Pre-Harvest Fruit Drop 247 



Other techniques that have been used include aerosol sprays 

 (Tukey and Hamner, 1945) and concentrated solutions applied from 

 airplanes, particularly in the large orchard areas of the Pacific North- 

 west. 



Experiments with naphthaleneacetic acid have shown that re- 

 peated applications at intervals of from 3 to 7 days may somewhat 

 prolong the effective control obtained (Gardner et al, 1940), but the 

 more recent use of 2,4,5-T and 2,4,5-TP with their long periods of 

 effective control have essentially obviated the need for repeated spray- 

 ing. 



The time of application of the spray is, of course, of great impor- 

 tance to its effectiveness. If the breakdown of the middle lamellae of 

 cells of the abscission zone proceeds too far, no auxin application 

 could be expected to prevent abscission. Therefore it is most impor- 

 tant that the auxin be applied when the first signs of fruit drop 

 appear or as soon as the grower realizes that a delay in picking time 

 will be necessary. 



FACTORS IN SUCCESS 



Severity of Natural Drop 



The usefulness of auxin sprays to control pre-harvest drop will 

 obviously be a function of the severity of the fruit drop which would 

 occur without the spray. Under particularly adverse conditions as 

 much as 90 per cent of an apple crop may be lost by fruit drop. 



Several environmental factors are recognized as playing a part in 

 causing fruit drop. A spell of warm weather at or shortly before 

 harvest time is the most common of these factors; also frost and wind- 

 storms can result in heavy drop (Roberts and Struckmeyer, 1943). 



Several factors inherent to the tree itself are also recognized as 

 favoring fruit drop. Among these are high levels of available nitrogen 

 (Southwick, 1940), a particularly heavy apple crop (Southwick and 

 Shaw, 1941), a low seed content in the fruits (Southwick, 1939) or 

 trees weakened by disease, animals, or insects (Enzie and Schneider, 

 1941). Some varieties are recognized as more susceptible to fruit drop 

 than others and these include Mcintosh, Duchess, Delicious, Williams, 

 and Winesap. 



Delay in Picking Schedule 



As apples mature and the auxin production in the seed drops 

 lower and lower (figure 103), the tendency to abscise becomes greater 

 and greater. Unless the fruits are picked before ripening proceeds 

 too far, fruit fall can be expected to be excessive. Frequently growers 



