2l6 



PLANT GROWTH SUBSTANCES 



the crop is over thinned. Hormone sprays, while not a panacea for all 

 of these hazards, do offer the possibility of thinning without injury to 

 the young foliage since their action is not necessarily a caustic one. 



A discussion of the role of growth substances in fruit thinning must 

 necessarily consider their effect on fruit set since one process is the 

 opposite of the other, and the concept of thinning by means of these 

 compounds was the outgrowth of the unsuccessful attempts with tree 

 fruits to increase fruit set. For the purpose of this discussion, fruit set is 

 considered to be the net resulting crop after all abscission, from flowering 

 through the June drop has occurred. 



The writer is not aware of any well-substantiated case in tree fruits 

 of an improvement in fruit set by means of growth substances. This 

 statement does not apply to the few cases of those fruits that can be 

 stimulated to set parthenocarpically and where an improved set is the 

 result of such stimulation coupled perhaps with faulty or complete lack 

 of pollination. On the other hand, there are many recorded failures of 

 the hormone chemicals to improve set and instances in which fruit set 

 was actually decreased. 



The success with growth substances in controlling mature fruit drop 

 naturally led to trials to increase crop production by preventing the 

 abscission of young fruits. Gardner, Marth, and Batjer (17) recorded the 

 failure of NAA and its amide, applied at petal fall, to improve the set 

 of several apple varieties but made no mention of any resulting decrease 

 in set. Burkholder and McCown (10) found that NAA sprays at 10 ppm. 

 applied to Starking at full bloom reduced the number of clusters that 

 held fruit past the June drop by 15.1 per cent, and at 50 ppm. the reduc- 

 tion was 77.7 per cent; the amide at 50 ppm. reduced the set by 34.0 

 per cent. Severe epinasty and leaf scorch accompanied the use of 50 

 ppm. of NAA but no injury attended the amide. If the thought of using 

 either of these substances as intentional fruit thinners occurred to these 

 workers at the time, it was not mentioned. 



With certain citrus varieties one might logically expect an improve- 

 ment in set with growth substances since many varieties are apparently 

 able to set parthenocarpically even without the benefit of applied stimu- 

 lation. Pomeroy and Aldrich (50), however, made extensive trials on 

 Washington Navel oranges and Marsh grapefruit without success. They 

 used several growth substances, both naphthalene and indole compounds, 

 at various concentrations and applied in different ways. Naphthalene- 



