FRANK E. GARDNER 217 



acetic acid at relatively high concentrations reduced the set rather than 

 increased it. Stewart and Heild (44) using 2,4-D also noted a reduction 

 in number of orange fruits set, although not necessarily a reduction in 

 number of harvested boxes because the fruits remaining grew to larger 

 size. The lack of response of citrus varieties in setting additional fruit 

 under hormone stimulation is especially puzzling in view of its naturally 

 parthenocarpic tendency. 



Failure of the hormones to prevent the abscission of young fruits is 

 not restricted to the fruits mentioned. Negative results are frequently 

 deemed of such little interest that they are never published. In Florida 

 the Haden mango blooms profusely but sheds its small fruits to the 

 degree that satisfactory crops are infrequent. The June drop of avocados 

 often converts a seemingly heavy set to a very light crop. Gardner and 

 others in Florida (unpublished results) used various hormone chemicals 

 on these two fruits without benefit. Smith (38), in the hope of controlling 

 the immature drop of pecans, found no effect on shedding from applica- 

 tions of indoleacetic acid or naphthaleneacetamide at 50 ppm, Naph- 

 thaleneacetic acid at this concentration, however, gave a definite increase 

 in the shedding of young nuts. 



If one were to generalize from the above record it would be concluded 

 that with tree fruits hormone treatments have either been without effect 

 on fruit set or, in the higher concentrations, have resulted in reduced set. 

 Schneider and Enzie (36,37) were apparently the first to utifize the 

 abscission-promoting effect of these compounds on young fruits with 

 the specific intent of fruit thinning. They reported that NAA sprays 

 at 100 ppm. on apples nearly eliminated the crop on all varieties tested, 

 but with marked injury to leaves and growing points. At 10 ppm. the 

 effect was more moderate on both scores. Naphthaleneacetamide at 

 80 ppm. performed well in thinning without damage to foliage. Indole 

 derivatives, however, were of no value. Hoffman, Southwick, and Edger- 

 ton (25,39) and also Batjer and Thompson (7) reported on the use 

 of the sodium salt of NAA for thinning apples and in general found it 

 a promising material although subject to variable results — overthinning 

 in some instances and underthinning in others. This may be an expression 

 of the variable state of the trees or of the conditions of spraying rather 

 than an inherent shortcoming of the compound itself. It would appear 

 that more attention should be given to naphthaleneacetamide as a 

 thinning spray rather than to the acid or the sodium salt despite the 



