208 PLANT GROWTH SUBSTANCES 



cellaneous cuttings that had been treated to stimulate rooting. Other 

 instances were also noted of delayed abscission of various treated organs, 

 particularly floral parts. Since the first report of control of apple drop in 

 1939 by Gardner, Marth, and Batjer (16) there have been not less than 

 seventy-five scientific publications dealing with hormone chemicals in 

 relation to fruit drop. As a result of these various reports hormone sprays 

 have become a standard orchard practice for apples and pears in most 

 sections where these fruits are grown, in many cases being applied by 

 airplane over large acreages. It should be noted that at the time the idea 

 was conceived plant-hormone chemicals were being produced in minor 

 quantities and consequently were sold at prices that would make their 

 use for orchard spraying appear to be fantastic. It has been the history 

 of most synthetic biological compounds, however, that chemical know- 

 how coupled with sufficient demand has resulted in great price reduc- 

 tions. Accordingly, a plant investigator should not discard, untried, his 

 ideas for practical applications simply because of the current price of the 

 compound under consideration. 



Little would be served by attempting to review all of the papers 

 relating to apple drop control. The first detailed report of the original 

 work (17) established the effectiveness of a-naphthaleneacetic acid 

 (NAA) and its amide at concentrations of 5 to 10 ppm. Many of the 

 subsequent reports served to confirm the early findings and to extend 

 the results to additional varieties and conditions. Omission of specific 

 mention of each of these numerous papers in no way reflects on their 

 importance and helpfulness. A number of reports have served to demon- 

 strate the usefulness of various carriers for the hormone compounds and 

 also methods of application. Still others have given techniques for 

 screening compounds for effectiveness and have furthered the knowledge 

 of penetration and movement of these substances within the tree. 



Despite efforts to find more effective or more adaptable compounds 

 for drop control, NAA and its amide and salts remain the most useful. 

 In searching for more effective compounds Batjer and Marth (3) found 

 that 2,4-dichlorophenoxyacetic acid (2,4-D) applied to Winesap apples 

 extended the effective period greatly beyond that of NAA, but that it 

 did not take effect as quickly. The results were so exceptionally good on 

 Winesap that many other varieties were subsequently tried by Batjer 

 and Thompson (4) and Harley et al. (19,20). Unfortunately, the only 



