244 Auxins in Agriculture 



abscission by auxin there must be a sufficiency of certain nutritive 

 materials — especially carbohydrates (Livingston, 1950). Indirect evi- 

 dence that this same factor has a bearing on pre-harvest drop control 

 has been produced by Thompson (1951). He found that auxins applied 

 to weak trees with yellowing foliage presumably low in carbohydrates, 

 could not effectively prevent pre-harvest drop. 



METHODS OF APPLICATION 



Selection of Auxins 



In the first study of pre-harvest drop control with auxins, Gardner 

 et ol (1940) found that of several different auxins tested, a-naphtha- 

 leneacetic acid was the best. The amide form of this same auxin per- 

 formed essentially in the same manner. For more than a decade this 

 auxin was the most effective one reported in the literature. After 

 being sprayed on the trees it took 2 or 3 days to control drop, and 

 effective control lasted from 7 to 14 days depending upon the variety 

 of apple. In some instances there was an extremely effective reduction 

 in fruit drop, as much as 78 per cent (Murneek, 1940). The major 

 limitations of this auxin are (1) it is not effective on several varieties 

 such as Rhode Island Greening, Jonathan and Delicious, and (2) its 

 effectiveness on Mcintosh is very short-lived, usually 7 or 8 days. This 

 second limitation is unfortunate since Mcintosh is particularly prone 

 to drop its fruits and is one of the commonest orchard varieties in 

 the United States. 



The compound 2,4-D was found to be particularly effective on 

 Winesap and closely related varieties (Batjer and Marth, 1949), but 

 was much less effective on more distantly related varieties and has 

 never achieved widespread use for this reason. This is an interesting 

 case of a genetic specificity for an auxin, in that one group of closely 

 related varieties responds well to a given auxin whereas other varieties 

 do not. 



2,4,5-T (2,4,5-trichlorophenoxyacetic acid) has more recently been 

 found to be highly effective in controlling drop of Mcintosh apples 

 (Hoffman and Edgerton, 1952). This auxin effectively controls apple 

 drop for the remarkably long period of 4 to 6 weeks after spraying and 

 appears to be the best controlling agent for Mcintosh apples. 



2,4,5-TP (a-2,4,5-trichlorophenoxypropionic acid) is another very 

 effective auxin for controlling drop on Mcintosh apples (Edgerton 

 and Hoffman, 1951), and while it does not produce as strong an effect 

 as 2,4-D in controlling drop of Winesap apples, it has the advantage 

 of affording effective control on most other apple varieties. Like 



y 



