306 Plums and Plum Culture 



existence. All these doubtless operate, though all 

 vary greatly with circumstances. In some instances 

 one plays the most important part; in other cases an- 

 other cause has the greatest effect. 



Studies made at the Vermont Experiment station 

 show that lack of pollination is certainly one of the 

 causes. A large number of the young plums which 

 are imperfectly pollinated at blossoming time never 

 begin to develop, but fall off within a week or ten days. 

 A certain number, however, usually a smaller num- 

 ber, remain on the tree and begin to grow. These 

 may hang on till they arc half or two-thirds grown, 

 but presently they stop growing, turn yellow, and fall. 

 This may occur from lack of pollination alone. Per- 

 haps such fruits are stimulated to a partial develop- 

 ment by a partial pollination. Perhaps they begin 

 their growth for some other reason. At any rate, they 

 do not reach maturity, but instead come to the ground 

 with the June drop. 



Examination of specimens leaves no room for 

 doubt that the curculio is also a factor in the June drop 

 of plums. The curculio sometimes works directly into 

 the seed, causing the certain and early fall of the fruit. 

 In other cases the larva works only in the flesh just 

 outside the stone. The fruit usually shows symptoms 

 of premature ripening, and earlier or later is apt to fall. 

 Many curcnlio-infested fruits remain on the tree till 

 ripening time, however. The majority of the curculio 

 drops seem to come toward the end of the season, 

 mostly in July, whereas the non-pollinated drops seem 

 to fall mostly in June. 



The third factor mentioned remains to be consid- 

 ered. Numbers of young plums fall even though well 

 pollinated and though unattacked by the curculio. 

 When plums set too thickly on the fruit spurs and 

 when they are not properly thinned by other causes, 



