42 CIRCULAR 867, U. S. DEPARTMENT OF AGRICULTURE 



The effective concentration of dinitro sprays is related to weather 

 conditions, tree vigor, variety, and other factors. The concentration 

 of Elgetol, most generally used, ranges from 1 pint to 1 quart per 

 100 gallons. In areas where DN No. 1 is used, effective concentration 

 varies from y 2 to 1 pound per 100 gallons. Quantities greater than the 

 above may result in appreciable foliage injury in some fruit sections. 

 particularly when applied during cool, humid weather. 



Success with dinitro thinning sprays is predicated on the assump- 

 tion that fruit set will be heavy. It therefore follows that these 

 sprays can be used with greater safety on varieties that consistently 

 set fruit considerably in excess of the amount required for a com- 

 mercial crop. The partially self -fruitful varieties of apples are con- 

 sidered best adapted for chemical thinning because of their heavy 

 fruit-setting characteristic. This group includes Yellow Transparent. 

 Duchess, Early Mcintosh. Wealthy, Jonathan, Golden Delicious. 

 Baldwin, York Imperial, Rome Beauty, and Yellow Newtown. In 

 the Northwest fruit sections self -unfruitful varieties (Winesap. De- 

 licious, Stayman, and Mcintosh) are safely and effectively thinned 

 with dinitro sprays when weather conditions are favorable for fruit 

 set and in orchards where there is adequate provisions for cross- 

 pollination. In Midwest and East fruit sections self -unfruitful va- 

 rieties are not generally considered adapted for chemical thinning 

 with dinitro sprays. 



Weather conditions existing during the bloom period are perhaps 

 the most important factor in the success of dinitro thinning sprays. 

 If the weather from the time of the first opening of blossoms until full 

 bloom (a period of about 3 to 7 days) is characterized by cool, humid 

 conditions, dinitro sprays are likely to thin much heavier than if 

 the spray application is preceded by warm, dry weather. When 

 adverse weather and restricted cross-pollination conditions prevail, 

 dinitro thinning sprays are likely to result in a greater set reduction 

 than desired. In such instances, a combination of both a light set and a 

 greater than average killing power of the spray may result in serious 

 overthinning. Failure to obtain sufficient thinning with dinitro sprays 

 is often associated with weather and other conditions that are con- 

 ducive to heavy 7 fruit set. 



Overthinning is less likely to occur when dinitro sprays are applied 

 to trees in a normal to above normal state of vigor. Trees are more 

 susceptible to the action of thinning sprays when suffering from the 

 effects of imperfectly drained soil, various types of root trouble, 

 winter injury, low nitrogen level, or any condition that ma}' affect 

 normal functioning of growth and fruit-setting processes. 



In many instances the application of dinitro sprays does not result 

 in sufficient reduction of fruit set. In sections where heavy thinning 

 is required for satisfactory size and quality dinitro sprays should 

 be more properly regarded as a supplement rather than a substitute 

 for hand thinning. 



Naphthaleneacetic acid or its sodium salt is an effective thinning 

 agent for most varieties of apples, particularly under conditions 

 prevailing in Midwest and East fruit sections. This material is 

 applied as a postbloom spray, and its chief advantages over the 

 dinitro spray are greater latitude in timing and in the opportunity 



