14 CIRCULAR 867, U. S. DEPARTMENT OF AGRICULTURE 



Dinitro sprays may cause considerable scorching of the young devel- 

 oping leaves if applied during rainy periods. This type of injury is not 

 too harmful, because the growing points are not usually damaged and 

 the tree recovers rapidly. Slight mottling and crinkling of some leaves 

 may result if cool, humid weather prevails at the time and following 

 the spray application. In Eastern fruit sections, particularly the 

 Northeast, DN No. 1 is frequently preferred to Elgetol, because there 

 is less likelihood of serious foliage injury resulting under conditions 

 prevailing in these fruit areas. In the Pacific Northwest foliage injury 

 from the use of dinitro sprays is rarely a problem. 



Tree Vigor 



Overthinning is less likely to occur when dinitro sprays are applied 

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

 susceptible to the action of these sprays when suffering from effects 

 brought about by imperfectly drained soil, various types of root 

 trouble, winter injury, low nitrogen level, or any condition that may 

 affect normal functioning of growth and fruit-setting processes. 

 Fruit set on trees suffering from the effects of any of the above con- 

 ditions is frequently lower than on normal trees. Experimental evi- 

 dence and repeated observations with trees of this type indicate that 

 use of dinitro sprays may result in reduction of fruit set beyond the 

 point of an adequate commercial crop. Overthinning is probably a 

 result of a much greater indirect effect of the toxicant than usually 

 occurs on normally vigorous trees. It should not be concluded thnt 

 weak trees preclude the possibility of using dinitro sprays. If heavy 

 fruit set can be depended upon, it is likely that lower than standard 

 concentrations (one-half to two-thirds pint of Elgetol per 100 gal- 

 lons) might prove safe and effective. In order to proceed safely, 

 however, it would be necessary to conduct trial tests. 



In certain situations high tree vigor is not necessarily associated 

 with less effectiveness of dinitro sprays. Under West Virginia con- 

 ditions (4) trees moderately low in nitrogen, but not low enough to 

 limit fruit set, were no more susceptible to thinning sprays than trees 

 of the same variety maintained at a much higher level of vigor. It 

 therefore might be concluded that the conditions or causes responsible 

 for a particular state of vigor may be important factors in determin- 

 ing the response of trees to dinitro sprays. 



Technique in Applying Sprays 



The amount of bloom below which dinitro thinning sprays are not 

 advisable varies so greatly as to preclude any specific recommenda- 

 tion. Usually the heavier the bloom the greater is the total fruit set. 

 When using these sprays a general rule is to avoid spraying trees or 

 portions of trees with less than about a 50-percent bloom. With some 

 varieties a bloom of considerably less than 50 percent can be safely 

 sprayed. With a majority of apple varieties the less the bloom the 

 greater the percentage set and the more resistant the blossoms are to 

 the action of the thinning sprays. This safeguard will prevent over- 

 thinning if sound judgment is used in avoiding trees that have such 



