246 Auxins In Agriculture i 



It is rather striking to observe that the effectiveness of an auxin 

 in controlling pre-harvest drop of apples is not in proportion to its 

 activity in growth. This has been pointed out by Edgerton and Hoff- 

 man (1951), and is a particularly interesting observation in view of 

 the fact that there is no clear-cut concentration effect observable in 

 the control of pre-harvest drop. The effects of auxin in nearly all its 

 known functions show a quantitative increase in effectiveness with in- 

 creasing concentrations. In the control of pre-harvest drop, however, 

 there is no consistent difference between concentrations ranging from 

 approximately 5 to 50 mg./l. Most consistent results have been ob- 

 tained with concentrations of 10 to 20 mg./l. of these various auxins 

 and, consequently, the use of such concentrations has become general. 



The relative rapidity of effectiveness and the duration of the 

 effect of naphthaleneacetic acid and 2,4,5-TP are compared in the 

 data shown in figure 104. It can be seen that the naphthaleneacetic 

 acid afforded good control of fruit drop from 2 to about 10 days after 

 the spray was applied, and after that the dropping rate was as high 

 or higher than with untreated fruits. The 2,4,5-TP, however, started 

 to control drop effectively after 9 days, and was still giving excellent 

 control 15 days after application. 



Auxin sprays to prevent fruit drop have been used effectively 

 on apricot (Hesse and Davey, 1942), pear (Davey and Hesse, 1942), 

 orange (Stewart and Klotz, 1947), grapefruit (Stewart and Parker, 

 1948), lemon (Stewart and Hield, 1950), and almond (Serr and Forde, 

 1952). Its use has been attempted unsuccessfully on grape (Pentzer, 

 1941) and peach (Hesse and Davey, 1942). 



Techniques of Application 



The commonest method of applying auxins for pre-harvest drop 

 control is in aqueous sprays. In their original study, Gardner et al 

 (1940) showed that the auxin applied to the stem ends of the apple 

 fruits was considerably more effective than auxin applied to the 

 distal (blossom) end of the fruits or to the leaves. Later studies of the 

 effects of different placement of the sprays have shown that not only 

 are materials effective when applied to the fruits, but also some effect 

 is obtained when applied to the leaves immediately adjacent to the 

 fruits (Batjer and Thompson, 1948). The effectiveness of the spray is 

 quite localized and is effective only for fruits within approximately 

 3 inches of the locus of application. 



Another technique which has been reported to be effective is the 

 use of dusts containing the auxin (Hoffman et al, 1942). Dust applica- 

 tions have not been uniformly successful, however, and have not re- 

 ceived widespread use. 



