Pre-Harvest Fruit Drop 245 



2,4,5-T, the effect lasts for from 4 to 6 weeks. This compound has es- 

 sentially displaced all others in the control of pre-harvest drop of 

 apples in the United States. 



The amide of 2,4,5-T has been reported as being particularly 

 effective in the control of fruit drop (Edgerton and Hoffman, 1953). 



Not only do the phenoxy acid auxins show a longer lasting effect 

 in the control of fruit drop, but they take a considerably longer time 

 to begin their effects after spraying. Naphthaleneacetic acid holds 

 fruits on after only 2 or 3 days from the spraying time, but 2,4,5-T 

 and 2,4,5-TP do not generally control drop effectively for some 10 

 to 17 days after spraying. Such delays in effectiveness can be seen in 

 the data of Erickson et al (1952). A few reports indicate a retarded 

 fruit drop as early as 4 days after application of 2,4,5-TP (Southwick 

 et al, 1953), but such early control is not general. 



A good many other auxins ha\e been tested for this use with less 

 success. It may be of interest to note that /i?-propionic and yg-butyric 

 phenoxy acids are ineffective in prevention of fruit drop (Osborne 

 and Wain, 1951). 



a. 

 o 



Q 





UJ 



o 

 a: 



Ld 

 Q. 



15 DAYS 



TIME AFTER AUXIN APPLICATION 



Fig. 104. The relative effectiveness of naphthaleneacetic acid (NAA) and 

 a-(2,4,5-trichloiophenoxy)-propionic acid (2,4,5-TP) in controlling drop of Mcintosh 

 apples (Southwick et al, 1953), showing the more rapid control by NAA and longer 

 duration of control by 2,4,5-TP. Both auxins used at 20 mg./l. 



