FRUIT THINNING WITH CHEMICAL SPRAYS 19 



June drop than they are after this drop is over. From the limited 

 data available it seems doubtful that any appreciable thinning could 

 be accomplished with a hormone spray after the June drop unless 

 abnormally high concentrations were used, which might result in 

 some form of injury to either leaves or fruit. 



No definite explanation of the physiological effect of naphthalene- 

 acetic acid in reducing the set of apples has as yet been offered. 

 However, there is a limited amount of information that seems to indi- 

 cate the possible mechanism of action. In thinning experiments it has 

 been frequently observed that naphthaleneacetic acid sprays result in 

 heavier thinning on weak spurs located in shaded parts of the tree 

 than on the more vigorous spurs of well-exposed wood. An experi- 

 ment conducted by Southwick and Weeks (40) confirms these obser- 

 vations. Their data show that dormant flower buds of "on-year" 

 Wealthy trees having a diameter of only 4.7 millimeters or less were 

 more severely thinned by postbloom sprays than those buds having 

 a diameter of 4.8 millimeters or more. 



It has long been recognized that a good supply of reserve food is 

 essential for fruit setting. Heinicke (18) has emphasized the im- 

 portance of the vigor of individual spurs as a factor in fruit setting. 

 His work shows that when the seed number is constant, the vigorous 

 spurs that can supply adequate food and water exhibit a greater 

 capacity for fruit setting than weaker spurs. If the seed content is low 

 the spur must be vigorous, else the young fruits will likely shed. On 

 the other hand, fruits having a high number of seeds may develop 

 on relatively weak spurs. 



When naphthaleneacetic acid is used as a harvest spray to delay 

 fruit drop, Smock and Gross (37) have shown that it causes a tem- 

 porary increase in the respiration rate of the fruit. If a post-bloom 

 spray for thinning has a similar effect on the young fruits and vege- 

 tative tissues of the tree, it could conceivably reduce the food supply 

 to the young fruits and in this way induce abscission of the "weaker" 

 fruits. Whatever the cause, naphthaleneacetic acid sprays tend to 

 eliminate the type of fruit that is likely to develop into low grade 

 apples at harvest. 



Concentration 



The concentrations of naphthaleneacetic acid that have been found 

 most appropriate for thinning apples range from 5 to 20 p. p. m. It 

 is not possible to make general recommendations, since the concentra- 

 tion that may prove best will depend on the variety, the stage of 

 fruit development when applied, and the environmental factors that 

 affect fruit setting. Some of the results pertaining to concentration 

 are summarized in table 6. In attempting to interpret these data 

 the variety, orchard, year, and area should be considered in compar- 

 ing the normal set of check trees with the amount of thinning result- 

 ing from a given concentration. In many areas of the East and Mid- 

 west apple sections the weather in 1945 and 1947 was unfavorable 

 for fruit setting. 



In general, under East conditions, varieties that consistently set 

 heavily, such as Wealthy, Baldwin, and Golden Delicious, require a 



