2 CIRCULAR 867, U. S. DEPARTMENT OF AGRICULTURE 



directly related to the earliness and thoroughness of the fruit -thin- 

 ning operation. 



The application of chemical sprays during the bloom or early 

 postbloom period in order to reduce fruit set, thereby partially or 

 completely overcoming the necessity of hand thinning, is a practice 

 that has received considerable attention in recent years. A wide 

 variety of different materials has been used with varying degrees 

 of success. However, most of the investigational work with chemical 

 thinning, as well as the commercial application of the practice, has 

 centered around the use of the dinitro materials. Considerable atten- 

 tion has also been devoted to the use of naphthaleneacetic acid and 

 its sodium salt. A number of other materials have been under 

 experimentation but are not used commercially to any appreciable 

 extent. 



The most extensive experimentation and commercial use of chemi- 

 cal thinning sprays have been with apples, although the possibilities 

 of using such sprays with pears and stone fruits, particularly peaches, 

 have been rather extensively explored. 



Th-e extent to which chemical thinning sprays are used in commer- 

 cial apple areas varies greatly according to the fruit section and 

 varieties grown. In some sections results have been so erratic as to 

 preclude commercial usage. In the New England States and New 

 York chemical thinning sprays are used extensively on the Wealthy 

 and Baldwin varieties. In the Pacific Northwest during the past 3 

 years, 5 to 20 thousand acres of apples (depending on the season) 

 have been chemically thinned annually. The sprays are applied to 

 all commercial varieties grown in the area. 



With the rapid expansion in the use of chemical thinning sprays 

 considerable variations in results have been obtained. The purpose 

 of this circular is to bring together both experimental and observa- 

 tional information that will aid in evaluating the various factors 

 affecting the results with these sprays. This evaluation should help 

 the fruit grower to decide the extent to which he may profitably test 

 or use fruit-thinning sprays. 



EFFECTIVENESS OF DINITRO CHEMICALS 

 IN THINNING APPLES 



Relative Effectiveness of Different Forms 



From 1932 to 1935 Auchter and Roberts (1, 2) x performed tests 

 with a number of different chemicals to determine their effectiveness 

 in preventing the fruit set on various varieties of apples. Of the 

 several materials tested by these workers, the tar oil distillates were 

 the most effective in killing flower buds when applied in the cluster 

 bud stage. 



The primary object of this early work was to find a practical method 

 of entirely preventing fruit set of certain apple varieties. In 1939 

 Magness, Batjer, and Harley (31) and Gardner, Merrill, and Petering 

 (llf) reported promising results in reducing fruit set with Dow 



i Italic numbers in parentheses refer to Literature Cited p. 44. 



