FRUIT THINNING WITH CHEMICAL SPRAYS 43 



to determine the initial amount of fruit set before applying- a thin- 

 ning spray. The latter advantage is an important factor in areas 

 that are subjected to spring frosts during the bloom period. 



Naphthaleneacetic acid sprays may be applied any time from the 

 petal-fall stage until 2 to 3 weeks after the calyx period. Applica- 

 tions made at the petal-fall and calyx stage are more desirable from 

 the standpoint of obtaining benefits of early thinning, but the danger 

 from foliage injury is largely avoided by delaying the application 

 until 2 to 3 weeks after calyx. 



The concentrations of naphthaleneacetic acid that have been found 

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

 p. p. m. Results indicate (though there are numerous exceptions) 

 that the later the spray is applied, within the limits stated above, 

 the stronger should be the concentration. Irrespective of timing, the 

 most desirable concentration will depend upon such factors as va- 

 riety, tree vigor, and environmental conditions. 



Varieties of apples considered best adapted for naphthaleneacetic 

 acid thinning sprays with the least possibility of foliage injury 

 include Jonathan, Wealthy, Golden Delicious, Grimes Golden, and 

 Baldwin. These sprays have proved generally more erratic on such 

 varieties as Delicious, Winesap, Early Mcintosh, and Yellow Trans- 

 parent. 



The choice between naphthaleneacetic acid and the dinitro mate- 

 rials as a thinning agent for apples depends upon a number of fac- 

 tors, such as fruit section, variety, and numerous environmental con- 

 ditions. In the Northwest the dinitro materials have proved more 

 consistent and satisfactory than naphthaleneacetic acid, while in 

 certain Midwest and East fruit areas (for certain varieties) the 

 reverse has been true. 



The use of naphthaleneacetic acid thinning sprays on stone fruits 

 is still in the experimental stage. Spray applications during the 

 blossoming period and shortly following have given negative results. 

 Preliminary results with peaches indicate that if the sprays are 

 delayed until approximately 1 month following bloom effective 

 thinning may be obtained. 



Outstanding benefits derived from chemical thinning of apples 

 when successfully executed include a substantial saving in hand- 

 thinning labor, greater fruit size, and a pronounced tendency toward 

 correction of alternate bearing. In many instances production has 

 increased 15 to 20 percent with the continued use of dinitro sprays. 

 The dinitro materials are generally more effective than naphthalene- 

 acetic acid in correcting biennial bearing and increasing fruit size, 

 principally because of the immediate effect of this material on fruit- 

 set reduction. Thinning effects from naphthaleneacetic acid sprays 

 may not take place until 3 to 6 weeks following bloom, depending 

 upon the time of spray application. 



It should be emphasized that chemical thinning sprays in their 

 present form are not foolproof and for this reason cannot be gen- 

 erally recommended. Because of various reasons outlined in this 

 circular some orchards and varieties are not adapted to the use of 

 thinning sprays. However, there are many conditions under which 

 these sprays can be safely and successfully used, provided the fruit 



