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POI'iOLOGICAL RESEARCH 



9t The Influence of Chemical Treatments on Flo\>eriiig, Fruiting, and Preharvest 



Drop of Fruit Trees 



Many apple and peach varieties tend to set an excessive nxmiber of fruits 

 when they blossom heavily provided that the weather at blossom time is not a 

 factor in reducing the set. Unless the leaf area per fruit is incressed in 

 such years the fruit size at harvest is apt to be so small that much of the crop 

 cannot be marketed at a profit. Also, if removal of excess fruits can be accom- 

 plished shortly after the trees bloom (before flovrer bud initiation commences) 

 it may be possible to "break" the biennial bearing habit of many of our apple 

 varieties. Hand thinning of apples is a long, expensive operation which is 

 usually done too late in the summer to obtain the maximum size benefits for a 

 given degree of thinning or to "break" the biennial bearing habit of the trees. 

 It was our hope in working on this project to find certain chemicals iirhich 

 could be sprayed on the trees early enough to reduce the set, improve size of 

 the remaining fruit, and make many of our alternating varieties more annual 

 in production. 



In addition, this project is concerned vrLth the testing of chemicals v/hich 

 may be suitable for preharvest drop control of apples. All growers are aware 

 that our prinicipal variety, l.iclntosh, may drop much of its crop just before 

 har-zest. We have been interested not only in measuring their drop control 

 effectiveness but also in determining their influence on fruit ripening and 

 storage life. 



For several years now we have been testing several grovfth regulating sub- 

 stances for thinning apples and peaches. For apples the materials naphthalen- 

 eacetic acid (NAA) and naphthaleneacetariiide (NA Amide) have been found to be of 

 considerable value in thinning many apole varieties and improving the size of 

 remaining fruit. Also, v^^hen thinning is sufficiently heavy, the weather favor- 

 able, and the variety not too inherently biennial, regular bearing has frequently 

 resulted from spray applications of these materials applied at late bloom or up 

 to a couple of weeks thereafter, depending on the variety » 



One of the early problems, encountered when M.k was the principle material 

 used for this purpose, was occasional overthinning and severe foliage injury on 

 certain varieties. The use of NA Amide has greatly reduced these risks, Hovrever, 

 we haven't reached the point where the degree of thinning obtained by chemical 

 means can be precisely predicted in advance. Even though chemical thinning of 

 apples lacks the precision desired, it has novr reached the point where it is a 

 rather common commercial practice. The situation with peaches is still in the 

 experimental stage. Neither NAA nor NA Amide are reliable materials for peach 

 thinning but Chloro IPC has shown some promise. 



In recent years the number of chemicals available for preharvest drop con- 

 trol of apples has increased. The principal materials now being used are NAA, 

 2,U,5- trichlorophenoxyacetic acid (2,1|,5-TA), and 2,U,5-trichlorophenoxypropionic 

 acid (2,ii,5-TP), NAA materials v;ere first found useful for this purpose about 

 15 years ago. In the last 5 years 2,li,5-TP and 2,l4,5-TA, to some degree have 

 replaced NAA since they usually control drop for a longer period than NAA« 



