning apples, the application has been made at the 

 calyx stage. Beneficial results have also been reported from later 

 treatments by Hoffman, Southwick, and Edgerton (22) and South- 

 wick and Weeks (4-0). 



From the standpoint of obtaining maximum benefits in fruit size 

 and annual bearing of biennial varieties, the earlier the thinning can 

 be accomplished the better will be the results. However, with many 

 varieties in certain seasons, it is just as difficult to appraise the need 

 for thinning at the calyx as at an earlier bloom stage. Since naph- 

 thaleneacetic acid preparations are effective in reducing the set of 

 apples for several weeks following the bloom, there is considerable 

 leeway in timing the spray. This offers definite advantages for those 



