FACTORS INFLUENCING THE ABSCISSION OF FLOWERS AND 



PARTIALLY DEVELOPED FRUITS OF THE 



APPLE (PYRUS MALUS L.)^ 



Arthur J, Heinicke 



Observations have shown that normally less than ten per cent of the 

 apple blossoms which open in spring produce fruit. Many of the flowers 

 are lost a few days after the petals fall, and a large number of the par- 

 tially developed fruits are thrown off during the next few weeks. A 

 rather conspicuous drop, commonly called the June drop, occurs in June 

 and July, when the fruits are from one to three centimeters in diameter. 



This June drop may or may not be beneficial to the fruit grower. If 

 more than five to ten per cent of the flowers on a tree producing a heavy 

 bloom set fruit, a large quantity of apples must be removed by hand so 

 that the remaining specimens can attain a desirable size and color. On 

 the other hand, apple trees frequently produce an abundance of flowers 

 but little or no fruit is harvested from them, practically all the apples 

 being lost during the June drop or before. 



Diseases, insects, and unfavorable weather are often held acco ntable 

 for the heavy loss of flowers and partially developed fruits. In unsprayed 

 orchards the flowers and fruits that have fallen from the trees often 

 show injury by scab and codling moth. Heavy losses sometimes result 

 from the effects of winter injury, frost, wind, and hail. But in many 

 cases the drop occurs even in the absence of such destructive agents. 



The failure of a large proportion of apple blossoms to set, and the 

 heavy loss of partially developed fruits during the June drop, are fre- 

 quently associated with poor pollination and lack of fertilization. The 

 fact that a large proportion of the apples that fall generally have fewer 

 seeds to the fruit than those that remain on the tree, indicates that the 

 development of seeds is an important factor in fruit setting. Never- 

 theless, many flowers set fruit even tho they are poorly pollinated, and 

 many fruits remain on the tree even tho they have relatively few seeds. 



Obviously, then, there are other factors, aside from the destructive 

 agents previously mentioned, and in addition to poor pollination and lack 

 of fertilization, which influence the abscission of flowers and partially 

 developed fruits of the apple. To study such factors was the object of a 



' Also presented to the Faculty of the Graduate School of Cornell University, September, igi6, as a 

 major thesis in partial fulfillment of the requirements for the degree of doctor ot philosophy. 



Ai'thor's Acknowledgment. The writer wishes to acknowledge his indebtedness to Professor W. H. 

 Chandler, who proposed the problem and who gave many helpful suggebtions during the course of the 

 investigations. 



