CHAPTER XII 



Flower and Fruit Thinning 



Many fruit trees produce such excessive quantities of flowers in one 

 year that sustaining the large number of fruits causes the trees to fail 

 to lay down normal numbers of new flower buds. Consequently, the 

 trees become biennial in their bearing habit, producing excessive num- 

 bers of blossoms one year and few or none the next year. To alleviate 

 the situation, growers have thinned flowers or young fruits by hand, 

 but this tedious and expensive process has many disadvantages. Besides 

 being expensive, hand thinning must be completed approximately 

 three weeks after bloom in biennial bearing apples, for the primordia 

 are initiated approximately three weeks after petal fall in mature trees 

 (Murneek, 1943). 



A second need for thinning is reduction of heavy fruit crops to 

 aid in effective insect and disease control and to increase fruit size and 

 quality. When excessive numbers of fruits are set in a tree it may be- 

 come difficult to obtain complete coverage of the fruit and foliage with 

 sprays, and the fruits harvested are prone to be low in quality and 

 small in size. Excessive set may also cause damage to the trees as the 

 fruits enlarge and weigh the branches down excessively. 



Auchter and Roberts (1935) were the first to demonstrate that 

 chemical sprays could be used effectively to thin apple flowers. From 

 their work there has evolved the use of toxic substances such as phenols 

 and cresols literally to kill off some of the blossoms. Burkholder and 

 McCown (1941) were the first to find that an auxin such as naphtha- 

 leneacetic acid could be effectively used to thin flowers. Subsequent 

 workers have found that auxins are effective in thinning of young fruits 

 as well. 



It seems rather paradoxical to find that in some cases auxin sprays 

 applied to flowers will hold the flowers on the plant and cause par- 

 thenocarpic fruit development, whereas the same material sprayed 

 onto other plants causes thinning of the blossoms and fruit. The dif- 

 ference in the effects obtained is apparently owing to differences in the 

 ability of various species of plants to set parthenocarpic fruit. A par- 

 ticularly good example of such differences in behavior among tree 

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