FRANK E. GARDNER 213 



of adequate coverage inherent in the dusting operation probably accounts 

 for its minor usage in commercial hormone control of fruit drop. 



Tukey and Hamner (49) and also Marth et al. (28), using a highly 

 concentrated solution of hormone in hand aerosol bombs, found this 

 method of application to be effective. Obviously its usefulness is re- 

 stricted to small trees, although Hamner and Rasmussen (18) found that 

 a concentrated oil solution of NAA applied as a vapor by a commercial 

 fog machine (Todd Insecticidal Fog Applicator) could be used success- 

 fully on standard-size trees. The uncontrolled drift of the fog with even 

 slight air currents is the chief limiting factor. 



Since the first trials in 1944 the apple and pear growers of the Pacific 

 Northwest have made increasing use of airplane applications. Naph- 

 thaleneacetic acid dissolved in an oil emulsion in high concentration is 

 applied by low-flying planes equipped to disperse the material in minute 

 droplets which are forced down through the trees by air turbulence 

 created by the planes. This method of application has obvious advantages 

 in covering large acreages quickly and, according to tests conducted by 

 Thompson and Batjer (48), it is quite effective in fruit drop control, 

 although apparently not the equivalent of a thorough, conventional 

 spray application. 



There is no question but that thorough coverage with the applied 

 growth substance is important for maximum drop control regardless of 

 the method of application. While there is some transmission effect 

 through the tissues for short distances, at least in the case of NAA, such 

 effects are quite limited. Batjer and Thompson (5), carefully applying 

 this compound by hand to fruit stems and cluster bases and to the sub- 

 tending foliage only, found that this foliage was the chief means of 

 transmitting the effect to the point of fruit abscission. There was no 

 evidence of transmission, however, from a completely sprayed spur 

 to unsprayed spurs nearby on the same branch. It is entirely possible 

 that in the case of 2,4-D the transmission effects take place over much 

 greater distances in the case of the few varieties on which it is effective. 

 The ability to affect abscission by transmission over considerable distance 

 within the plant tissues will be an important characteristic of new 

 compounds destined for fruit drop control. 



Varietal differences. — While all apple varieties are apparently affected 

 by NAA and its amide, the response varies greatly depending on the 



