OVERCOMING THE PROBLEM 



. . .With NAA 



Use of a plant-growth regulator to control fruit drop became practical 

 in 1939 when horticulturists at the U.S. Department of Agriculture's Plant 

 Industry Station reported that NAA--naphthaleneacetic acid or its salts 

 or esters--would retard the drop of apples. Soon after, NAA was found 

 effective for retarding the drop of Bartlett pears, through experinnents 

 at the California Agricultural Experiment Station. 



The chief limitation of the NAA compounds is their brief effective- 

 ness, which usually lasts not more than 10 days or Z weeks. When the 

 effect wears off, fruit on the tree is likely to drop quickly, often in quan- 

 tities overnight. Repeating the spray to prolong effectiveness has not 

 proved very helpful. 



Despite its limitations, NAA has, fronn the first, been widely used 

 and valued by growers. It is still chosen in some situations. It is the only 

 fruit- sticking chemical found practical for airplane spraying. 



.With 2,4-D, 2,4,5-T, and 2,4,5-TP 



Initial success of NAA has led to many further experiments with 

 growth -regulating chemicals in a search for longer -lasting control of 

 fruit drop. Thus far, 3 phenoxy compounds have proved their practical 

 value. They can be applied conveniently early--3 to 4 weeks ahead of the 

 rush work of harvest- -with expectation of controlling drop during harvest, 

 even if picking takes longer than the usual 3 weeks. Each of the longer- 

 lasting compounds has some limitations in use: One protects only certain 

 apple and pear varieties; others protect the commercial varieties of 

 apples but not pears. None is satisfactory for spraying from the air. 



Experiments with 2,4-D (2,4-dichlorophenoxyacetic acid) at USDA's 

 Plant Industry Station, Beltsville, Maryland, dennonstrated strikingly that 

 the growth-regulating chemicals can be specific in their fruit-sticking effect. 

 This chenriical controls drop of 2 apple varieties-- Winesap and Stayman-- 

 and also of Bartlett pears. But when 2,4-D was tried on apples other than 

 Winesap and Stayman, the trees dropped fruit almost as though no treat- 

 ment had been applied. 



More recently, research initially begun at the Agricultural Experi- 

 ment Station of Cornell University has established wide usefulness of the 

 phenoxy compounds 2,4,5-T (2,4,5-trichlorophenoxyacetic acid) and 2,4,5- 

 TP (2,4,5-trichlorophenoxyproprionic acid). Both can be used in salt and 

 amide forms as well as the acid. Both are satisfactory for sticking fruit 

 on all commercial varieties of apples. Neither of these compounds, how- 

 ever, is suitable for pears because it may damage foliage and fruit. 



SPRAYING VERSUS DUSTING 



Spraying is commonly preferred to dusting as a means of applying 

 stop-drop chemicals. Ground spraying is somewhat more effective than 

 spraying from airplane. For ground spraying, growers use any of the 

 conventional machines used in pest control. 



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