22 CIRCULAR 867, U. S. DEPARTMENT OF AGRICULTURE 



at bloom or calyx time than at a later stage of development. There 

 were a few instances however, where either the reverse was true or 

 there was seemingly no difference in the amount of thinning obtained 

 as related to time of application. The amount of thinning probably 

 depends on absorption of the chemical as well as concentration and 

 susceptibility of the flowers and young fruits to abscission. There is 

 a continuous increase in leaf surface for absorption as the season 

 advances, and the size and condition of the foliage as well as the 

 weather at the time of application may influence the results. 



Varietal Response 



Although the concentration of naphthaleneacetic acid required for 

 adequate thinning varies with the variety, spray applications have, 

 under many conditions, resulted in a significant reduction in set for 

 practically all varieties, with the possible exception of Kome Beauty. 

 The lack of effectiveness of naphthaleneacetic acid in thinning Rome 

 Beauty under New York conditions in 3 years out of 4 is illustrated 

 by the fruit-set records for this variety in table 6. In the 1947 test the 

 weather was cool and extremely wet during the bloom period and for 

 several weeks following petal fall. Under these conditions of limited 

 sunshine, the foliage was quite succulent and a calyx spray of naph- 

 thaleneacetic acid at 20 p. p. m. seriously overthinned Rome Beauty 

 despite the fact that the unsprayed trees set heavier than desirable. 

 In 1946, 1948, and 1949, when bloom weather was favorable for fruit 

 setting, sprays of 10, 15 and 20 p. p. m. applied at the calyx stage 

 resulted in little or no thinning. 



Environmental Factors Affecting Results 



As in the case of dinitro bloom sprays for thinning, when environ- 

 mental factors tend to make a light set the use of a postbloom hormone 

 spray usually results in overthinning. In this connection, it may be 

 assumed that any treatment that can be depended upon to thin apples 

 will overthin the crop if the normal set is less than or no more than 

 that required for a commercial crop. This is illustrated in the 

 thinning experiments conducted on Duchess and Wealthy in Michi- 

 gan by Stebbins, Neal, and Gardner (1^1 and table 6.) Assuming that 

 25 to 30 fruits per 100 blossoming points constitute a normal crop 

 for these varieties in that area, the fruit-set records in table 6 indi- 

 cate that an excessive set did not occur in 1945. Similarly, over- 

 thinning of Early Mcintosh (orchard 1) in Massachusetts (Ifi and 

 table 6) was accompanied by a much lighter set on untreated trees 

 than in orchard 2 where the same concentration resulted in a more 

 satisfactory degree of thinning. 



In seasons when cool, cloudy or wet weather occurs during a part 

 of the bloom period, apples may set heavily even though the average 

 number of seeds per fruit is lower than when the bloom weather is 

 exceptionally favorable for bee activity. This is especially true in 

 vigorous well-cared-for orchards. Under conditions of a low average 

 seed content, a postbloom spray of naphthaleneacetic acid may thin 

 heavier than when the average seed content is high following strong 



