Circular no. 867 



March 1951 • Washington, D. C 



UNITED STATES DEPARTMENT OF AGRICULTURE 



Fruit Thinning With Chemical Sprays 



By L. P. Batjee, principal physiologist, Division of Fruit and Vegetable Crops 

 and Diseases, Bureau of Plant Industry, Soils, ^vd Agricultural Engineering, 

 Agricultural Research Administration, and M. B. Hoffman, pomologist, Cornell 

 University Agricultural Experiment Station. 



CONTENTS 



Introduction 



Effectiveness of dinitro chemicals 



in thinning apples 



Relative effectiveness of differ- 

 ent forms 



Time of application and mecha- 

 nism of action 



Concentration 



Varietal response 



Number of spray applications _ 



Pollination and bee activity 



Weather conditions during 



blossoming period 



Tree vigor 



Technique in applying sprays _ 

 Effectiveness of growth-regulat- 

 ing chemicals in thinning 



apples 



Effectiveness of different chemi- 

 cals 



Timing of sprays and mecha- 

 nism of action 



Concentration 



Varietal response 



Environmental factors affect- 

 ing results 



Page 



Other chemicals used for thinning 

 apples , 



Effects obtained by thinning- 

 apples with chemical sprays 



Fruit size and yield . 



Biennial bearing 



Reduction in thinning cost 



ffectiveness of chemical thinning 



sprays on peaches 



Dinitro thinning sprays 



Naphtha leneacetic acid thin- 

 ning sprays 



Effectiveness of chemical thin- 

 ning sprays on pears, apri- 

 cots, and plums 



Pears 



Apricots and plums 



Effect of thinning sprays on 

 pollinating insects 



Summary and conclusions 



22 Literature cited 



Page 



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27 

 27 

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 29 



31 

 31 



34 



36 

 36 

 38 



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41 

 44 



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INTRODUCTION 



Thinning of certain species and varieties of tree fruits is essential 

 to avoid a tendency toward biennial bearing, especially of most apple 

 varieties, and also to obtain increased fruit size, color, and quality. 

 In some sections, particularly the Western States, hand thinning 

 represents one of the greatest single costs in producing fruit. In a 

 number of apple varieties, adequate thinning by hand cannot be clone 

 early enough to curb the tendency of the tree to become alternate in 

 its fruiting habit. Ultimate size of apples and other fruits is also 



