Damon Boynton 289 



of the color may be poor and preharvest drop tends to be very heavy. 

 Injuries to the leaf surface from insects, diseases, spray materials, or 

 mineral deficiencies modify nitrogen responses in different ways, de- 

 pending on the time and severity of loss of effective leaf area. Early 

 damage or defoliation such as that caused by severe primary infections 

 of apple scab or lime-sulfur injury, may reduce vegetative growth, fruit 

 set, and flower initiation for subsequent crops. Later leaf injuries, such 

 as those caused by mites, by leaf hoppers, by arsenical injury, or by 

 potassium or magnesium deficiencies may have little or no influence on 

 growth, set, or subsequent crop, but tend to decrease fruit size and 

 quality and to predispose the trees to heavy early fruit drop. 



DIAGNOSTIC OBSERVATIONS ON NITROGEN RESPONSE 



These, then, are some of the main practices which the grower may 

 manipulate in the attempt to control nitrogen effects on the Mcintosh 

 apple tree and some of the other conditions that influence nitrogen 

 responses. Under the variable weather conditions of the northeastern 

 United States, it seems unlikely that very close control will be possible 

 but some measure of it may be obtained by adjusting practices accord- 

 ing to careful diagnostic observations. Because of the perennial nature 

 of the apple tree, these diagnostic observations need to take into ac- 

 count separately : tree responses to conditions of the current season and 

 tree responses to conditions of previous seasons. 



The first and most important observation to be made is an evaluation 

 of the crop on the tree, in relation to the bearing capacity of the tree and 

 the bloom, and in relation to the best average fruit quality that is practi- 

 cally attainable. Next, measurements of shoot growth, leaf size, and 

 leaf color appear to be particularly useful in diagnoses of this sort. 

 These are interrelated more frequently than not but furnish in com- 

 bination useful quantitative evidence of the nitrogen status of an apple 

 tree. Table V illustrates the effects of four rates of nitrogen fertiliza- 

 tion on some of these measurements on shoots from two New York 

 Mcintosh apple orchards. For such measurements to be useful under 

 field conditions, they have to be easily made and to be referred to a 

 set of growth standards. Leaf color may be determined by reference to 

 color standards for apple leaves now available or may be made from 



