FRANK E. GARDNER 215 



available that the petiole-drop test may be a simple and useful means 

 not only of screening varieties for response to a particular compound 

 but also of screening new compounds for effectiveness in controlling 

 drop of a particular kind of fruit. 



Temperature. — It is recognized that hormone responses in general are 

 favored by high temperatures and impeded by cold. Without any 

 precise information available, it was thought that temperatures lower 

 than 70°F. at the time of spraying were unfavorable for drop control 

 even though a temperature rise took place later on. In this connection 

 the work of Batjer (i) is of considerable interest. He found that NAA 

 sprays applied at midday with the temperature at approximately 8o°F. 

 were consistently more effective than sprays apphed in the early morning 

 of the same day with the temperature approximately 20 degrees lower. 

 From a practical standpoint the difference in control was not great, and 

 there was some indication that the unfavorable influence of the lower 

 temperatures might be compensated for by using a higher concentration 

 of the hormone. The work of Overholser et al. (35) is in complete 

 agreement on the point of temperature effects at time of spraying. 

 Batjer, in attempting to study the temperature relationship more closely, 

 used the apple petiole technique in which the leaf blades were removed 

 and the petioles sprayed at a controlled range of temperatures. The 

 subsequent rate of abscission of the petioles was recorded at a uniform 

 temperature for all lots. The results indicated that above 72°F, tempera- 

 ture effects were negligible. Below 72°F. the control of abscission was 

 increasingly poorer with lowering of the temperature. 



Fruit Thinning and Fruit Set 



As all fruitgrowers know, thinning by hand is a tedious and expensive 

 operation. Thinning is frequently necessary, however, in order to secure 

 fruit of marketable size and to insure regular annual bearing with certain 

 varieties that are prone to overcrop one year and fail to set the next. 

 Numerous studies have been made to establish a practical thinning pro- 

 cedure by use of certain caustic sprays. The results have been variable 

 because the process is dependent on kiUing or injuring only a portion 

 of the flowers and young fruits, causing them to abscise. The procedure 

 is not without hazard since the margin of safety is narrow before injury 

 to the fohage and twigs occurs. Early thinning is also complicated by the 

 risk that frost may subsequently kill additional flowers or fruit and thus 



