6 CIRCULAR 867, U. S. DEPARTMENT OF AGRICULTURE 



germination when applied to the stigmatic surface of apple flower?. 

 Later MacDaniels and Hoffman (29) reported that Elgetol sprays 

 applied at concentrations of 0.1 to 0.2 percent during the bloom period 

 effectively reduced the fruit set of several varieties of apples. From 

 their initial orchard trials these authors concluded that closed blossoms 

 were not killed by the spray, nor were blossoms that had been pollinated 

 sufficiently long to permit fertilization. Hildebrand (19) has presented 

 data to indicate that a 0.3 percent concentration of Elgetol applied to 

 open flowers is able to inactivate pollen tubes that have already grown 

 halfway down the style of the pistil. This would approximate the 

 depth of pollen penetration about 1 day after the pollen has been 

 applied to the stigmatic surface. 



In subsequent experiments involving chemical thinning attention 

 was directed toward obtaining additional information relative to the 

 length of time during the bloom period that these sprays would 

 effectively reduce fruit set. In table 2 typical results are presented 

 relating to the effectiveness of Elgetol sprays applied at different 

 stages of bloom development. These data show that greater thinning 

 was obtained when the sprays were applied at full bloom as compared 

 with pre-full-bloom when only 35 to 75 percent of the flowers were 

 open. It is interesting to note, however, that about the same amount 

 of thinning resulted when the spray was applied 1 to 3 days following- 

 full bloom (early petal fall) as was obtained when applications were 

 made at the full bloom stage. 



The fact that appreciable reduction of set is obtained when Elgetol 

 sprays are applied from 1 to 3 days following full bloom would 

 indicate that a large number of flowers already fertilized are pre 

 vented from developing fruit by the action of the spray material. It 

 is believed that under many conditions reduction in fruit set is due 

 to the Elgetol acting as a pollinicide or as a direct causal factor 

 (MacDaniels and Hildebrand (28) and Hildebrand (19)). The 

 spray may also prevent many of the fertilized flowers from setting 

 indirectly through temporary alteration of growth processes resulting 

 from toxicant absorption by both flower and leaf tissue. Although 

 information regarding weather and tree condition under which this 

 type of thinning may take place is lacking, observations indicate that 

 reduction of set is more prevalent during cool, humid bloom periods 

 or when trees are lacking in vigor as a result of imperfect soil 

 drainage, winter injury, inadequate nutrition, and other types of 

 trouble that interfere with normal tree functioning. 



For best results Elgetol sprays generally should be applied as near 

 the full-bloom stage as possible. By full bloom is meant that stage 

 at which an occasional petal will fall from the earlier opening 

 blossoms when a small branch is gently shaken but not prior to the 

 opening of approximately 90 percent of the blossoms. At this stage 

 usually most of the center, or "king," blossoms of apples have been 

 open for 12 to 36 hours and under favorable pollinating conditions 

 have been pollinated and fertilized. When the "king" blossom has 

 reached this stage, it has a greater tendency to survive spray treatment 

 than the freshly opened or side blossoms, many of which are killed 

 by the spray. If the spray is applied after the later opening blossoms 

 have become pollinated and fertilized, less thinning may result. As 



